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DIY top mounted intercooler


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Hey all,

Iv looked and looked at all the options for intercooler upgrades for my LGT below... The child's writing about the wheel weight was for a US forum sorry...


261 Jaemyn & Chris smaller by flamshackle, on Flickr

As far as intercoolers go- I have not been overly impressed. What I have discovered is that most if not all the manufacturers I have researched make small to large compromises in the flow of the cores and tanks based on mass production, men compensating for something, easy of manufacture and the bottom line design value (MAKING $ OUT OF US)

With a little time and TLC I feel its wont be hard to do away with some of the shortcuts and build something that not only cools well but also FLOWS BETTER (more on this later...)

Because of this I have decided to build my own top mount intercooler ;D

Being one of the tight in the rear ended variety of persons I will develop and build three of this design, selling off two (to some lucky customers here in New Zealand) hopefully making my one essentially cost free! (Bar the stupid amount of time that will be spent on R+D)

So this thread will cover the development and build over the next few months. My background is fabrication and I previously fabricated my own end tanks and front mount setup for my WRX race car.

I am a busy man and dont have oodles of time but all that said it will be fun to get back into some fab work and doco the build for all to hassle and heckle!

Its important to note as I dive into this that my project car is the JDM LGT (2.0 engine) with the tincey wincey twin scroll huffer (love that early boost!)

So in light of this I have made the call to go with a top mount upgrade in keeping with the size of the 2.0 engine and my love of the sweet twin scroll response. A front mount would add lag and make this tiny wee cute little huffer of mine loose much of its shining qualities (in that it comes on boost VERY early).

So if you interested keep in touch to see some dodgy jigs get built (custom wood I am thinking), wild exaggeration, ruthless pursuit of function over form and all of this will unfold in my living room! haha!

PS I am presently also single turbo'ing my mates TT so the time is not easy to come by...

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Anyway I got my jig sorted! I know its average but im using custom wood for the jig! (cos its free) standard intercooler is a super restrictive unit! I can see loads of potential improvements already... anyway here she is! the jig almighty!


IMG_3191 by flamshackle

internal shot of heinous internal passage for the air to "flow" through...


IMG_3171 by flamshackle

the jig with standard cooler on it...


IMG_3190 by flamshackle

So next job is to get the cores I ordered on there and start fabing the mock plastic end tanks to make my patterns from... also a couple of other ideas Im working on to make this bad boy flow like a river! ;D

will update soon...

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 scottiescottiescottie said:

Hey soopersubaru, yea I did see it. looks great :~)

How much did it cost you to make?

$ 250 for the top mount rework, Radiator for front $275 (brand new) Intake manifold thermally coated $350 then a circ pump, and water lines to be added.

All up around $950.

The one shown is the Mk3 version.

Would like to fit another one and hopefully recover some costs once others realise it does work extremely well for the road car. (Soon to fit to our rally car too)

I did initially suggest $2000 as a retail cost installed.

THEN the bagging began!! No doubt some will climb into me again on this thread.


It works and that's that!!

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The uprated MRT top mount is over $1100 AUD and looks hugely overpriced. Will be very interested in how you go.

You can buy that direct from the manufacturer "hyperflow" and IMHO is one of the best top mount upgrades on the market... I personally hope to make my design flow slightly better than this with a couple of small improvement.

As for the cost I really don't know but will work out costs and keep it as cheap as possible :~)

[quote name='soopersubaru said:

$ 250 for the top mount rework, Radiator for front $275 (brand new) Intake manifold thermally coated $350 then a circ pump, and water lines to be added.

All up around $950.

The one shown is the Mk3 version.

Would like to fit another one and hopefully recover some costs once others realise it does work extremely well for the road car. (Soon to fit to our rally car too)

I did initially suggest $2000 as a retail cost installed.

THEN the bagging began!! No doubt some will climb into me again on this thread.


It works and that's that!!

Well I guess some conclusive and clear results would win people over to see if it really works well or not. After reading your thread (and knowing what I know about infer oolong) it looks like it would work very well!

$2000 does seem like a lot of $ though? It would have to bs considerably better than an air to air upgrade to justify it being twice the price I would think.

Good stuff for developing something cool for your own car though!!! I love a bit of home made action! Keeps the bigger performance upgrade companys real!

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Scottie....Thanks for your comments! positive feed back is great!!

It DOES work exceptionally well. Clear and conclusive results are possible if I can find another car to compare with,, on the same day on the same track with laptops hooked up.

As yet no takers.

Maybe when you finish yours we can team up and see.

My WAIC is very good in the real world of 100kph road driving, IAC remains at 8 deg above ambient.

This IAC temp falls further when you slow down and come to a stop for intersections,lights etc.

TMIC allow the IAC to rise, so do FMIC.

Therefore when you want power ,,,You certainly get it!! be that from a standstill or when overtaking.

My 1500kg wagon performs easily as well as those 6 cylinder Turbo fords (4.0 litre) and those V8 Holden thing's with over 5.0 litres/NA

So for a 2.0 litre 4 cylinder turbo its fantastic! it does 14km per litre cruising and 3km per litre on the track.

Cold air is great for any high compression ratio turbo.

So the spin off for my "expensive" unit is...better economy and higher performance...For a ROAD car!

I have offered to fit one to a car at cost.......Still just doubters no takers!! ( just [email protected]#%K$&^RS!! who bag it. )

Now if it was stamped STI, HKS, RAGE, TURBO SMART, or something similar i reckon it would sell quick as!!

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 soopersubaru said:

I have offered to fit one to a car at cost.......Still just doubters no takers!! ( just [email protected]#%K$&^RS!! who bag it. )

Now if it was stamped STI, HKS, RAGE, TURBO SMART, or something similar i reckon it would sell quick as!!

Seriously, I think if you got some conclusive comparible results instead of hostile posts like your one above you would sell units.

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 scottiescottiescottie said:

But yes! very keen to chat and compare some data :~) I think my theory at this stage is that the water to air will be better in stop start environments but air to air with water sprey will bs better on constant running. Open to being wrong however.

You are correct in this assumption.!

FMIC particularly good for track work. The trouble with the TMIC is getting enough surface area for required cooling and also flowing enough air through it the achieve the cooling required. Particularly bad on hot days and low speeds.

Water spray also works very well as the latent heat of vaporisation is utilised.

My basic data trouble is that i have no prior to mod readings.

Over the next few months we shall be fitting one to our rally car. Readings are going to be taken prior to mod and post mod.

Small WRX TMIC intercooler circa '93 first, Followed by the larger twin turbo Top mount.

Both with spray and without.

Then the Soopercooler also with and without spray. Daytime temp will be the only variable. (front radiator will have spray fitted same as my wagon)

These will be thermal overload tests......That is:- Holding 20 psi at 100 kph on the brakes until 60degC IAT is reached. Over Time taken to achieve. A test anyone can do!

In this way we hope to further prove the unit.

Will be happy to discuss this further and compare with your data.

P.S Just get wound up with peeps baggin what they do not understand,, Then I instinctively react too it!

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FMIC particularly good for track work. The trouble with the TMIC is getting enough surface area for required cooling and also flowing enough air through it the achieve the cooling required. Particularly bad on hot days and low speeds.

Water spray also works very well as the latent heat of vaporisation is utilised.

My basic data trouble is that i have no prior to mod readings.

Over the next few months we shall be fitting one to our rally car. Readings are going to be taken prior to mod and post mod.

Small WRX TMIC intercooler circa '93 first, Followed by the larger twin turbo Top mount.

Both with spray and without.

Then the Soopercooler also with and without spray. Daytime temp will be the only variable. (front radiator will have spray fitted same as my wagon)

These will be thermal overload tests......That is:- Holding 20 psi at 100 kph on the brakes until 60degC IAT is reached. Over Time taken to achieve. A test anyone can do!

In this way we hope to further prove the unit.

Will be happy to discuss this further and compare with your data.

My plan is to compare mine to another after market TM and FM cooler (air to air) The option to compare it to yours will be great too! flick me a PM later on when the coolers made up and we can sort out how to pull that off :)

That heat soak test is actually a stroke of genius for comparable results under WOT.

[quote name='soopersubaru said:

P.S Just get wound up with peeps baggin what they do not understand,, Then I instinctively react too it!

All good sounds like typical kiwi behavior to me... We are the worst in the world for baggin stuff before we even listen...

got some pics to upload tonight of the progress! its coming along slow but steady.

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 slystiguy said:

I don't think the issue it really weither or not it works sooper, more just the fact that for the price you ask it would be smarter/cheaper to go air to air

Cheaper not smarter.

Ya only gets what you pay for in this world.

Will fit at cost price $ 650 if inlet manifold does not need replacing or require thermal coating.

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 soopersubaru said:
Cheaper not smarter. Ya only gets what you pay for in this world.

Will fit at cost price $ 650 if inlet manifold does not need replacing or require thermal coating.

Far out! your total build cost is $650??? thats what I am thinking my costs will be for an air to air top mount... Thats a flippin reasonable price mate!

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the cooler inlet and outlets were the same external diameter but different internal diamters but only by 3mm...

Got some inlet outlet mounting rounds turned up as per images... also got my cores!!!


IMG_2951 by flamshackle

They are $460 for the core alone from Heat exchangers christchurch... I am not entirely happy with them at this price but its the only local vacuum brazed bar and plate I could find.


I intend to weld the tanks flush with the internal tube entry points as below pic shows...


IMG_3170 by flamshackle

The core weighs double the factory item WITHOUT TANKS! this will ensure good heat sink qualities... good for road use upgrade... will be very interesting to see the test results when it comes to speed of heat transfer on this core :-\


IMG_3174 by flamshackle

I am planning to make up a rectangle to oval for the end tank to work for the side as seen below...

will be a mission but should flow better than the "Hyperflow" end tank design I think...


IMG_3180 by flamshackle

Anyway I am working on the end tanks in plastic first then will transfer patterns to sheet alloy and tip the round sections... should look ace! :)

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You may find this interesting...I actually made one, Copied...Not my story!

In this story I want to tell you about a brand new concept in road car intercooling.

It has the potential to be very efficient, cheap to put together, compact and can keep charge-air plumbing very short. It is also, AFAIK, unique in that no-one else has ever previously used the approach. The design is suitable for cars where the off-boost turbo outlet temperature is below 50 degrees C in nearly all weather conditions. That is, when driving down the highway, when driving in urban areas and when stopped in traffic, the temperature of the un-boosted air coming out of the turbo needs to be less than 50 degrees C. That's the case in many turbo cars - but not all of them. How much above 50 degrees the boosted air temp rises to doesn't really matter, because these temps will be reduced by the intercooler.

Intrigued? Well, hang on for the ride as we cover stuff that you haven't heard of since high school or university chemistry and physics!

Intercooler Functioning

But first, some background. Not many people realise it, but road car intercooling on turbo cars works in a way that's very different to the common perception. We said it in a previous article (the first part of our series on our Intelligent Intercooler Water Spray), but it's worth repeating here.

It seems straightforward enough. An intercooler acts as an air/air radiator for the intake air, cooling it after the compression of the turbo has caused it to get hot. The compressed air passes through the intercooler, losing its heat to the alloy fins and tubes that form the intercooler core. This heat is immediately dissipated to the outside air that's being forced through it by the forward movement of the car. (We'll get to water/air systems in a moment.)

The trouble with this analysis is that - for a road car - it is not entirely correct. Huh? So what actually happens, then?

In road cars, intercoolers act far more often as heat sinks rather than as radiators. Instead of thinking of an intercooler as being like the engine coolant radiator at the front of the car, it's far better to think of it as being like a heatsink inside a big sound system power amplifier. If an electric fan cools the amplifier heatsink, you're even closer to the mark. Importantly, because the power spike is just that (a spike, not a continuous high output signal), the heat that's just been dumped into the amplifier's heatsink is dissipated to the air over a relatively long period. This means that the heatsink does not have to get rid of the heat at the same rate at which it is being absorbed.

Now, take the case of a turbo road car. Most of the time in a turbo road car there's no boost occurring. In fact, even when you're driving hard - say through the hills on a big fang - by the time you take into account braking times, gear-change times, trailing throttle and so on, the 'on-full-boost' time is still likely to be less than fifty percent. In normal highway or urban driving, the 'on-full-boost' time is likely to be something less than 5 per cent!

So the intercooler temperature (note: not the intake air temp, but the temp of the intercooler itself) is fairly close to ambient most of the time. You put your boot into it for a typical quick spurt, and the temperature of the air coming out of the turbo compressor rockets from (say) 40 degrees C to 100 degrees C. However, after it's passed through the intercooler, this air temp has dropped to (say) 55 degrees. Where's all the heat gone? Traditionalists would say that it's been transferred to the atmosphere through the intercooler (and some of it will have done just that) but for the most part, it's been put into the heatsink that's the intercooler. The temperature of the alloy fins and tubes and end tanks will have risen a bit, because the heat's been stored in it. Just like in the amplifier heat sink. Then, over the next minute or so of no boost, that heat will be transferred from the intercooler heatsink to both the outside air - and, importantly, also to the intake air going into the engine. Since the engine's now off boost, that heating of the intake air is of no consequence.

A Real-Life Example

I once had a high-boost Daihatsu Mira Turbo in which I ran a water/air intercooling system. The water/air heat exchanger comprised a highly modified ex-boat multi-tube copper heat exchanger, with a few litres of water in it. An electric pump circulated the water through a separate front-mounted cooling core. Intake air temp was measured using a thermistor and a dedicated LCD fast-response meter.

In normal point-and-squirt urban driving, the intake air temp remained the same with the intercooler pump switched either on or off!

Why? Because when the car was on boost, the heat was being dumped into the copper-tube-and-water heatsink, and when the car was off-boost, this heat was fed back into the (now cooler) intake air flow. Of course, if I was climbing a long hill (ie on boost for perhaps more than 15 seconds) the pump needed to be operating to give the lowest intake air temps. But even in that tiny car, 15 seconds of constant full boost would achieve over 160 km/h from a standstill...

The latter shows why water/air intercooling in road cars is so successful - but why most race cars use air/air intercooling. Water has a very high thermal mass, so easily absorbing the temp spikes caused by a road car's on/off boost driving. However, race-style boost (say on full boost for 70 per cent of the time) means that the system has to start working far more as a real-time heat transfer mechanism - which is best done by very large air/air intercoolers.

Let's run that point by you again: in a water/air intercooling system being used on a road car, the measured intake air temps are much the same whether the intercooler water pump is running or not. The point-and-squirt style of boost usage in a road car simply means that the heat gets dumped into the water/air heat exchanger (reducing the intake air temps over the turbo outlet temps) when the car is on boost, then gets slowly fed back into the intake airstream when the car is off-boost. The water/air heat exchanger knocks off the temp spikes that occur on-boost, at the cost of slightly elevating the off-boost temps.

So is it possible to build an intercooler - really, a heatsink - that has no external cooling? That is, its whole purpose in life is to absorb the on-boost heat and then feed it back into the intake air stream when the car is off-boost?

Thermal Mass

The limiting factor in such a 'closed-loop' heatsink intercooler design is the amount of heat it can absorb. In the small water/air core being used in the Mira Turbo, the thermal mass (the amount of heat able to be absorbed for a given temperature rise) was sufficient for all but the duration of boost used on long hills. And that was with only a few litres of water (and the copper tubes) for the heat storage.

Water has a very high thermal mass, or more technically, a high specific heat. Think of it like this: when you place a saucepan of water on the stove it takes the input of a lot of energy before the temperature of the water changes much. (Try heating that saucepan of water with a single candle!) In fact the specific heat of water is 4.18 kilojoules per kilogram per degree C. So, to raise the temp of 1 litre of water (1 litre of water has a mass of 1kg) by one degree C requires 4.18 kilojoules of energy. As I said, that's a lot.

In fact, as a comparison, have a look at some specific heat values of common materials:

Material Specific Heat

(kJ/kg/degree C)

Water 4.18

Aluminium 0.94

Copper 0.39

Air 1.01

Concrete 0.88

So if you were using a solid block of aluminium as your heat storage mechanism, you'd need 4.4 times the mass of aluminium to get the same heat storage as water. (As you can see, specific heat doesn't have much to do with how good a conductor the material is.)

If you want to make a heatsink that's capable of absorbing lots of heat without increasing much in temperature, you could use lots of water. For example, if the heat of the boosted intake air could be conducted to - say - 20 litres of water, I'd bet that in a road car the intake air temp on boost would never get very high. But 20 litres of water is 20kg, and because water is a poor conductor of heat, you'd also need a really good heat exchange mechanism.

Hmmm, too big and heavy.

So is there a commonly available substance with a much higher specific heat than water? The short answer is 'no'.

Doing it Differently

These sorts of questions are also being covered extensively in an industry that has nothing to do with turbocharged cars. In solar house design, adding thermal mass is important because it knocks off the highs and lows of temperature extremes that are experienced by the occupants. For example, because water has a much higher specific heat than say concrete (see table above), some designers place storage containers of water within the house. This water gradually rises in temp on the hot days (keeping the house cooler) then feeds the heat back out as the temperature drops (keeping it warmer). In short, the water containers act as heatsinks, knocking off the peaks and troughs in the temp variations.

But the same problems apply to using large bodies of water in a house as they do to a turbo engine heatsink/intercooler application-lots of water is needed if you want to absorb lots of heat. So solar house designers are exploring a completely different way of storing that heat.

They are now using materials that absorb a lot of energy as they change in state.

Huh? That's it? Yes - now let's look at what that means. It is incredibly significant.

We've covered the idea that materials have a specific heat - the property of the material that determines how much heat it can absorb for a given temperature increase. But there's also another characteristic of materials, called the specific heat of fusion.

Let's have a detailed example. We'll start with a solid (rather than a liquid or a gas) which we'll call 'Performal'. We get a chunk of Performal and put it in a saucepan on the stove. We then place a thermometer probe in the Performal and turn the stove to 'high'. Every minute we take a temp reading, and as expected, the stuff starts getting warmer. It must have a pretty high specific heat, because even though the stove is set to high it warms up only slowly. In fact, the graph here shows the temp increase that we measured over the first ten minutes.

We're getting pretty bored with this experiment (what is this, a performance car magazine or a science class?!) and so when we jot down '50 degrees C' as the temp after 10 minutes we're thinking more of that night's cruise than anything else. And so when the next reading after another minute is also 50 degrees C, we get the uncomfortable feeling that we've stuffed up the readings. The stove is still running on high, pouring heat into the Performal, but a further minute later the temp of the Performal is still 50 degrees C!

What the hell is going on? Have the laws of physics and chemistry gone out the window? We're continuing to add heat - lots of it - but the substance isn't getting any hotter!?

What is happening is that the Performal is melting - it is changing from a solid to a liquid at 50 degrees. And when it undergoes that change in state, it can absorb lots of energy without altering in temperature. Instead of heating the material up, the energy from the stove is being used to separate the material's molecular bonds.

Until all of the Performal has changed from a solid to a liquid, its temperature will not change. That's what the above graph shows - and you can see that the temp is being held constant, even though we're continuing to pour in the heat energy from the stove. It's only when the Performal has completely melted that its temp will start to rise again - and then the rate of temp increase will be dependent on the specific heat of Performal in liquid form, which might be different to its specific heat in solid form.

So here's a graph that showed what happened as we heated the Performal. When it started to change state, it had the capability of absorbing a huge amount of energy without getting any hotter. And the amount of energy it can absorb during this change of state is called its specific heat of fusion.

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Some Definitions

Specific Heat - the quantity of heat required to raise the temperature of one gram of a substance by one degree Celsius.

Specific Heat of Fusion - the quantity of heat required to convert a substance from the solid to the liquid state with no temperature change.

Melting point - the temperature at which the substance changes from a solid to a liquid.

Back to Cars

So where does this leave us? Well, let's pack a conventional air/air intercooler core with Performal. We'll first heat the Performal up until it melts, then pour it through all the fins of the intercooler, filling them right up. After that, we'll place a water-tight jacket all round the core (so the Performal can't leak out when it melts) and then we'll insulate the assembly so that under-bonnet heat can't affect it. Finally, the new heatsink will be installed in the turbo-to-intake plumbing.

Remember, Performal has four different characteristics that interest us:

• Its specific heat as a solid

• Its melting point

• Its specific heat of fusion

• Its specific heat as a liquid

To remind you, Performal's melting point is 50 degrees C.

So, the car's cold and so you only gently drive it down the street on this 30-degree C day. But after half an hour of this gentle driving the temps have stabilised: at idle the temp of the air coming out of the turbo is 40 degrees C, and the temp of the Performal heatsink is also 40 degrees.

Then you put your foot down. The turbo spools up to 15 psi boost and the air coming out of the turbo rockets from 40 degrees to 80 degrees. But a lot of this heat is absorbed as the air passes through the aluminium and Performal honeycomb heatsink, so the intake air temp at the engine remains much cooler - say (with some heat exchange efficiency losses) it's 50 degrees C. After 5 seconds of full boost the heatsink has risen in temp to 45 degrees C, with the heat all being absorbed by the Performal's specific heat capability as a solid.

You get back off the throttle and go back to a cruise. The heat from the heatsink is now fed back into the airstream, which is now cooler than the heatsink. Effectively, the heatsink is being internally cooled.

But that five seconds of boost has whetted your appetite: this time you wind it right out through the first three gears at full boost. The temp of the heatsink rises: soon it has reached 50 degrees C and the Performal starts to change state, to melt. Its ability to absorb energy without rising higher in temp is now taking effect: despite a huge amount of heat being pumped into the heatsink, the outlet air temp stays just the same, at (say) a constant 55 degrees C. Again, when you get off the throttle and the air flowing through the heatsink is lower in temp than heatsink temp, the heat will be fed back into the intake airstream and the heatsink will cool. As it cools the Performal will start to solidify, until when it has all turned back into a solid, its temp will start to drop below 50.

The speed bug has bitten and you decide to go for a top speed run: on full boost continuously for a minute. The Performal then warms up, reaches melting point and holds the intake air temp steady. But it can only do this while the material is melting, and after 40 seconds it has all turned to a liquid. At this point, its temp will again start to rise, but even as a liquid it will be absorbing heat and so reducing the turbo outlet temp. Of course, when the police catch you and you are having a roadside discussion, that heat will be being fed back into the airstream: it is likely to stay warm for some time.

Ice/Water Fusion Intercooler

Drag racers who use a mixture of ice and water in a heat exchanger core are already using a fusion intercooler. The specific heat of fusion for ice (ie how much energy per kilogram is required to melt it) is 334 kJ/Kg. That's why ice/water systems are so effective - a lot of energy is required to melt the ice and the ice/water mix will stay at 0 degrees C until all of the ice is melted. Trouble is for a road car, the water doesn't turn back into ice when the car's back off boost.....

The Potential

You can see now why some 2700 words ago I said that for the system to be effective, the off-boost intake air temp must be below 50 degrees. Otherwise, with the normal off-boost heat the Performal would be melted all of the time, and so its capability to absorb heat during its change of state would be gone. (That is, it would already be changed in state!)

That off-boost turbo outlet temp will be dependent on a number of things:

• The temp of the air being breathed by the turbo

• The heating of the compressor side of the turbo by the exhaust manifold and turbine

• The size and design of the turbo

• The temperature of the day

• The airflow through the engine bay

In areas of cold climate the off-boost turbo outlet temp is very unlikely to ever exceed 50 degrees C, however, when the ambient is 40 degrees C the air coming out of the turbo will often be above 50 even when off-boost.

I had intended fitting a Performal heatsink to my Nissan Maxima VG20DE turbo, however the transverse engine location (the turbo is heated by the radiator airflow), the very small turbo and the long intake path all conspired to give a turbo outlet temp of about 30 degrees above ambient! Since where I live the ambient seldom drops below 20, the Performal would be frequently already changed in state, even before boost hit. It's therefore not suitable for this car and climate, and it shows how important direct measurement of the actual temps really is.

(Of course, I could place a free-flowing but very small air/air intercooler in front of the Performal heatsink - this would cool the off-boost air sufficiently to keep the Performal as a solid, while the small air/air intercooler's performance on boost wouldn't matter - it would just need to flow enough air to not be a restriction. However, space constraints mean that such an approach on my car would be very difficult.)


The potential on the right car is huge: but what actually is bloody "Performal"? Time to let you into a secret - and some of you may have already guessed. "Performal" doesn't exist but a substance with very similar characteristics is commonly available. It is called paraffin wax, and is sold for use in making candles. Specifically, its typical characteristics are:

• Melting point: 52 degrees C

• Specific heat: 3.27 kJ per kg per degree C

• Specific heat of fusion: 210 kJ/kg

So to increase the temp of 1kg of the wax from 47 to 52 degrees takes 16.35kJ, but to push it past 52 degrees takes nearly 13 times as much energy. (Or, to risk causing confusion, you could dissipate in it a power of 14kW for 15 seconds to melt 1kg.)

Paraffin wax is non-toxic, doesn't explode (although it will catch fire if you expose it to a naked flame) and is easily handled. Special waxes designed specifically for this change-of-state heat storage purpose are also available with melting points in 10-degree C increments from 50 degrees to 100 degrees C, however their availability is obviously less than simple candle wax.

Constructing One?

The easiest approach to making a fusion intercooler would be to obtain a small air/air intercooler core, which has adequate airflow for the application. The core could be sealed with sheet aluminium welded into place, much as the water/air design shown here was constructed. The assembly could then be heated to perhaps 60 degrees C in an oven and before having its core completely filled with molten wax - but with a small gap left for expansion. The resulting assembly would then need to be insulated from underbonnet heat, perhaps by being placed in a larger box and having the gaps filled with expanding foam applied with a spray can.

From Here?

As I said above, the car that I had hoped to apply the technology to isn't suitable: its off-boost intake air temp is too high. However, in the past I have owned turbo cars where the off-boost intake air temp was only 5-10 degrees above ambient, and these would be suitable for the technique. Remember, this approach would not be appropriate for race cars or for cars spending long times on boost on a dyno; it would however be very suitable for turbo cars where packaging is very tight (or a rear- or mid-mounted engine is used) and where in normal road use the duration of boost is limited.

It's a fascinating concept, which as the URLs below show is now being used in other industries and applications. (To find more links do a web search under 'phase change materials heat storage' or similar.) Equipped with a wax or other phase change material that melted at (say) 65 degrees C, the fusion intercooler could even be used as a 'safety' heatsink, able to knock the top of peaks that only rarely occur, or occur for only a very short period.

Either as a main intercooler/heatsink, or as an additional safety device, it's certainly something with huge real-world potential.

Made one....It does not work in the real world of a road car

I reckon it would be great for drags....But it weighs a bit!!(sooper sub)

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Then there is this too

An intercooler is a heat exchanger. That means there are two or more fluids that don't physically touch each other but a transfer heat or energy takes place between them. Turbo Regals made in 1986/87, Turbo TAs, GMC Syclones and Typhoons all came with intercoolers to cool down the hot compressed air coming from the turbocharger. Turbo Regals and Turbo TAs use outside air as the cooling media; Syclones and Typhoons use water. Turbo Regals made in 1985 and before did not have intercoolers as original equipment.

At wide open throttle and full boost the hot compressed air coming from a turbocharger is probably between 250 and 350 deg F depending on the particular turbo, boost pressure, outside air temperature, etc.. We want to cool it down, which reduces its volume so we can pack more air molecules into the cylinders and reduce the engine's likelihood of detonation.

How does an intercooler work? Hot air from the turbo flows through tubes inside the intercooler. The turbo air transfers heat to the tubes, warming the tubes and cooling the turbo air. Outside air (or water) passes over the tubes and between fins that are attached to the tubes. Heat is transferred from the hot tubes and fins to the cool outside air. This heats the outside air while cooling the tubes. This is how the turbo air is cooled down. Heat goes from the turbo air to the tubes to the outside air.

There are some useful equations which will help us understand the factors involved in transfering heat. These equations are good for any heat transfer problem, such as radiators and a/c condensers, not just intercoolers. After we look at these equations and see what's important and what's not, we can talk about what all this means.

Equation 1

The first equation describes the overall heat transfer that occurs.

Q = U x A x DTlm

Q is the amount of energy that is transferred.

U is called the heat transfer coefficient. It is a measure of how well the exchanger transfers heat. The bigger the number, the better the transfer.

A is the heat transfer area, or the surface area of the intercooler tubes and fins that is exposed to the outside air.

DTlm is called the log mean temperature difference. It is an indication of the "driving force", or the overall average difference in temperature between the hot and cold fluids. The equation for this is:

DTlm = (DT1-DT2) * F


where DT1 = turbo air temperature in - outside air temperature out

DT2 = turbo air temperature out - outside air temperature in

F = a correction factor, see below


The outside air that passes through the fins on the passenger side of the intercooler comes out hotter than the air passing through the fins on the drivers side of the intercooler. If you captured the air passing through all the fins and mixed it up, the temperature of this mix is the "outside air temperature out".

F is a correction factor that accounts for the fact that the cooling air coming out of the back of the intercooler is cooler on one side than the other.

To calculate this correction factor, calculate "P" and "R":

P = turbo air temp out - turbo air temp in

outside air temp in - turbo air temp in

R = outside air temp in - outside air temp out

turbo air temp out - turbo air temp in

Find P and R on "Fchart.jpg" (attached) and read F off the left hand side.

This overall heat transfer equation shows us how to get better intercooler performance. To get colder air out of the intercooler we need to transfer more heat, or make Q bigger in other words. To make Q bigger we have to make U, A, or DTlm bigger, so that when you multiply them all together you get a bigger number. More on that later.

Equation 2

We also have an equation for checking the amount of heat lost or gained by the fluid on one side of the heat exchanger (ie, just the turbo air or just the outside air):

Q = m x Cp x DT

Q is the energy transferred. It will have the exact same value as the Q in the first equation. If 5000 BTU are transferred from turbo air to outside air, then Q = 5000 for this equation AND the first equation.

m is the mass flowrate (lbs/minute) of fluid, in this case either turbo air or outside air depending on which side you're looking at.

Cp is the heat capacity of the air. This is a measure of the amount of energy that the fluid will absorb for every degree of temperature that it goes up. It is about 0.25 for air and 1.0 for water. Air doesn't do a great job of absorbing heat. If you put 10 BTU into a pound of air the temperature of it goes up about 40 degrees. If you put 10 BTU into a pound of water, the temperature only goes up about 10 degrees! Water is a great energy absorber. That's why we use water for radiators instead of some other fluid.

DT is the difference in temperature between the inlet and outlet. If the air is 200 deg going in and 125 deg coming out, then DT = 200 - 125 = 75. Again, on the cooling air side the outlet temperature is the average "mix" temperature.

If you know 3 of the 4 main variables on one side of the exchanger (the amount of heat transferred, the inlet and outlet temperatures, and the fluidís flow rate) then this equation is used to figure out the 4th. For example, if you know the amount of heat transferred, the inlet temperature, and the flow rate you can calculate the outlet temperature. Since you canít measure everything, this equation is used to figure out what you donít know.


These equations are all for steady state heat transfer, which we probably don't really see too much under the conditions that we are most interested in - drag race! Cruising on the highway you would definitely see steady state. Perhaps at the big end of the track you may see it too, I don't know. As various people on the mailing list have pointed out in the past, the material of the intercooler itself will rise in temperature when you hit full throttle, absorbing more heat than what these equations would lead you to believe. For example, at steady state idle the intercooler body may be at 100 deg F. At steady state full throttle it may be 175 deg F. The energy it takes to heat it up to that temperature comes from the turbo outlet air, and so the cooling of that air is what is removed by both the flowing outside air and the absorption of the intercooler body. How long does it take to get to the new steady state? Beats me, but the graphs I've seen of intercooler outlet temperatures over the course of a quarter mile run lead me to believe that it is approached before you get to the end of the quarter mile, since the intercooler outlet temperatures reached a steady level.

So, now that we've got these equations, what do they REALLY tell us?

Heat transfer goes really well when there is a large temperature difference, or driving force, between the two fluids. This is shown in equation 1 as a large DTlm. It doesn't go as well when there is a small temperature difference between the two fluids (small DTlm). The closer you get the intercooler outlet temperature to the outside air temperature the smaller DTlm gets, which makes the heat transfer tougher.

The difference between the intercooler outlet temperature and the outside air temperature is called the approach. If it is 100 degrees outside and your intercooler cools the air going into the intake manifold down to 140 degrees, then you have an approach of 40 degrees (140 - 100 = 40). To get a better (smaller) approach you have to have more area or a better U, but there is a problem with diminshing returns. Lets rearrange the first equation to Q/DTlm = U x A. Every time DTlm goes down (get a better temperature approach) then Q goes up (transfer more heat, get a colder outlet temperature), and dividing Q by DTlm gets bigger a lot faster than U x A does. The upshot of that is we have a situation of diminishing returns; for every degree of a better approach you need more and more U x A to get there. Start with a 30 deg approach and go to 20 and you have to improve U x A by some amount, to go from 20 to 10 you need to increase U x A by an even bigger amount.

I would consider an approach of 20 degrees to be pretty good. In industrial heat exchangers it starts to get uneconomical to do better somewhere around there, the exchanger starts to get too big to justify the added expense. The one time I checked my car (stock turbo, stock IC, ported heads, bigger cam) I had an approach of about 60 deg. The only practical way of making the DTlm bigger on an existing intercooler is to only drive on cold days; if you buy a better intercooler you naturally get a better DTlm.

You can transfer more heat (and have cooler outlet temps) with more heat transfer area. That means buying a new intercooler with more tubes, more fins, longer tubes, or all three. This is what most aftermarket intercoolers strive for. Big front mounts, intercooler and a half, etc... are all increasing the area.

A practical consideration is the fin count. The area of the fins is included in the heat transfer area; more fins means more area. If you try to pack too many fins into the intercooler the heat transfer area does go up, which is good, but the cooling air flow over the fins goes down, which is bad. Looking at the 2nd equation, Q = m * Cp * DT, when the fin count is too high then the air flow ("m") drops. For a given Q that you are trying to reach then you have to have a bigger DT, which means you have to heat up that air more. Then THAT affects the DTlm in the first equation, making it smaller, and lowering the overall heat transfer. So there is an optimum to be found. Starting off with bare tubes you add fins and the heat transfer goes up because you're increasing the area, and you keep adding fins until the it starts to choke off the cooling air flow and heat transfer starts going back down. At that point you have to add more tubes or make them longer to get more heat transfer out of the increased area.

Make U go up. You can increase the U by adding or improving "turbulators" inside the tubes. These are fins inside the tubes which cause the air to swirl inside the tube and makes it transfer its heat to the tube more efficiently. Our intercoolers have these, but I understand that more efficient designs are now available. One of the best ways to increase the U is to clean the tubes out! Oil film (from a bad turbo seal or from the stock valve cover breather) inside the tubes acts as an insulator or thermal barrier. It keeps heat from moving from the air to the tube wall. This is expressed in our equation as a lower U. Lower U means lower Qs which mean hotter turbo air temperatures coming out of the intercooler.

Air-to-water. If we use water as the cooling medium instead of outside air, we can see a big improvement for several reasons: Water can absorb more energy with a lower temperature rise. This improves our DTlm, makes it bigger, which makes Q go up and outlet temps go down. A well designed water cooled exchanger also has a much bigger U, which also helps Q go up. And since both DTlm and U went up, you can make the area A smaller which makes it easier to fit the intercooler in the engine compartment. Of course, there are some practical drawbacks. The need for a water circulation system is one. A big one is cooling the water down after it is heated (which means another radiator). This leads to another problem: You heat the water, and cool it down with outside air like the Syclone/Typhoon. You can't get it as cool as the outside air, but maybe you can get it within 20 degrees of it. Now you are cooling the turbo air with water that is 20 hotter than the outside air, and you can only get within 15 degrees of that temperature so coming out of the intercooler you have turbo air that is 35 degrees hotter than outside! (turbo air is 15 deg over water temp which is 20 deg over outside temp). You could have easily done that with an air to air intercooler! But... if you put ice water in your holding tank and circulate that... Then maybe the air temp coming out of the intercooler is 15 deg above that or 45 to 50 deg. Hang on! But after the water warms up, you're back to the hot air again. So, great for racing, not as good for the street.

Lower the inlet temperature. The less hard the turbo has to work to compress the air then the lower the temperature the air coming out of the turbo is. This actually hurts the DTlm, but still if it's cooler going in it will be cooler coming out. You can work the turbo less hard by running less boost, by improving the pressure drop between the air filter and the turbo, or by having a more efficient compressor wheel. You can also reduce the pressure drop in the intercooler, which allows you to run the same amount of boost in the intake manifold while having a lower turbo discharge pressure. More on this later. If you can drop the turbo outlet pressure by 2 psi, or raise the turbo inlet pressure by 1 psi, that will drop the turbo discharge temperature about 16 degrees (depending on the compression efficiency and boost level). If the turbo air is going into the intercooler 16 degrees colder then it may come out only 10 degrees colder than before, but that is still better than what it was.

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Pressure Drop

Another aspect of intercoolers to be considered is pressure drop. The pressure read by a boost gauge is the pressure in the intake manifold. It is not the same as the pressure that the turbocharger itself puts out. To get a fluid, such as air, to flow there must be a difference in pressure from one end to the other. Consider a straw that is sitting on the table. It doesn't having anything moving through it until you pick it up, stick it in your mouth, and change the pressure at one end (either by blowing or sucking). In the same way the turbo outlet pressure is higher than the intake manifold pressure, and will always be higher than the intake pressure, because there must be a pressure difference for the air to move.

The difference in pressure required for a given amount of air to move from turbo to intake manifold is an indication of the hydraulic restriction of the intercooler, the up pipe, and the throttle body. Let's say you are trying to move 255 gram/sec of air through a stock intercooler, up pipe, and throttle body and there is a 4 psi difference that is pushing it along (I'm just making up numbers here). If your boost gauge reads 15 psi, that means the turbo is actually putting up 19 psi. Now you buy a PT-70 and slap on some Champion heads. Now you are moving 450 gm/sec of air. At 15 psi boost in the intake manifold the turbo now has to put up 23 psi, because the pressure drop required to get the higher air flow is now 8 psi instead of the 4 that we had before. More flow with the same equipment means higher pressure drop. So we put on a new front mount intercooler. It has a lower pressure drop, pressure drop is now 4 psi, so the turbo is putting up 19 psi again. Now we add the 65 mm throttle body and the pressure drop is now 3 psi. Then we add the 2.5" up pipe, and it drops to 2.5 psi. Now to make 15 psi boost the turbo only has to put up 17.5 psi. The difference in turbo outlet temperature between 23 psi and 17.5 psi is about 40 deg (assuming a constant efficiency)! So you can see how just by reducing the pressure drop we can lower the temperatures while still running the same amount of boost.

I have seen some misunderstandings regarding intercooler pressure drop and how it relates to heat transfer. For example, one vendor's catalog implies that if you had little or no pressure drop then you would have no heat transfer. This is incorrect. Pressure drop and heat transfer are relatively independent, you can have good heat transfer in an intercooler that has a small pressure drop if it is designed correctly. It is easier to have good heat transfer when there is a larger pressure drop because the fluid's turbulence helps the heat transfer coefficient (U), but I have seen industrial coolers that are designed to have less than 0.2 psi of drop while flowing a heck of a lot more air, so it is certainly feasible.

Pressure drop is important because the higher the turbo discharge pressure is the higher the temperature of the turbo air. When we drop the turbo discharge pressure we also drop the temperature of the air coming out of the turbo. When we do that we also drop the intercooler outlet temperature, although not as much, but hey, every little bit helps. This lower pressure drop is part of the benefit offered by new, bigger front mount intercoolers; by the Duttweiler neck modification to stock location intercoolers; by bigger up pipes; and by bigger throttle bodies. You can also make the turbo work less hard by improving the inlet side to it. K&N air filters, free flowing MAF pipes, removing a screen from the MAF, removing the MAF itself when switching to an aftermarket fuel injection system, the upcoming 3" and 3.5" MAFs from Modern Muscle, these all reduce the pressure drop in the turbo inlet system which makes the compressor work less to produce the same boost which will reduce the turbo discharge temperature (among other, and probably greater, benefits).

What about my Intercooler?

Wondering if your intercooler is up to snuff? The big test: measure your intercooler outlet temperature! When I did this I got a K type thermocouple, the thin wire kind, slid it under the throttle body/up pipe hose and down into the center of the up pipe, and went for a drive. On an 80 to 85 deg day I got a WOT temperature of 140 deg, for a 55 to 60 deg approach. That tells me that I need more intercooler. If I can get the temperature down to 100 deg, the air density in the intake manifold goes up by 7%, so I should flow 7% more air and presumably make 7% more hp. On a 350 hp engine that is 25 hp increase. On a 450 hp engine that's a 30 hp increase. Damn, where's my check book…

Another check is pressure drop. Best way to check it is to find a pressure differential gauge, which has 2 lines instead of the single line a normal pressure gauge has. It checks the difference between the 2 spots it is hooked up to, as opposed to checking the difference in pressure between the spot it is hooked up to and atmospheric pressure, which is how a normal pressure gauge works.

Hook one line of the gauge to the turbo outlet and one to (preferably) the intercooler outlet. The turbo outlet/intercooler inlet pressure is easy, just tee into the wastegate supply line off the compressor housing. It would be nice to get the intercooler outlet pressure directly, but there's no convenient spot to hook up to. Hooking into the intake manifold (such as via the line to the boost gauge) is quite convenient, but gives the total pressure drop: intercooler + up pipe + throttle body. That'll give you a pretty good idea though.

Instead of the differential pressure gauge you could use 2 boost gauges, one in each spot, but then you have to worry about whether both gauges are calibrated the same, try to read both at the same time while driving fast, etc AND you may spring (ie, ruin) the gauge on the turbo outlet since when you close the throttle you get a big pressure spike that your normal boost gauge never sees.

If you find more than 4 or 5 psi difference between the intercooler inlet and intake manifold (and I'm just giving an educated guess here, you'd probably want to refer to one of the intercooler manufacturers for a better number) then I would suspect that a larger, lower pressure drop intercooler would offer you some gains.

Comparing competing Intercooler Designs

How to compare competing intercooler designs: Well, ultimately you want the one that will give you the coldest air possible into the intake manifold. This will be the one with highest UA value. When you multiply the heat transfer coefficient by the area (U x A) you get the UA value. This value doesn't really change much with reasonable changes in flow rates or temperatures, so if you could get the data to evaluate the UA for an intercooler in one car then you can use that to extrapolate how it would work in another car.

To evaluate the UA you need enough info to calculate the heat transferred (Q) and the DTlm. Then UA = Q/DTlm. Sounds easy, right? It would be, if the data was available. To properly evaluate an intercooler you would need: the turbo air flow through the intercooler; the pressure and temperature of the air from the turbo; the intercooler outlet temperature and pressure; the outside air temperature; and either the mix temperature of the cooling air as it leaves the intercooler or the flow rate of that air. That's a lot of info, and I'm not going to pretend that a vendor would make all that available to you, or that they would even collect all that data. I'm sure that the majority of the vendors selling bigger intercoolers have a trial and error process that they use to design their offerings rather than putting forth a real engineering effort anyway. But, if they did and they would release the info I would then use that data to figure out the amount of heat transferred (Q) and the DTlm, and then calculate the UA value for the intercooler. I would compare various intercooler's UA values and choose the one with the highest UA since that will give you the highest Q (most heat transferred) and the best DTlm (closest approach).

Formula Examples

Well, you've made it this far. If you'd like to see some examples using the formulas outlined in the beginning, read on. If not, well, I'm done. It's pretty easy to make a spreadsheet up to do all these calculations. Please remember that all these numbers have been made up! Any resemblance to real life is a happy coincidence.

Stock intercooler, stock turbo.

Given 40 lb/min air flow @ 300 deg F and 19 psig from the turbo to make 15 psig boost in the intake manifold; 85 deg F outside temperature; an intercooler outlet temperature of 140 deg F has been measured, as has the cooling air temperature of 160 deg. What is the UA of the stock intercooler?

First, calculate Q

Q = m * Cp * DT

Q = 40 lb/min * 0.25 BTU/lb-F * (300-140 F) = 1600 BTU/min

Calculate DTlm

DT1 = turbo air temperature in - outside air temperature out = 300 - 160 = 140

DT2 = turbo air temperature out - outside air temperature in = 140 - 85 = 55

P=0.74, R=0.47, F=0.875

DTlm = F*(DT1-DT2)/ln(DT1/DT2) = 0.875*(140-55)/ln(140/55) = 74.4/0.934=79.6 F

Calculate UA

UA = Q/DTlm = (1600 BTU/min)/79.6 F = 20.1 BTU/min-F

What is the cooling air flow?

Q = m * Cp * DT, or Q/(Cp * DT) = m,

m = (1600 BTU/min)/[0.25 BTU/lb-F * (160-85 F)] = 85.33 lb/min of outside cooling air

Stock intercooler, big turbo

How will the same stock intercooler perform with a bigger turbo and more boost?

Given 53 lb/min @ 350 deg F and 27 psig from the turbo to make 22 psig in the intake; 85 deg F outside temperature. Cooling air flow is still 85.33 lb/min.

This requires some trial and error to solve since we don't know the intercooler outlet temperature. There IS a way to calculate it directly, but that involves some more equations and is a little tedious so I'll skip it and do it the hard way, by assuming an intercooler outlet temperature and then checking to see if it is right. I'll do that by calculating Q for the overall exchanger and then Q for just the turbo air; if they come out the same then my guess was correct.

m=53 lb/min, Cp=0.25, U*A=20.1

lets start by assuming that the intercooler outlet temp = 140

Q = m * Cp * DT

Then DT = (350 - 140 ) = 210 and Q = 2782.5 BTU/min

Cooling air flow = 85.33 lb/min

DT for the cooling air = Q/(m*Cp)

DT = 2782.5 BTU/min / (85.33 lb/min * 0.25 BTU/lb-f) = 130.4 F

since DT = T out - T in, then 130.4 = T out - 85 and T out = 215.4 F

So the cooling air inlet is 85 F and the outlet is 215.4 F, and the turbo air inlet is 350 F and the outlet is assumed to be 140 F. Now calculate DTlm:

P=0.792, R=0.62, and F=0.75

DT1=134.6, DT2=55

DTlm=(134.6-55)/ln(134.6/55) * 0.75 = 66.7

Now calculate a new Q, Q= UA * DTlm


Since this isn't the same Q we got when we assumed an outlet temp of 140 deg, we have to get a new outlet temp and run through all this again.

I'll assume a new intercooler outlet temp of 170.


cooling air DT = 2385/(85.33*0.25) = 111.8

Cooling air outlet = 85 + 111.8 = 196.8

P=0.68, R=0.62, F=0.84


Q=1954.7 still not close enough

Last try!

T IC out = 182


cooling air DT = 2226/(85.33*0.25) = 104.4

Cooling air outlet = 85 + 104.4 = 189.4

P=0.63, R=0.62, F=0.88


Q=2232 close enough

Well, this time the Q we guessed at (by guessing the IC outlet temp) and the Q we calculated from the overall equation are pretty close, so we can say we've found the answer. It appears that this intercooler, which worked fine in a basically stock application (cooling the air to the intake manifold to 140 deg F) isn't working as well in this high HP application, being able to cool the air down to only 182 deg!

Last example: same turbo and air flow as before, but we have a new intercooler with the same heat transfer coefficient but 50% more area (intercooler and a half). We'll assume that it also flows 1.5 times the cooling air flow.

U * A old IC = 20.1

U * 1.5 * A = 1.5 * 20.1 = 30.15 = UA for new intercooler

m turbo air = 53 lb/min, Cp = 0.25 BTU/lb-F, T in = 350 deg

m cooling air = 1.5 * 85.33 = 128 lb/min, Cp = 0.25 BTU/lb-F, T in = 85 F

Assume intercooler outlet temp = 140 F

Q = m*Cp*DT = 53 * 0.25 * (350-140) = 2782.5

cooling air DT = 2782.5/(128*0.25) = 87

Cooling air outlet = 85 + 87 = 172

P=0.79, R=0.41, F=0.85


Q=2684 not too bad, we'll try it once more

Assume intercooler outlet temp = 142 F

Q = m*Cp*DT = 53 * 0.25 * (350-142) = 2756

cooling air DT = 2782.5/(128*0.25) = 86.1

Cooling air outlet = 85 + 86.1 = 171.1

P=0.78, R=0.41, F=0.86


Q=2763 close enough

So this tells us that in this high performance car the intercooler-and-a-half outlet temperature is about the same as the outlet temperature of the stock turbo/stock intercooler car.

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