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Science Behind Intercooler Piping Sizing


Niran

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Been thinking about this recently while planning for my FMIC setup. Why are cold side pipes larger than hot side pipes?

 

Let's use my build as an example:

- Turbo compressor outlet: 2"

- Intake manifold inlet: 2.75"

- FMIC inlet / outlet: 2.75"

 

Sizing of the above is obviously a big influencing factor, but why would cold side pipes be bigger? Isn't the science that hot air is less dense and takes up more space? Therefore after being chilled through the intercooler, the space required for that same volume of air should be less, because it's more dense? I.e larger cold side pipes could cause lag because air velocity is now slower due to the bigger pipe diameter.

 

Smaller hot side pipes make sense.

 

Lastly what would you do in my application? 2" pipe from turbo to core, or have a transition somewhere in between or at the turbo even. Seen 2.25" on many intercooler kits.

 

Likewise with the cold side, 2.75" from core to inlet or step it down? 

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Now I'm been a couch scientist here but maybe its bigger so the engine through the throttle body can take in more and create a vacuum from the size change, so it helps "pull" the air from the turbo through the system. 

But at the same time, this could just be because most cars the throttle body is pretty large, on my ST 2.5" and the intercooler piping is all 2" but all the modded cars get the best out of all pipes going 2.5" 

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Hmmm. Pressure, density & velocity.

 

Higher pressure would increase response (thought test this using extremes of density).

 

Velocity is where we lack understanding, right? From Murch's old post, thin headers are the win. Was this due to velocity or pressure, and as you state @Not_Sean, due to the interaction with another part of the system in regards to either temperature or a bottleneck/step change?

 

Almost need a napkin and a mech engineer who understands fluid dynamics 101 ¬¬

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16 minutes ago, Gripless said:

My guess is air speed and expansion as it cools could be a factor.

the velocity out or the turbo and heat are both high, at the throttle cooler and slower.

 

According to thermodynamics, air contracts or becomes more dense as it becomes cooler. The particles move closer together when cold, and further apart when hot. That's the confusing part, because then the cold pipes should also be small to keep velocity up. Although that is not the only factor as mentioned above, i think the 'more air available cold side' is a factor but don't understand exactly why. 

 

Smaller pipes should mean greater pressure and greater velocity (pressure = force / area) 

Edited by Niran
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That is the theory, I did write that poorly, meant area to expand into to drop the speed.

 

But to have increased velocity won't you have more pressure and therefore heat since it is a confined space?

 

Also wouldn't it be ideal to have matched to throttle size for when the throttle opens then the pressure wouldn't drop from the expansion of going through the larger throttle area. Like using it as a storage tank for the already pressurized and cooled air.

 

it may be a balance between less ideal in the pipe and more ideal in the cylinder. maybe that also gets balanced with throttle response. Ok at this point it does my head in as too many factors.

 

Need someone with flow modeling software to try it. Someone must have somewhere on the Internet.

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on a high level, doesn't it just increase the area to pressurize, which directly affects your throttle response? in theory allows you to reserve a higher volume of pressurized air for a larger turbocharger application? why would better velocity in the pipe matter when your throttle body / intake manifold would take care of all that where it's really important? is that a bottle neck anyway?

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I've read many threads on this before, these two are good value. 

 

https://forums.nasioc.com/forums/showthread.php?t=2399163

https://forums.nasioc.com/forums/showthread.php?t=2648685

 

 

My 2c, I think at lower power levels like we are all dealing with 400kw- then its not a massive contestant in the game. As long as you are not going smaller than your turbo outlet things should be good. Smaller pipe is easier to work with and route through the engine bay. I went 2 hotside and 2.5 cold into a 2.75TB

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Read all that and got this as well.

tight bends are bad

intercooler core design is the biggest factor for real world. Which makes sense as there are some terrible designs with nasty bends at the entry and tiny passages through them.

pipe size makes little difference compared to those two if like @A_J_T said you stay about the outlet size.

 

other than that's is complex physics interactions wizardry no one really can explain well that makes little difference.

 

 

 

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Thanks guys. Really interesting discussion.

 

So i was gonna go 2.75'' cold side (same as TB and intercooler outlet).

 

Hotside i'm not sure though, as turbo outlet is 2" and intercooler inlet is 2.75". Feel like 2" piping might be too small for my power level - maybe step to 2.25" or 2.5" straight from the turbo?

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Also struggling to get my head around this. I think the bold below is the most accurate/best statement from the top thread, I'm unsure about his filling calc but I suspect the extra difference in time to fill from 2" to 2.75" over the short length of hot side IC piping would be barely noticeable.

 

I think basically we are trying to get the air from the turbo to the intake with as little pressure drop as possible.

Pressure loss is generally a factor of velocity and roughness/restrictions (with the same flow - smaller pipe means higher velocity, higher velocity means higher friction losses)

When talking about restrictions other than friction on IC pipe walls, the smaller the diameter pipe has a bigger the restriction (i.e 2" 90 degree bend causes more pressure drop than a 2.75" 90 even at the same velocity, when you take into account the higher velocity from smaller pipe it compounds)

 

Imo you want to be at the point where it’s not causing a significant pressure drop, but also not overkill. But I don't know where that is xD

I think you are right on cold side, I'd probably go 2.75" hot side just to know it's not causing any pressure drop/restriction.

 

Quote

A higher volume/diameter tube of the same length will have less head loss and require less energy per unit airflow, however it will require more time and energy to fill and pressurize initially.

 

So just to be clear:
Bigger diameter: lower head loss, higher energy lost to filling.
Longer length: higher head loss, higher energy lost to filling.

Regarding filling loss, here's a quick example.
2" dia. vs 3" dia. on 2 meters worth of pipe, constant 120F temp (very low/generous number), using pv=nrt.
From atmo. pressure -> 2 atm relative (29psi+), the 3" pipe requires only ~11.2 more grams of air to fill. Looking at a random log/rom from a stock turbo car, airflow during spool at atmospheric pressure is around 100 grams per second.

 

Edited by pl0x
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@pl0x so from that rotating the manifold to shortening the pipe could make more difference then diameter if length is the bigger factor.

 

one thing I did remember is missing is the transitions in size. I know the formula sae and rally restrictors where they smoothly curve the size change have very little loss in power. Same way you have bellmouth trumpets on carbs.

this would be the hardest and most costly to produce part though.

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People have always said flipping manifolds reduces the "lag", I've never seen any back to back results so unsure if it's true or if it's mainly placebo,.

I guess it would take out around 180 degrees of bends and 1m of piping, so definitely reducing some area to fill and pressure loss.

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11 minutes ago, pl0x said:

People have always said flipping manifolds reduces the "lag", I've never seen any back to back results so unsure if it's true or if it's mainly placebo,.

I guess it would take out around 180 degrees of bends and 1m of piping, so definitely reducing some area to fill and pressure loss.

And then people say there is no noticeable difference between TMIC and FMIC ¬¬ soooo now what haha. To figure this all out, surely you need to take into account the whole journey the air goes on before going bang- intake mani plenum size, valve sizes and head intake ports etc. regardless it is interesting to think and talk about for sure. 

 

Probably not in this case but It makes me smile when alot of effort is put into something like perfect flowing intercooler and pipe setup and then the intake mani still has its TGVs in the plenums or there is an angry half kinked silicone 180deg bend coming off the comp housing. 

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On 5/9/2018 at 9:47 AM, A_J_T said:

And then people say there is no noticeable difference between TMIC and FMIC ¬¬ soooo now what haha. To figure this all out, surely you need to take into account the whole journey the air goes on before going bang- intake mani plenum size, valve sizes and head intake ports etc. regardless it is interesting to think and talk about for sure. 

 

Probably not in this case but It makes me smile when alot of effort is put into something like perfect flowing intercooler and pipe setup and then the intake mani still has its TGVs in the plenums or there is an angry half kinked silicone 180deg bend coming off the comp housing. 

I'm sure there has been much more advanced study/calc's done on this, would be good to find and have a read. But i'm sure for the majority pretty much what @Gripless said above, bends are bads, pipe sizes should be in between the turbo outlet and the TB inlet size.

 

Mines not quite what you're describing, but the hot side does have about 450 degrees of bends in it. After having reading through this and getting my head around it, I think i'll have another attempt.

Edited by pl0x
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See Prestige Tuning & Motorsport's post on facebook today?

Additional front mount lag looks like 100rpm or so. Factory location turbo and manifold, basically longest routing and large volume to fill.

Given those results I'd be trying to get as little pressure drop and not really be concerned with volume to fill (with in reason), so 2.75" all round shouldn't hurt.

 

Quote

The old top mount vs front mount debate. Subaru owners, I'm looking at you!

Tyson's Subaru pictured here runs a forged motor, Tomei turbo and a Process West top mount intercooler (TMIC) (one of the best top mounts we have used). Although when pushing 23psi we see air temps increasing from 30 to 75 degrees+ during back to back power runs or at the track.

This results in inconsistent power and a hot running engine. A issue we see often, our solution? A good quality front mounted intercooler! (FMIC)

Boost is our friend and we don't want to be waiting forever for it to arrive. A good quality FMIC will cool the air charge increasing density, dense cool air is easier to compress than hot thin air. Thus making additional lag basically non existent and with a higher concentration of oxygen vs hotter air, this means more power!

So off came the TMIC and on went a Process west FMIC. The results? Air temps which stay at 26-28 during back to back power runs, a cooler engine, a 35kw gain on the same boost level and better yet, the same response as the TMIC!

Be warned though, not all FMIC's are created equal, we often see people getting poor quality intercoolers which end up making the situation worse!

If you're looking at upgrading your intercooler give us a call/send us a message to see how we can help and what intercoolers we recommend for your car, for proven results!

Prestige Tuning & Motorsport
[email protected]
021 0299 4612

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  • 3 months later...

Hmmm. I have replaced my holy plastic hot side pipe with one made of rad hose and some Silicon hose. From memory the Silicon is 45mm diameter. Granted this is a cheap and temporary fix due to my goal being just get car road legal and to my new home 500km away. 

 

Already it feels a bit more laggy due to the gentler bends I have in my new ugly unit, but probably because there's less restriction in the piping ie bottle necks. 

 

I was just wondering if I increased this diameter by Say 20mm with a fabricated stainless flange and aluminum piping/silicon hose, and in theory fitted a larger throttle body and welded larger diameter flange to the tmic, in addition to repairing damaged cooling fins.. 

 

Would I see 

a)  neglible performance gains, or

b)  need a tune?

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