another bov question

[pl0x]

 boostin said:

Twin turbo's seem to run ok with atmo bov's though. Single turbo's don't really like them.

which single turbos? ive heard v5/6 eat AFMs when using full venting bov's

same AFM as be/bh and when i brought my BOV i was told to not run it on full vent for to long because the AFM is very sensitive?

[Pappu1]

always used a full venting one and never had issues..just adjust the spring thing correctly for proper idle thats all..

get a good quality one woud be my recomendation as the cheaper ones tend to cause issues

[Guest boostin]

I'm not a fan of the surge they create at low boost levels. The BOV needs to be adjusted too tight to allow for the low throttle low boost pressure relief and you can hear the compressor surge when you back off.

[cuscogtb]

Been running full atmo bov on my BH since 2006 with no issues. Have used a racepro and non branded ones at first but been using a hks ssqv for the past year and a half. Best sounding one too imo.

[Volcanix]

gfb mach 1 on my rs 93 runs fine id prefer hybrid but the way the turb inlet is its a bit hard to align

[Guest]

I like a DV rather than BOV! I just don’t get why enthuses would want to vent their intake charge to atmosphere your poor turbo’s.. ahhh I’m just getting old I guess, i wuld do a speed density conversion befor anyway

[Guest Oscar]

 WiLd02StI said:

I like a DV rather than BOV! I just don’t get why enthuses would want to vent their intake charge to atmosphere your poor turbo’s.. ahhh I’m just getting old I guess, i wuld do a speed density conversion befor anyway

My head hurts just reading this. Were in the same English class as RAYDEO?

[Koom]

 Oscar']

[quote name='WiLd02StI said:

I like a DV rather than BOV! I just don’t get why enthuses would want to vent their intake charge to atmosphere your poor turbo’s.. ahhh I’m just getting old I guess, i wuld do a speed density conversion befor anyway

/quote]

My head hurts just reading this. Were in the same English class as RAYDEO?

Funny thing is that its been edited too.

Don't know why they're against venting excess pressure to atmosphere as thats as much as I can gather from their post.

[Guest Oscar]

 Koom']

[quote name='WiLd02StI said:

I like a DV rather than BOV! I just don’t get why enthuses would want to vent their intake charge to atmosphere your poor turbo’s.. ahhh I’m just getting old I guess, i wuld do a speed density conversion befor anyway

/quote]

My head hurts just reading this. Were in the same English class as RAYDEO?

Funny thing is that its been edited too.

Don't know why they're against venting excess pressure to atmosphere as thats as much as I can gather from their post.

Probably from NASIOC, they are dead against VTA setups. Something about USDM cars blowing up just by looking at them funny.

[Guest]

Not to make things complicated and hurt your head as you put it, if your venting boost pressure out into the atmosphere IMO you loosing that boost IMO thats bad.

A dv keeps the turbo boost recalculating thus keeping the turbo spinning reducing lag thats good IMO.

ok so it’s hot air your plumbing back but is it enough to change the performance of your car’s output by much to little? That’s the question you should ask.

Ps BOV are good for pisssssssst sounds and peeeeshhhh sounds, and 400hp+ car’s with large turbo, s, well not really they are ok I just prefer dv is all is the point of all this

[Guest boostcut]

what a complete mick. a bov is to stop compressor surge/stall and it dont matter weather its venting or plumbed back.

[Guest]

hi boostcut

There is much discussion that has been talked about these DV vs BOV and undoubtedly will be until the end on time, I’m well aware what a bov is for so thank you for stating the obvious I was mealy taking the mickey, I just prefer the dv system it makes more sense to me. I did say that BOV are ok

[Guest]

 Omegah said:

ive got a greddy type rs, full venting, hasn't given me any problems.

me wants a HKS SSQV though

Hello Omegah, this is one of my first few posts, I recently purchased a 2001 B4 Legacy 5sp manual. I didn't know until I went to the yard to pick up the car when I popped the bonnet to find a HKS SSQV. Im no professional in Subaru's nor car modifying lol.. but I'm a huge enthousiest... (maybe wrong spelling lol) hope maybe sometime we can meet up for a chat or something so I can learn a few things here and there. ;D

[Guest boostcut]

 WiLd02StI said:

hi boostcut

There is much discussion that has been talked about these DV vs BOV and undoubtedly will be until the end on time, I’m well aware what a bov is for so thank you for stating the obvious I was mealy taking the mickey, I just prefer the dv system it makes more sense to me. I did say that BOV are ok

do you not like vta bovs because they dont "recalculate" boost? hahaha maybe you should find out how an engine and turbo works before commenting any further...

as soon as you take your foot off the throttle it stops producing boost, thus producing vacuum. the vacuum then sucks the bov off its seat and releases the boost the turbo is producing and dead heading against the throttle plate. as the turbo is spinning at 150,000rpm it WILL still produce boost but if the throttle is closed where can it go?

you may be old but it doesnt mean you can spell or know what your talking about. i hope this cleared up the hazey view you had of vent to atmosphere bovs.

cheers. adam.

[Durty-Sanchez]

Here's a good reason to stick with a plumb-back setup (sorry for those who've heard me bleat on about this before, seems older posts were lost in forum fails). In anycase, you can take my word for it or onwards science geeks for todays lesson in BOV thermodynamics..

The Turbo/IC/BOV thermodynamic system is very similar to that of a refridgerator, so much so that air being vented from your BOV is cold - very cold (well at least colder than the surrounding air). In some cases it could even be well below 0°C. How?.. well.. (numbers refer to crude MSPaint PV diagram below)

1. At first we have some air sitting around outside at a given ambient temperature. The turbo sucks this air in and compresses it through an adiabatic process (a process that adds no heat energy to the system). Yes the air does heat up, but that is because of the increased pressure (as given by the ideal gas law). For simplicities sake, no heat is exchanged from the turbo to the air. Looking at the 1st law of thermodynamics (dU = dQ-dW or the change in the systems internal energy, in this case the airmass, is related to the increase in its temperature - the work done by the airmass) we see that no heat energy is added, but work is done on the airmass by an external system (the turbo), so the internal energy of the system increases. (note, dW is only positive if the airmass does work on its surrounds, if work is done on the airmass by something external then dW is a negative value. Negative minus a negative is positive, so dU is positive). The final result is that the airmass is hotter, of greater pressure, and occupies a smaller volume than before.. you'll see the path it takes as the compression stage of our thermodynamic cycle.

2. Straight out of the turbo the air is then passed through the intercooler, or in other words a heat exchanger. The heat exchanger is designed to remove heat from our system, so we get a negative dQ in our thermodynamic equation and the overall energy of the airmass decreases. The air itself loses heat energy and the pressure of the airmass decreases. The volume however remains the same (roughly), so this is a constant volume process. This is usually the point where the air enters the engine and that'd be the end of this part of the cycle.. however, the case of a BOV venting causes something else to happen.

3. When your BOV vents you then have two systems suddenly combining - one at a much higher energy than the other. From the 2nd law of thermodynamics these two systems have to reach an equilibrium, which can only be done if the pressurised airmass (from the BOV) does work on the ambient airmass. Because the ambient airmass is so large in comparison, it will see very little in the way of changes, however the BOV airmass will do something quite interesting. Because of the speed of the BOV releasing this is considered an adiabatic expansion (no heat is exchanged from the BOV air to the atmospheric air), so the only method by which the BOV air can release it's energy is to do work. The work the BOV air needs to do to expand in to the outside atmosphere is roughly equal to the work done by the turbo to compress it in the first place.. however the intercooler has already taken away some of the airs internal energy. The result is that in order for the BOV air to expand fully in to the atmosphere it has to borrow from its internal energy - or heat - meaning its temperature drops below ambient.

4. So the air shooting out of your BOV is actually cold, much colder than the surrounding air (depending on how efficient your I/C is). This happens regardless of whether or not your BOV is plumb back or atmospheric venting.. however you wont get any benefit out of it if you vent it. Might as well plumb it back in and cool the airflow through the system for that split second! This is, of course, one cycle of the vapour compression refridgeration cycle. Your fridge does this repeatedly to cool the refridgerant down to well below freezing and keep your beers cool.

Anyway, hope that was somewhat interesting for someone, and next time someone claims that the BOV air is hot so you're better to vent it you can pwn them with science.

[Guest boostin]

Ooohh, can you redo that for the 1st gens with the BOV before the intercooler??

Nice write up. So recirc BOV's are cooler!!

[Durty-Sanchez]

Yea BOV needs to be post-cooler in order for sh*t to happen that way.. if it's pre-cooler the air coming out is (assuming no heat exchange) the same temperature..

[GTBLTD]

so glad i got a single turbo now i got an ARC BOV with single trumpet no problems with. used to run a HKS SSQ on the GTB drove me insane on the twin turbo haha got so sick of it. each to there own i suppose whether you like having a venting to atmosphere bov or not as long as ya happy with your car.

[madmike]

 Durty-Sanchez said:

Here's a good reason to stick with a plumb-back setup (sorry for those who've heard me bleat on about this before, seems older posts were lost in forum fails). In anycase, you can take my word for it or onwards science geeks for todays lesson in BOV thermodynamics..

The Turbo/IC/BOV thermodynamic system is very similar to that of a refridgerator, so much so that air being vented from your BOV is cold - very cold (well at least colder than the surrounding air). In some cases it could even be well below 0°C. How?.. well.. (numbers refer to crude MSPaint PV diagram below)

1. At first we have some air sitting around outside at a given ambient temperature. The turbo sucks this air in and compresses it through an adiabatic process (a process that adds no heat energy to the system). Yes the air does heat up, but that is because of the increased pressure (as given by the ideal gas law). For simplicities sake, no heat is exchanged from the turbo to the air. Looking at the 1st law of thermodynamics (dU = dQ-dW or the change in the systems internal energy, in this case the airmass, is related to the increase in its temperature - the work done by the airmass) we see that no heat energy is added, but work is done on the airmass by an external system (the turbo), so the internal energy of the system increases. (note, dW is only positive if the airmass does work on its surrounds, if work is done on the airmass by something external then dW is a negative value. Negative minus a negative is positive, so dU is positive). The final result is that the airmass is hotter, of greater pressure, and occupies a smaller volume than before.. you'll see the path it takes as the compression stage of our thermodynamic cycle.

2. Straight out of the turbo the air is then passed through the intercooler, or in other words a heat exchanger. The heat exchanger is designed to remove heat from our system, so we get a negative dQ in our thermodynamic equation and the overall energy of the airmass decreases. The air itself loses heat energy and the pressure of the airmass decreases. The volume however remains the same (roughly), so this is a constant volume process. This is usually the point where the air enters the engine and that'd be the end of this part of the cycle.. however, the case of a BOV venting causes something else to happen.

3. When your BOV vents you then have two systems suddenly combining - one at a much higher energy than the other. From the 2nd law of thermodynamics these two systems have to reach an equilibrium, which can only be done if the pressurised airmass (from the BOV) does work on the ambient airmass. Because the ambient airmass is so large in comparison, it will see very little in the way of changes, however the BOV airmass will do something quite interesting. Because of the speed of the BOV releasing this is considered an adiabatic expansion (no heat is exchanged from the BOV air to the atmospheric air), so the only method by which the BOV air can release it's energy is to do work. The work the BOV air needs to do to expand in to the outside atmosphere is roughly equal to the work done by the turbo to compress it in the first place.. however the intercooler has already taken away some of the airs internal energy. The result is that in order for the BOV air to expand fully in to the atmosphere it has to borrow from its internal energy - or heat - meaning its temperature drops below ambient.

4. So the air shooting out of your BOV is actually cold, much colder than the surrounding air (depending on how efficient your I/C is). This happens regardless of whether or not your BOV is plumb back or atmospheric venting.. however you wont get any benefit out of it if you vent it. Might as well plumb it back in and cool the airflow through the system for that split second! This is, of course, one cycle of the vapour compression refridgeration cycle. Your fridge does this repeatedly to cool the refridgerant down to well below freezing and keep your beers cool.

Anyway, hope that was somewhat interesting for someone, and next time someone claims that the BOV air is hot so you're better to vent it you can pwn them with science.

Nice little write up and diagram but it has some flaws....

1st rule of thermodynamics in refrigeration law states that for a substantial transfer of energy (Heat) there must be a change of state. ie change from liquid-gas or liquid-solid and vise versa.

Based on that fact.

The whole venting air charge being cooler than the surrounding air theory is impossible given the fact that the air being expelled by any blow off valve has no possibility whatsoever ( based on known rules of gas thermodynamics) of producing air that is cooler than the air originally bought in through the intake.

If anything you are more likely to see cooler temps in the intercooler when venting to atmosphere because the BOV is able to more Efficiently release boost pressure into the atmosphere pressure than recycle it back into the intake chamber.

Don't want to bleat..

but being a refrigeration engineer/designer for 15 odd years. And designing quite a few larger systems gives me some clout to refute this evidence.

It does sound plausible at first look but experience tells me otherwise.

[Durty-Sanchez]

Ahh.. then why do we run an intercooler if for "a substantial transfer of energy (Heat) there must be a change of state"? Temperature drops through an air-air intercooler of anywhere from 30-50°C, which is more than enough to cause these effects from a adiabatic expansion.

Granted there will probably be cooling in the intercooler itself, but I am assuming that the turbo is still supplying pressure to the system, so most of the adiabatic cooling is taking place outside of the vent hole (BOV). To simplify (apologies for the rant), if you increase pressure the temperature goes up. If you then decrease the pressure the temperature goes down. If you increase a fluids pressure then decrease it back to its original state, the temperature will be the same as before you started. Now increase the pressure (higher temperature), then remove heat from the system (intercooler), then decrease the pressure.. the temperature drop is the same as before, which leaves you with air cooler than you started with because you took heat energy out during the heat exchange (intercooler stage).

Perhaps you may have overlooked what is really going on here.. have a read up here.

Example: Coke - Coke is bottled with plenty of fizz in it. The bottle itself can be considered a thermodynamic system as the working fluid (the trapped gas/fizz) is trapped within the bottle. The disolved gas expands and escapes the liquid until the bottle reaches a equilibrium pressure whereby no more disolved gas can escape (hence why when you look at a new bottle of coke it's not fizzing inside). This pressure increase leads to an increase in temperature (as given by the ideal gas law). Now take your new bottle of coke (in equilibrium) and put it in to a fridge, or even just leave it at room temperature. As heat is exchanged from the bottle to the surrounding atmosphere the temperature of the gas decreases.. now open it. As the trapped gas (under pressure) needs to do work on the atmosphere in order to expand it must decrease it's heat energy.. however, much of that heat energy has been removed through the heat exchange process. This is an adiabatic process as there is not enough time for the heat to be exchanged in to the trapped gas. The result? The temperature drops below the ambient temperature, which you can prove by looking at the top chamber of the bottle where there is now mist, which is water vapour in the air that has dropped below dew/condensation temperature.

Find a atmospheric venting BOV on a car under load (i.e. on a dyno) and you'll see the same condensation forming as the BOV vents.

[madmike]

 Durty-Sanchez said:

Ahh.. then why do we run an intercooler if for "a substantial transfer of energy (Heat) there must be a change of state"? Temperature drops through an air-air intercooler of anywhere from 30-50°C, which is more than enough to cause these effects from a adiabatic expansion.

Granted there will probably be cooling in the intercooler itself, but I am assuming that the turbo is still supplying pressure to the system, so most of the adiabatic cooling is taking place outside of the vent hole (BOV). To simplify (apologies for the rant), if you increase pressure the temperature goes up. If you then decrease the pressure the temperature goes down. If you increase a fluids pressure then decrease it back to its original state, the temperature will be the same as before you started. Now increase the pressure (higher temperature), then remove heat from the system (intercooler), then decrease the pressure.. the temperature drop is the same as before, which leaves you with air cooler than you started with because you took heat energy out during the heat exchange (intercooler stage).

Perhaps you may have overlooked what is really going on here.. have a read up here.

Example: Coke - Coke is bottled with plenty of fizz in it. The bottle itself can be considered a thermodynamic system as the working fluid (the trapped gas/fizz) is trapped within the bottle. The disolved gas expands and escapes the liquid until the bottle reaches a equilibrium pressure whereby no more disolved gas can escape (hence why when you look at a new bottle of coke it's not fizzing inside). This pressure increase leads to an increase in temperature (as given by the ideal gas law). Now take your new bottle of coke (in equilibrium) and put it in to a fridge, or even just leave it at room temperature. As heat is exchanged from the bottle to the surrounding atmosphere the temperature of the gas decreases.. now open it. As the trapped gas (under pressure) needs to do work on the atmosphere in order to expand it must decrease it's heat energy.. however, much of that heat energy has been removed through the heat exchange process. This is an adiabatic process as there is not enough time for the heat to be exchanged in to the trapped gas. The result? The temperature drops below the ambient temperature, which you can prove by looking at the top chamber of the bottle where there is now mist, which is water vapour in the air that has dropped below dew/condensation temperature.

Find a atmospheric venting BOV on a car under load (i.e. on a dyno) and you'll see the same condensation forming as the BOV vents.

A transformation of a thermodynamic system can be considered adiabatic when it is quick enough that no significant heat is transferred between the system and the outside.

I suggest that the condensation seen at the outlet of a "vent to Atmo" blow off valve on a static dyno could also be a symptom of a higher "dew point" caused by the compression of atmospheric pressure air at any given humidity and not necessarily caused by any cooling effect of pressurized (compressed) air passing out to atmosphere.

And you will also find that the mist seen sometimes coming out of a cold coke or beer bottle when initially opened is actually CO2 carrying an amount of water with it released from the liquid at the time the bottle is opened.

At any rate.. the cooling effect of venting back into the intake vs venting to atmo is in my opinion completely offset by the fact that the pressurized air charge will be pressurizing the intake piping and hence will not be able to relieve the unwanted boost as efficiently.

This is another of these disputes that really can never be fully cleared up one way or the other. (Even with an army of little dudes in white coats.. LOL)

[Durty-Sanchez]

Without being "that forum guy" here are some links to help (sorry, I was one of those white lab coat guys for 6 years).. in any case, I give you the Reverse Brayton Cycle aka the Gas/Air Refrigeration Cycle aka Bell Coleman Cycle:

Gas Refrigeration Cycle

Brayton Cycle

And one more link, which shows that the cold discharge, when re-compressed and discharged again, continues to drop in temperature, as used on aircraft

[madmike]

 Durty-Sanchez said:

Without being "that forum guy" here are some links to help (sorry, I was one of those white lab coat guys for 6 years).. in any case, I give you the Reverse Brayton Cycle aka the Gas/Air Refrigeration Cycle aka Bell Coleman Cycle:

Gas Refrigeration Cycle

Brayton Cycle

And one more link, which shows that the cold discharge, when re-compressed and discharged again, continues to drop in temperature, as used on aircraft

LOL Good call..

I love a good debate.

I understand all the principals of the specific systems described in those links and have used these to design secondary refrigeration systems in larger cool stores. using a mix of glycol and brine as the secondary refrigerant.

(aiming to keep the ammonia side of the particular system as small as possible refrigerant charge wise due to council restrictions and OSH protocols).

Still none of these specifically relate to my comment regarding the pressurization and subsequent disruption of airflow in the air intake before the turbo when a plumb back blow off valve is employed.

Surely this disruption to the laminar airflow stream would typically negate the cooling benefits of any chilling effect of air charge being returned to the induction side of the turbo..

[smokn4]

geez you fallas know alot on this ;D,

from blow offs to fridges to dogg gone air planes

[GTBLTD]

yeah hard out mine just goes ppsshh i know which guys to talk to if mine starts crapping out ;D