Recirculation vs Blowoff
11-13-2002, 04:16 PM
#1
Three Wheelin'
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Recirculation vs Blowoff
Could someone explain to me the reasoning behind having the Recirculation valve blow off boost back into the intake track instead of into the atmosphere? I am designing my own intake boot, and I want to know if it is possible to vent the blowoff valve to the atmosphere.
Thanks
Thanks
11-13-2002, 04:34 PM
#2
Three Wheelin'
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If you're using a MAF or the stock barndoor air metering device, the air has already been "metered", so you have to recirculate the air back into the intake stream. If it's blown off to the atmosphere, when you open up the throttle again, more air will be metered, so the ECU will give fuel for the vented air and the newly measured air, thus causing the A/F ratio to go very rich.
On MAP setups (standalones, GURU MAP), the air's measured as manifold pressure inside the intake manifold, so air isn't measured until it gets past the throttle body. Anything before the air measuring point, you can vent whereever you like.
On MAP setups (standalones, GURU MAP), the air's measured as manifold pressure inside the intake manifold, so air isn't measured until it gets past the throttle body. Anything before the air measuring point, you can vent whereever you like.
11-13-2002, 04:44 PM
#3
Rennlist Junkie Forever
The factory turbo systems on our cars with either the stock flapper vane or a MAF measure that "X" amount of air has entererd the intake tract, and has added the proper amount of fuel.
If you dump to atmosphere, then you have a situation where that measured air is now missing. This results in a rich mixture.
Further, the bypass valve is open under vacuum conditions. Why? Because your engine can suck in air "bypassing" the turbo, intercooler piping, and intercooler. Much less restriction. This is why it is called a bypass valve.
If you have a bypass valve that you dump to atmosphere, then you will have a situation where that valve is also open to atmosphere under vacuum conditions... and that's a real problem because not only do you have unmetered air entering the engine, but you have unfiltered air entering the engine.
You can get an adjustable bypass valve, then you'd have to have it so tight that it wouldn't open up under vacuum conditions, thereby eliminating the advantage in the first place.. which is to eliminate having to suck air through the turbo and intercooler while not under boost.
Hope this helps.
If you dump to atmosphere, then you have a situation where that measured air is now missing. This results in a rich mixture.
Further, the bypass valve is open under vacuum conditions. Why? Because your engine can suck in air "bypassing" the turbo, intercooler piping, and intercooler. Much less restriction. This is why it is called a bypass valve.
If you have a bypass valve that you dump to atmosphere, then you will have a situation where that valve is also open to atmosphere under vacuum conditions... and that's a real problem because not only do you have unmetered air entering the engine, but you have unfiltered air entering the engine.
You can get an adjustable bypass valve, then you'd have to have it so tight that it wouldn't open up under vacuum conditions, thereby eliminating the advantage in the first place.. which is to eliminate having to suck air through the turbo and intercooler while not under boost.
Hope this helps.
11-13-2002, 04:49 PM
#4
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When the throttle is let off when there is boost it causes back-pressure because of the turbo's turbine still spinning and the throttle body door being shut. The trapped air is then pressed through the BOV and recirulated back into the intake manifold and other areas. The 951 is designed to read the wasted (trapped) air and adjust your fuel system accordingly so there is no rough idle or loss of power when shifting to the next gear. When you release that trapped air into the atmosphere instead of recirculating it you will most likely lose some initial boost going into the next gear and possibly other things. I hope this help you better understand the purpose of the BOV. All the best, Gabe <img src="graemlins/burnout.gif" border="0" alt="[burnout]" />
11-13-2002, 09:54 PM
#5
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"Could someone explain to me the reasoning behind having the Recirculation valve blow off boost back into the intake track instead of into the atmosphere? "
"When you release that trapped air into the atmosphere instead of recirculating it you will most likely lose some initial boost going into the next gear and possibly other things"
Yup, to expand on Blurry951's performance benefits of recirculation, think of it from the turbo's perspective. When you're shifting, there's minimal exhaust pressure so the turbo's spinning out of shear momentum. There's three things you can at this point with the outlet air from the turbo.
1. Do nothing - this is the worse case scenario. When the throttle is closed, you've got a whole column of air at 15psi moving towards it. The air will then slam into the plate and the tail-end of the column will pressurize the head of the column up to 30-40-50psi+. Then this high-pressure point builds up backwards as more and more air piles up into the closed throttle. The pressure point (shockwave) eventually makes it back to the turbo and can shatter the compressor blades. A couple of Toyota's ceramic-composite turbos are particularly sensitive to this. You can image the resistance the turbo feels as it tries to force more and more air into the dead-end path. This resistance slows down the turbo tremendously.
2. Vent the compressed turbo outlet to atmosphere - this is the simplest solution. So when the throttle is closed, you open up a valve ahead of the throttle to vent the turbo's outlet. This prevents the pressure build-up ahead of the plate and saves the turbo from shockwave damage.
The turbo can continue to spin and doesn't slow down as much because it doesn't have to try and pressurize more air into a dead-end tube. However, the turbo inlet is still coming in at atmospheric pressure (or a slight vacuum). So the turbo still has to do work and compress this new air up to the max-boost level you've set. This pressure-differential will slow down the turbo as it compresses the incoming air and dumps it out the blow-off. Additionaly the incoming air will have been measured by the AFM/MAF sensor and fuel will be injected to match, yet that air never makes it to the engine. The result will be a rich condition until you re-open the throttle or until the turbo has stopped spinning.
However, this configuration does keep the turbo spinning longer than the previous one and boost-lag upon re-opening the throttle is less.
3. recirculate pressured air - This is the best solution because it provides the highest performance possible. By recirculating the pressurized outlet back to the inlet of the turbo, you reduce the pressure-differential between the inlet & outlet, slowing down the turbo the least. In fact, if you can recirculate enough air fast and smoothly enough you can almost keep that turbo spinning forever. If the turbo outlet is 15psi and turbo inlet is 15psi, you'll have zero pumping losses and will extract minimal work out of the turb.
An additional benefit is you'll have accurate fuel-metering. Any air that goes past the AFM/MAF sensor goes into the engine 1:1. The fuel that is metered will match the air that's ingested by the engine.
So this last configuration is the best from a performance point of view. That's why on our Stg.2 J-pipes, we've moved the bypass valve as close to the turbo as possible and aimed the recirculation port 45-degrees so it has a straight shot at the ocmpressor-blades to keetp it spinning as much as possible between shifts. We've also removed on 90-degree bend from the recirculation path as well to aid in flow-velocity.
Here's a picture I drew up a while back to illustrate the differences between #2 & 3 but I can't find the original thread anywhere...
<img src="http://boards.rennlist.com/upload/BOV-CompareDiagram3.gifZ
"When you release that trapped air into the atmosphere instead of recirculating it you will most likely lose some initial boost going into the next gear and possibly other things"
Yup, to expand on Blurry951's performance benefits of recirculation, think of it from the turbo's perspective. When you're shifting, there's minimal exhaust pressure so the turbo's spinning out of shear momentum. There's three things you can at this point with the outlet air from the turbo.
1. Do nothing - this is the worse case scenario. When the throttle is closed, you've got a whole column of air at 15psi moving towards it. The air will then slam into the plate and the tail-end of the column will pressurize the head of the column up to 30-40-50psi+. Then this high-pressure point builds up backwards as more and more air piles up into the closed throttle. The pressure point (shockwave) eventually makes it back to the turbo and can shatter the compressor blades. A couple of Toyota's ceramic-composite turbos are particularly sensitive to this. You can image the resistance the turbo feels as it tries to force more and more air into the dead-end path. This resistance slows down the turbo tremendously.
2. Vent the compressed turbo outlet to atmosphere - this is the simplest solution. So when the throttle is closed, you open up a valve ahead of the throttle to vent the turbo's outlet. This prevents the pressure build-up ahead of the plate and saves the turbo from shockwave damage.
The turbo can continue to spin and doesn't slow down as much because it doesn't have to try and pressurize more air into a dead-end tube. However, the turbo inlet is still coming in at atmospheric pressure (or a slight vacuum). So the turbo still has to do work and compress this new air up to the max-boost level you've set. This pressure-differential will slow down the turbo as it compresses the incoming air and dumps it out the blow-off. Additionaly the incoming air will have been measured by the AFM/MAF sensor and fuel will be injected to match, yet that air never makes it to the engine. The result will be a rich condition until you re-open the throttle or until the turbo has stopped spinning.
However, this configuration does keep the turbo spinning longer than the previous one and boost-lag upon re-opening the throttle is less.
3. recirculate pressured air - This is the best solution because it provides the highest performance possible. By recirculating the pressurized outlet back to the inlet of the turbo, you reduce the pressure-differential between the inlet & outlet, slowing down the turbo the least. In fact, if you can recirculate enough air fast and smoothly enough you can almost keep that turbo spinning forever. If the turbo outlet is 15psi and turbo inlet is 15psi, you'll have zero pumping losses and will extract minimal work out of the turb.
An additional benefit is you'll have accurate fuel-metering. Any air that goes past the AFM/MAF sensor goes into the engine 1:1. The fuel that is metered will match the air that's ingested by the engine.
So this last configuration is the best from a performance point of view. That's why on our Stg.2 J-pipes, we've moved the bypass valve as close to the turbo as possible and aimed the recirculation port 45-degrees so it has a straight shot at the ocmpressor-blades to keetp it spinning as much as possible between shifts. We've also removed on 90-degree bend from the recirculation path as well to aid in flow-velocity.
Here's a picture I drew up a while back to illustrate the differences between #2 & 3 but I can't find the original thread anywhere...
<img src="http://boards.rennlist.com/upload/BOV-CompareDiagram3.gifZ
11-14-2002, 10:53 AM
#6
Drifting
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Not that I'd ever doubt you Danno 
, but why do most of the cars in the WRC vent to atmoshere if it' a performance detriment to power delivery when shifting. It's not like those guys aren't shifting every couple of seconds. Plus I don't want to have to lose the cool "phhtphhtphhtphhtphht" sound my Forge dump valve makes when shifting. 
, but why do most of the cars in the WRC vent to atmoshere if it' a performance detriment to power delivery when shifting. It's not like those guys aren't shifting every couple of seconds. Plus I don't want to have to lose the cool "phhtphhtphhtphhtphht" sound my Forge dump valve makes when shifting.
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11-14-2002, 12:13 PM
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I agree with Danno theoretically, but in reality, I'm not sure how much the recirculation helps. The other half of the turbo is going to affect it's spool down also and may outweighs most of the benefit on the compressor side. The exhaust housing had all of this high pressure gas in it while you were on boost but "bam" you close the throttle, you probably go to a vacuum condition fast in the crossover as little air is coming in and the turbo is still spinning. I think that would slow the turbo down...
11-14-2002, 02:10 PM
#9
Three Wheelin'
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I actually ran my car with both a recirculating setup for about a month before switching to vent to atmosphere setup. The turbo is definately spooling quicker after shifts in the vent to atmosphere setup.
[quote]Originally posted by Russ Murphy:
<strong>Not that I'd ever doubt you Danno , but why do most of the cars in the WRC vent to atmoshere if it' a performance detriment to power delivery when shifting.</strong><hr></blockquote>
There may be a different reason behind this... in Rally car setups, they actually dump fuel by not igniting it in the combustion chamber, which dumps fuel into the exhaust housing of the turbo. It burns in there (not sure if it's autoignition from heat or if they actually have a spark plug or something in there) and spins the turbo up even quicker when off throttle, allowing them to have huge boost when they're back on the throttle. I don't think in this setup it would work in a recirculating setup, with boost building up higher and higher until there's a lot of pressure on the compressor side. Just my wild *** theory anyways...
[quote]Originally posted by Russ Murphy:
<strong>Not that I'd ever doubt you Danno
, but why do most of the cars in the WRC vent to atmoshere if it' a performance detriment to power delivery when shifting.</strong><hr></blockquote>There may be a different reason behind this... in Rally car setups, they actually dump fuel by not igniting it in the combustion chamber, which dumps fuel into the exhaust housing of the turbo. It burns in there (not sure if it's autoignition from heat or if they actually have a spark plug or something in there) and spins the turbo up even quicker when off throttle, allowing them to have huge boost when they're back on the throttle. I don't think in this setup it would work in a recirculating setup, with boost building up higher and higher until there's a lot of pressure on the compressor side. Just my wild *** theory anyways...
11-14-2002, 02:51 PM
#10
Drifting
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Talk about pressure build-up! Imagine raw fuel burning afterburner-like in the hotside with the throttle body closed. No wonder you can hear those car's blowoff valves from the next county over.
11-14-2002, 03:06 PM
#11
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Rage that sound like a good idea you mad sientist. I have another idea. We sould find a way to extened the shaft on the hot side of the turbo through the down pipe and add a electric motor that would keep the turbo up to speed on vacume conditions and throttle lift. In addition this shaft sould have a electric clutch that would disengage the motor when back on throttle so the turbo would not carry the mass off the motor and hinder the performance of the turbo.this could also be done with a gear on the shaft shaft between the hot and compressor section so the impellas would not be carrying additional weight. My two cents of engineering. <img src="graemlins/c.gif" border="0" alt="[ouch]" />
I have another idea. We sould find a way to extened the shaft on the hot side of the turbo through the down pipe and add a electric motor that would keep the turbo up to speed on vacume conditions and throttle lift. In addition this shaft sould have a electric clutch that would disengage the motor when back on throttle so the turbo would not carry the mass off the motor and hinder the performance of the turbo.this could also be done with a gear on the shaft shaft between the hot and compressor section so the impellas would not be carrying additional weight. My two cents of engineering. <img src="graemlins/c.gif" border="0" alt="[ouch]" />
11-14-2002, 03:25 PM
#12
Three Wheelin'
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haha that's not my idea, that's how the turbo systems in the rally cars are designed. In fact, with SDS, u can setup your car to do the same thing (they call it Anti-Lag). From the SDS page :
[quote]Turbo Anti-lag Programming for E and F Systems
For racing applications only, E and F systems may be programmed to reduce turbo lag by retarding the ignition timing and increasing the fuel supplied under high manifold vacuum conditions. The fuel is forced to burn in the exhaust system which keeps the turbine spooled up to some degree during closed throttle conditions. This function should be used only on race applications and only on cars equipped with very strong straight through, stainless steel mufflers or no mufflers as extreme temperatures and pressures may result. Turbocharger and exhaust system life may be seriously reduced as well.
Programming is accomplished by entering a MAP fuel value of 20-40 under the lowest MAP range available and entering an IGN RET/MANPRESS value of 20-30 in the lowest range available.<hr></blockquote>
[quote]Turbo Anti-lag Programming for E and F Systems
For racing applications only, E and F systems may be programmed to reduce turbo lag by retarding the ignition timing and increasing the fuel supplied under high manifold vacuum conditions. The fuel is forced to burn in the exhaust system which keeps the turbine spooled up to some degree during closed throttle conditions. This function should be used only on race applications and only on cars equipped with very strong straight through, stainless steel mufflers or no mufflers as extreme temperatures and pressures may result. Turbocharger and exhaust system life may be seriously reduced as well.
Programming is accomplished by entering a MAP fuel value of 20-40 under the lowest MAP range available and entering an IGN RET/MANPRESS value of 20-30 in the lowest range available.<hr></blockquote>
