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Started by R-A-Y, March 14, 2005, 11:01:51 PM

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Hope this can help a bit for those looking at BOVs as an aftermarket mod for their car :) Anyone more knowledgeable, please feel free to correct my statements :)

Blow Off Valve FAQ

Term usage: "Blow off valves" go by several names, among them are compressor bypass valve (CBV), air by-pass valve, bypass valve (BPV), blow off valve (BOV), Diverter valve, and possibly a few others. BOV is the common and incorrect term that lumps true blow off valves and bypass valves under the same term. For the sake of correctness, this post will refer to either aftermarket BOV, aftermarket BPV or OEM BPV as these are the most correct terms.

What is the function of a blow off valve (BOV)? To release pressure from the intake tract of a turbo car when the throttle closes. It is a vacuum-actuated valve designed to releases the air to the atmosphere.

What is the function of a bypass valve (BPV)? To release pressure from the intake tract of a turbo car when the throttle closes. It is a vacuum-actuated valve designed to recirculate the air back into the intake before the turbo inlet, but after the airflow sensor.

What is the purpose of a BOV/BPV? When the throttle closes and the intake system is under pressure, the high-pressure air entering the motor will bump into the closed throttle plate, and in the absence of a BOV/BPV, a pressure wave will travel back to the turbocharger. The result is that the compressor wheel will stall (a phenomenon known as "compressor surge") and slow down very quickly. This is hard on the bearings and decreases the turbo's lifespan, but it also means the turbo will take longer to spin up the next time the throttle is opened.

Which manufacturer is best? This topic is highly debated. There have been no reported consistent "bad" aftermarket Bovs. Obviously, there may have been bad ones sold, but not enough to report as "bad" overall.

What are the different types of aftermarket BOVs/BPVs? Different manufacturers use different methods. There are three basic types:
1. Aftermarket BPV: Similar in function to the OEM BPV where 100% of the air is recirculated.
2. Atmospheric BOV: 100% of the air is vented to the atmosphere.
3. Hybrid BOV: These depend on the manufacturer and end user settings. These can either be adjustable or manufacturer set for different percentages of atmospheric/recirculation dumping. They can also be set to work as recirculation during lower boost conditions and 100% atmospheric during higher boost conditions.

Are there any downsides to aftermarket BOVs? There have not been significant amounts of problems with BOVs. Aftermarket BOVs can and do require some light end user maintenance to keep them performing perfectly. For aftermarket hybrid BOVs that have end user defined settings, there will be an initial period of adjustment to obtain the desired recirculation/atmospheric ratio as well.

Are there any negative effects with aftermarket BOVs? Yes. The downside of releasing the air to atmosphere is that it has already been metered by the mass air sensor, and when it blows off, the ECU will be injecting the wrong amount of fuel into the cylinders. The engine temporarily runs extremely rich, meaning too much fuel is injected into the cylinders.

This temporary rich condition isn't usually that harmful, but it can cause bucking or hesitation on lift-throttle. If the condition is really bad, it can eventually foul spark plugs and even clog the catalytic converter as unburned fuel on the catalytic converter burns very hot, and too much of it can melt the cat.


let's sticky this for a couple of weeks ;D
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very informative and timely. salamat  8)


nice article ray!  is it okay if i also post some of the BOV FAQ from  
"for those who know, there is no finish line"


Quote from: nugundam93 on April 02, 2005, 10:21:45 AM
nice article ray!  is it okay if i also post some of the BOV FAQ from  

Of course :D ! Now one said that I am the only voice here ;D


hmmm, i checked your post ray, almost same na rin pala.  anyway, if you guys wanna read, it's at

the site also needs registration, btw.   ;D
"for those who know, there is no finish line"


Quote from: nugundam93 on April 04, 2005, 06:31:04 AM
hmmm, i checked your post ray, almost same na rin pala.  anyway, if you guys wanna read, it's at

the site also needs registration, btw.   ;D

Probably it came from NASIOC just like mine ;D


heheheh.  anyway, all is good :D
"for those who know, there is no finish line"


I have limited experience boosting cars but I would like to add briefly..

Blow-off valves are like "condoms" ;D, meaning there is always a choice between having one installed or not.

IMO, during long-term use, BOV can save your car or the turbo system itself from damage.

BOVs will enable excess pressure to release unto the atmosphere, minimizing turbo backspin. Turbo Backspin will cause the impeller on the turbine shaft to abruptly stop or stall, primarily bacause the boost pressure needs to release.

HTH :)
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thanks dondie!

now if i could only find a way for the 4m40 to be quieter so that the planned bov would make itself heard better.  hahahaha...
"for those who know, there is no finish line"


well, as for the BOV's for most turbocharged cars there are the ups and downs. for engines that is running with a MAP sensor, a BOV would be OK to use compared to a AFM equiped engine. since that the AFM is before the throttle body, there would be some slight effect on the engines idle and or performance since that the metered air is sipping through the BOV (some Greddy type S & Blitz BOV's suffer from leakage) and its causing the ECU to read inaccurate readings. therefore affecting its performance. in the other hand, A MAP sensor equiped engine can still maintain & give accurate air readings to the ECU since that the MAP sensor sits behind the throtlle body (ex. all D,B,H series Honda engines and other MAP equiped JDM engines from Toyota, etc.) so that the BOV can do its job without affecting anything to the ECU. anywayz, BOV are cool, specially if your boosting. but where I live though, BOV's needs to be in the hush hush, since some cops & law enforcers out here in most Califorina city's are really strict now on modified cars. it sucks, but its all good. its part of the game. anywayz, keep it kool out there in the streets of Manila and anywhere else aite, laterz! -KBR


oh yeh, also I would like to recommend ya'll using HKS SSQV and Turbo XS RFL BOV. both BOV's are good between 5-25 psi (as tested on some of our cars: 1991 240SX w/ SR20DET-s14 black top, 1991 MR2 w/ 3SGTE, 1995 Supra w/ 2JZGTE.) both BOV's gives good results and sounds. aite, cheers! -KBR


Yeah I heard all about those noise rules. You guys can't even run an HKS exhaust wihtout getting pulled over at least twice in a block ;D You guys should just move here ;D


yeh man, I wish I could move back. but anywayz, you cant really use a non CARB (California Air Resource Board) exhaust system or any high performance parts without a CARB approved sticker, or else you will get a citation if you get caught up and pulled over. but still alot of people out here are still doing it the ILLEGAL way and thats OK. as long as you dont get caught up. anywayz, I use a BORLA XR-1 3.5" race muffler w/ 3" straight through exhaust piping & 3" high flow CAT on my Supra. its pretty loud, loud enough that you can hear me rolling even though I'm like 2-3 blocks away. but I'm lucky so far that I dont get a ticket yet. anywayz, I know out there in the Philippines you get to do whatever you want in terms of modifying cars and ya'll still can get away from the cops! haha! aite, cheers! -KBR


AFM engines can use recirculating BOV's so as not to affect the metered air.
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