Greddy Type s blow off valve
The Greddy Type-S is offered either as a blow off or bypass valve. It is intended to protect a turbo's bearings from undue stress caused from compressor surge and to improve boost response between shifts. It is adjustable by the small screw in the center.  If the valve is too loose, you may leak boost pressure. If it is too tight, you may experience compressor surge.

Hi Sarah,

I'm an engineering student at Georgia Tech and I'm part of an automotive engineering team at the school. We're currently designing a turbo system for our competition car and one point that was recently brought up was the usefulness of a blow-off/bypass valve. I've always thought that it was necessary to run one for turbo longevity, yet one of our members found a forum post saying that they are only used for NVH purposes and that any reflected pressure waves that occur after the throttle is closed do not actually harm the turbocharger.

We've checked out some racing setups (80s F1, LeMans, Champcar) and aside from regulation pop-off valves, we haven't seen any applications of blow off valves in these cars. I read your blog on MotoIQ and I was wondering if you had any insight on this question? I'm trying to do some calculations to see if this is true, but I thought you would be more knowledgeable on the subject.

Thanks alot!


Sweet- I love a little controversy to spark some conversation! I have no empirical evidence or have yet to find a calculation to apply here suggesting a blowoff valve is necessary. Then again, I've never run a turbo car without one. Starting with the facts, as you accelerate in a turbo car, the engine takes in air from the turbo.

Let's look at a turbo like the Garrett GTX3071 boosting 30psi. It's spinning in the neighborhood of 140,000 rpm and flowing 55 lbs/min. When you let off the gas to shift/brake/play innocent near a cop or when the turbo is pushing out more air than the engine can suck up, you run into a compressor surge situation. Imagine now when the throttle plate closes quickly. The turbo isn't as quick to respond and takes longer to spool down. It continues to flow at a high rate but the air flow (which has both momentum and mass) is cut to basically nothing in no time. That compressed air has no place to go except back into the compressor wheel and thrust bearing at a destructive rate.

At this time, there is very little pressure on the exhaust side but there is a spike in charge pressure exerted on the compressor wheel's full surface area which creates a huge thrust load on the turbo bearings that could cause severe damage. A conventional journal bearing would never be able to withstand the shockwaves assaulting the compressor. A ball bearing is more resilient against this damage but the compressor surge could still generate enough force to even throw the wheel off balance, causing premature wear. Those bearings are more durable against forms of ricer monkey business without certifiable turbo failure but I'd still run a BOV.

The blow off valve (bypass valve, diverter valve) protects the turbo from these harmful shockwaves of boost pressure. It also helps to maintain boost pressure and keep the lag down. The BOV is located on the intake side of the turbo. When the throttle plate closes and the turbo continues to spin and build pressure, the intake manifold senses a vacuum and the blow off valve opens to vent the pressurized air from the intake to the atmosphere. A bypass valve accomplishes the same thing but it recirculates the pressurized air back into the intake tract instead.  

Corky Bell has even gone on record stating he's never seen a turbo damaged due to running no BOV and sh... (poop)- he wrote the book! He says a BOV is simply something to curb the "funny noise" (the lovely turkey gobble noisethe turbo makes caused by lifting quickly and creating an unstable air flow and pressure fluctuations. Diesel cars don't run BOV's because they have no throttle plate that closes when the engine comes off boost. WRC cars don't tend to run them because they run a buttload of boost and don't want a leak point. Then again, they also like to run Garrett ball bearing TR30R's which often come in for service or replacement.

Some setups include anti lag or boost tubes to try to get around it. Anti-lag systems function like blow off valves in that they have a valve that opens when the driver lifts off the throttle to dump the excess boost into the exhaust manifold. But the spinning turbo still has to thrust its air somewhere- right back at the compressor wheel. Listen to videos of turbo cars running hillclimbs or rallies. That chirping sound is a clear indication of compressor surge. Crawford Performance doesn't run them on their turbo setups and make amazing power and reliability. A perfectly matched ball bearing turbo shouldn't have too many issues but...

I don't know about you- I'd rather spend $200 on something that protects my $2000 investment. Think about what those racing car setups have that you don’t- the ability to swap out engines pretty regularly and fistfuls of sponsor money. The pressure fluctuations and inconsistent flow realized by turbo setups may still cause a quick demise to any race car build but in all likelihood, their engines aren’t long for this world anyways. 

Compare those setups to most “race” cars of privateers, daily drivers, or those without deep pockets. They run blow-off valves because no sponsor is going to swap in a new motor every two races. And let's be honest- forget the wear factor alone. When the blow off valve opens, it gives off that cool whoosh that lets you know all is good with the world and (Kool-Aid man) “oh yeah”- you gots a turbo; as if you could forget! Go fast or suck!

Turbonetics blow off valve
The Turbonetics Raptor blow off valve can be set up to blow off or bypass boost, making it a good choice for either MAP or MAF equipped cars. It is capable of handling a decent amount of boost, but Turbonetics also offers the Godzilla blow off valve for extreme boosting applications. 



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Wednesday, June 26, 2013 6:10 AM
I really would have liked to see a rebuttal to all the accusump failures. I'm sure a lot is due to improper installation, but they just seem problematic when you start digging. I realize that's not what the reader asked though.
Wednesday, June 26, 2013 7:31 AM
Sarah, I think you're confusing compressor surge and compressor stall. Compressor surge is a result of running an improperly sized turbo and can't be helped by a BOV. Compressor stall is what you get when you don't run a BOV (and is the chirping sound). I am in the anti-BOV camp, with the opinion that they're there on OEM applications for noise reduction and sold as aftermarket to get you to spend money.

I'm putting my money where my mouth is with my own "experiment." I plan to run without a BOV on a freshly rebuilt and balanced HT-18/T-04E hybrid that I'm installing on my FC (the same car that won the MotoIQ Reader's Ride contest...while still on a rotisserie). I'll be measuring both axial and thrust play before the turbo ever sees 1 mile and I'll re-check it at regular intervals (250, 500, 1000, etc). If your theory holds true, my journal bearing turbo should see disastrous amounts of thrust play in a very short period of time (and lots of smoke in the exhaust). If I'm correct then I should see no diminished service life or increased oil consumption.

If anyone can think of ways to improve this experiment, I'd love to hear them. Maybe then, this baby can be put to bed.
Mike Kojima
Mike Kojimalink
Wednesday, June 26, 2013 8:20 AM
Compressor stall is surge. When you close the throttle the engines swallowing capacity is greatly reduced and the compressor goes into surge.
Wes Dumalski
Wes Dumalskilink
Wednesday, June 26, 2013 8:26 AM
Great installment and hits home for our GR STi project. We are working on the next installment where we explore this very area of using a BPV.
Wednesday, June 26, 2013 9:16 AM
The important distinction is that off-throttle comp. surge is nowhere near as disastrous as on-throttle comp. surge. In off-throttle surge, the exhaust pressure/flow is reduced significantly (from throttle-lift).

Josh, you should mount pressure transducers (http://www.msefi.com/viewtopic.php?f=38&t=32648&p=200918&hilit=pressure+transducer#p200918) in the intake tract right after the compressor, right before the throttle, and in the manifold (unless you already have a MAP sensor). Log the data and record the pressure spike everyone talks about. You will find it to be only a few psi higher than boost pressure, as has been recorded by others who have done this.
Wednesday, June 26, 2013 9:37 AM
Bowenaero, I can't say the BOV is needed or not, I really don't know. but something that bugs me is that you're only looking at the pressure in the intake.

the compressor wheel is a bit bigger than the turbine normally but they are in the same general size. under normal conditions the backpressure on the exhaust side pushes against the boost pressure and prevents the turbo from jamming from one side to the other. when you remove pressure from one side but not the other there's a large pressure difference.

Wednesday, June 26, 2013 10:21 AM
I was thinking more along the lines of ways to ensure my measurements of play don't get skewed by something I'm not thinking about.

Anecdotal observation of a friend's turbo, after BOV removal, reinforce my thoughts on the whole thing.

Interestingly enough, I've seen data logs that appear to indicate that with the BOV removed, boost comes on earlier and returns faster after shifts. I'll attempt to hunt down the logs.

Mike: I guess I got caught up in the misconception that the two are different terms for different conditions...not that they are two terms for the same condition. I may be an automotive professional, but am definitely an engineering layman. :)

Wednesday, June 26, 2013 11:54 AM
You are right, Marc. Increased thrust load does exist. But how much it increases and the extent of damage caused by the increase is what's being debated.
Marty Staggs
Marty Staggslink
Wednesday, June 26, 2013 12:03 PM
Put the added trust wear and turbo "protection" aside for the moment.

The benefits to transient response (on/off throttle during shifting or racing) have been proven time and time again.

Put a shaft speed sensor on your turbo and log it as well as MAP.
The data is amazing.

paul 01
paul 01link
Wednesday, June 26, 2013 12:25 PM
Josh, I don't doubt that removing the BOV can result in increased spool for certain turbochargers (The 4" compressor housings for the GT30/GT35 series turbochargers with the surge ports come to mind). But a big factor in this finding was also probably due to a BOV/DV that doesn't respond quick enough due to spring load or port size. A proper BOV/DV should respond fast enough and flow enough to where the turbocharger wouldn't see any difference between flowing air into the engine or flowing air through the valve. This is where the transient response part comes into play ESPECIALLY for BB and billet turbos. If there is nothing there to keep the wheel from spinning, it will continue to spin on. Just ask Newton, "An object in motion stays in motion unless acted upon by an outside force". The only outside force in a proper setup being oil viscosity slowing it down slightly until the next combustion act.

For an all out setup, if the turbo is expendable I say toss it. Less things to worry about. But if it is required to be used for some time, add a valve.

I don't even want to get into the pros and cons of recirculating the discharge haha
Dan DeRosia
Dan DeRosialink
Wednesday, June 26, 2013 3:00 PM
Here's how I look at it. The Audi R8 is one of the most successful LeMans Prototypes ever; it ran a twin turbo 3.6L V8 with direct (gasoline) injection in later years. It won the LeMans 24 hours 5 times, ALMS championship 7 years straight, and so on - a total of 63 wins out of 79 races. Engineered for ultimate no compromise reliability over 24 hours.

No BOVs.

I seriously cannot believe that if there were any chance of lack of BOVs killing the turbos, Audi wouldn't spring for the couple hundred dollars to fit some.
Wednesday, June 26, 2013 3:40 PM
The thing that I hate about BOV, is they leak. They are another point of failure in a system. Most of the failures involve leaks, which in theory can overspeed the turbo. At "big boy" boost, I have seen way too many failures. My old GMC Typhoon with the 4.3 and auto trans ran a Mitsubishi turbo, for lots of miles, never had a failure without a BOV. Now onto a Vortech supercharger on a car, with a failing BOV, we had several failures under road course conditions. Once we had a stronger BOV to match the increase in boost, we had a lot less failures.

On my personal cars, between running a BOV, and not running a BOV, I will pick not running one. Its more important for me, not to have another leak, or potential failure point in the system.
Shifter Kart
Shifter Kartlink
Wednesday, June 26, 2013 4:25 PM
not just the Audis...none of the turbocharged Group C/LMP cars run BOVs. i dont think the turbo F1 cars ran any either.
Wednesday, June 26, 2013 8:36 PM
Keep in mind that race cars have very quick service intervals. The current Indycar engines have a 2000 mile life. There was some freaking out in F1 back in 2004 when the new regulations said the engine had to last a whole race weekend! Turbos on race engines are service items. They get rebuilt very often just like the engines.

Also, with modern race engines, I imagine most if not all have WOT upshifts. Therefore, the throttle plate stays open. I don't know if any race cars do this, but with modern electronic throttles, the throttle can be left open even if you get off the throttle which eliminates the need for the BOV.
Mike Kojima
Mike Kojimalink
Wednesday, June 26, 2013 9:46 PM
Keep in mind that race engines have other ways of reducing surge on throttle lift, like ECU engine/throttle control, antilag and modified variants of this that are easier on the turbo than pure antilag.

If you experiment with a good highly adjustable BOV, there is no doubt in my mind that it reduces lag between shifts and improves throttle response. Synapse BOV's for instance can respond so fast that the car is too responsive in corners and upsets the chassis. I end up slowing them down and making them less responsive for road racing and drifting.

They don't leak either.
Friday, June 28, 2013 12:53 PM
You guys also have to remember, a college student building a racecar with a turbo is almost certainly going to be building an FSAE car. The rules for FSAE dictate that the throttle body and intake restrictor must both be placed on the intake side of the turbos compressor. So, assuming a properly sized turbo a throttle transient will never be able to cause the compressor to experience flow conditions that would create compressor surge.

Also the audi lemans cars that dominated were diesels, most diesels dont have a throttle plate. Some have butterfly valves in the intake for emergency shutoff in the case of motor oil fueled runaway combustion though.
Dan DeRosia
Dan DeRosialink
Monday, July 01, 2013 4:29 AM
Hmmm, comment I made went somewhere with whatever switched over on the site. Whichever. I would bring up that, no, the Audi R8 wasn't diesel; as I said it was a twin turbo gasoline engine. Also the old IMSA Nissan GTP ZX-Turbo cars didn't have BOVs either, and given its vintage, I don't know that you can make the claim that they were doing anything revolutionary with ECU strategies, as 1980s vintage ECUs probably didn't have the capability. I'm picking the examples I am because they're cars I've had a chance to look up the skirts of in person, not third-hand or going off pictures on the internet.

I just really would like to see hard data. I've heard anecdotes on lack of BOVs aiding response between shifts, and I know that the way the FD's sequential turbo setup works, it momentarily puts one of the turbos into surge conditions to get it spooled faster... though looking at the FD may not be a good example on how something has a good longevity.
Wednesday, July 03, 2013 7:57 AM
Back when I didn't know better, my GReddy Type R BOV was adjusted way too tight and would surge on lift (gobbling turkeys and all). That turbo (Garrett) lasted about a month of driving and road racing before thrust play allowed oil into the exhaust.

Cutting the spring and adjusting it with a vacuum gauge so that the valve opened at the correct pressure, my replacement turbo has lasted years now.

Despite being set so that the BOV opens under normal idle vacuum, it has never leaked, even when pressure testing the intake tract to 50psi. Unless your diaphragm is leaking air or you've got it plumbed incorrectly, a good BOV should never open under throttle due to simple physics.
Saturday, October 26, 2013 11:11 AM
I ran 20+psi through a TD05/6 frame turbo for 5 years without a blowoff valve. I tore down the turbo several times (without replacing bearings) in that time and never saw any signs of thrust damage. And I live in the mountains, so my turbo works significantly harder (higher pressure ratio) than your turbo does for any given gauge pressure. Maybe MHI turbos are just that much more resilient? They do run factory 360° thrust bearings...

Back in 2008, when I originally took the valve off, it was definitely weird hearing the turbo surge, but based on the research I had done I knew it would be ok and eventually got used to the noise. Every case of a turbo failure that I found that had been attributed to a lack of/failure of blowoff valve was very obviously misdiagnosed. Most were from blowoff valves that had failed partially open (or had their vacuum reference lines removed), resulting a moderate boost leak and damage from overspeed. Some were oiling issues (wrong pressures, low ZDDP oil, etc). Some were FOD. Yet people were very quick to blame off-throttle surge. And they always fixed more than just the off-throttle surge condition, which erroneously confirmed in their mind that it really was the off-throttle surge that caused the failure in the first place.

Bear in mind too that a turbocharger on a car running LeMans or Daytona will see more abuse in those 24 hours than most any street car turbo will see in its lifetime. They see more full boost shifts than any street turbo likely ever will, yet longevity without a blowoff valve was not a concern to them either. Thus, the "they change out the turbo regurlarly" argument against race cars is moot.

The difference in response between shifts on a larger turbo is drastic. It was amazing on a TD06 turbine. Not as noticeable on a TD05h. The reason IMSA, F1 and Indycar did not use them was because they cost literally seconds per lap. On my car, it was the difference between waiting for the turbo to spool again (with a blowoff valve) and immediately lighting up the tires in the next gear if I chose to do so (without a blowoff valve).

Datalogs have shown boost recovery occurs in about half the time without a blowoff valve. The bypass valves on one particular car tested respond based on a pressure differential across the diaphragm created by a special vacuum transfer valve, not vacuum. As a result, they respond instantly as soon as pressure in the manifold starts to fall from the throttle closing. Vacuum is not necessary, only falling manifold pressure. They are also a pull-type valve and cannot leak, as boost holds them shut. This rules out "proper valve response" as a reason why boost might recover more quickly without a blowoff valve.

People seem to forget that a turbocharger is a positive feedback device. The more boost you have, the more boost you make and the faster you make it until the wastegate steps in and says "hang on a minute.." Opening the throttle with 10psi still in the charge pipes supplies a lot more air (and the accompanying fuel energy) through the engine to re-spool the turbo than starting over from atmospheric pressure. 80s race engineers knew this. Datalogs across platforms have confirmed this.

The impeller's deceleration between shifts is based primarily on the loss of drive pressure across the turbine. The energy stored in the charge pipes of a typical system at 30psi and dissipated across the compressor through reversion is miniscule compared to the energy stored in the spinning impeller at full boost. The numbers I got when I crunched them were in the neighborhood of 2-4%. While there is likely some additional deceleration of the impeller from reversion, the energy available from what's left in the charge system after a shift more than compensates for any angular velocity differential between the two setups. Trying to preserve angular velocity on the cold side of the turbo is a losing battle, as it's the loss of energy on the hot side that dictates impeller speed, yet this is what the aftermarket wants us to believe.

I do think it's better to keep the valve on a car with a MAF sensor that's capable of reading airflow in both directions, however. At lower boost it's probably not an issue (I never had an problem on my A4), but it could cause the same symptoms as venting metered air to atmosphere at higher boost levels.
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