Ask Sarah: BOV’s and Accusumps
 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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