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BOV
"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!
Ramitha"
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.
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