BOVs
 

Now, I don't know much about turbocharged cars or turbochargers in general.. But I was thinking the other day..

What is the point of a blow off valve? Wouldn't you want the air to stay in the turbo, so it doesn't have to spool up again when you shift?

But then, I suppose, what happens when you're not shifting but rather taking your foot off the gas to slow down.. I guess I just answered my own question?

Oh well.. maybe someone can explain it to me in detail.

As I understand it, it's all about the backfire - you run the risk of cracking (exploding) you airbox, the valve takes care of that failure mode.

A blow off valve essentially controls the amount of boost pressure created from the turbo unit.

Assume that a turbo can produce 30lbs. of pressure. However you want to be able to control how much of that pressure goes into the engine. The Pop off valve bleeds off pressure until it reaches a certian amount and closes. It's basically a relief valve that works the same way as a fuel pressure regulator.

Are you guys talking about a CIS pop-off valve or a turbo blow-off valve? The pop-off is for NA CIS airbox protection in the event of a backfire. The blow-off valve is to vent boost from the intake when you quickly let off the throttle. This creates a backpressure wave which can damage a turbo, and create lag the next time you get on it. BOV's do not control boost pressure or spikes in boost pressure - the wastegate does that.

what Tony said. That backpressure wave will cause a turbo spinning at tens of thousands of RPM to suddenly drop a LOT, and will shorten the life of the bearing.

This sounds like the wastegate.

Clowd,

It helps to have some background with turbo's to understand why they work it this way.

Turbo lag is something that the earlier systems had, where the turbo would spin down a bit inbetween shifts. Lower RPM means lower boost and you want all the boost you can get for more power. After shifting to the next higher gear you would then at times have a slight wait for the turbo compressor to spool up and be able to provide boost.

Enter the pop-off valve. It opens when a pre-set limit is reached and dumps excess pressure out of the system, saving the airbox, pistons, rods etc from overboost and damage. It ALSO keeps the turbo spinning at a higher RPM which then means that its ready to provide boost faster with each shift.

You are correct that you are dumping boost pressure overboard when it hits the max limit pressure but aside from protecting the engine you are keeping the turbocharger ready at a faster speed for the next gear, which means that the boost (read power!) comes on faster.

Hope this helps!

JoeA