I was reading the book Maximum Boost, and in there I got the impression that you only needed a wastegate or to vent the compressed air in the intake(BOV), in order to control the boost pressure. It said that the wastegate was the best route to go, so my question is do you need a BOV too or is it just like a backup for safety. Sorry if i sound like an idiot, I'm just confused here. Well thanks in advance, Shawn.

97 Eagle Talon Esi}( }

The wastegate is on the hot side of the turbo, or on the exhaust manifold(external). It allows the exhaust to vent, controling the boost pressure on the compressor side of the turbo.
No, you do not 'need' a bov, but I don't know anyone who is not using one.
Read the book carfully, you will get it. :)

You actually need both.

After you let off the gas to shift (and your turbo is spooled up), obviously your turbo is still spinning very fast. That means it's still forcing air through your intake piping. Since you let off the gas to shift, a throttle plate closed in the opening in your throttle body, not allowing hardly any air past it (just enough air to idle). All this air that the turbo has moved since you let off the gas has no where to go. It slams into the closed throttle plate, and sends a shockwave of air back towards the turbochargers compressor wheel. This brute shock to the compressor wheel over and over again can cause premature failure of the compressor wheel (breaking off fins and such). Also, this air keeps piling up inside your intake pipe, making more and more pressure inside. This pressure is working against your turbocharger, rapidly slowing it down. Once you shift gears, and resume acceleration, you want the turbo to be spinning as fast as possible to build up boost. With a BOV, between shifts and once the pressure has built up in your intake pipe, it forces open the blow off valve, and releases pressure above a predetermined level, and effectively lets the turbocharger "freewheel" (continue spinning at a high rate of speed without creating excess pressure in the intake system). This keeps the turbo spooled up and ready to make boost after your shift.

The Blow Off Valve has nothing to do with controlling how much pressure you want your engine to intake.

The wastegate serves that purpose. In a similiar design theory, also working by means of air pressure, the wastegate (at a predetermined pressure level) opens up to allow your EXHAUST gasses to go around the turbine wheel. This happens to keep the turbo from spinning any faster because you've reached your desired level of boost, and you don't want any more. If there was no wastegate, your turbo would continue building up more and more boost. Not really a good idea.

The wastegate is on the exhaust side of the turbo, and is your sole means of controlling boost. The wastegate itself is only a small flap coving a hole in the turbine housing. It is the actuator that determines when that flap is allowed to open (at a given boost pressure).

Hope that helps. I also reccomend taking a look at the "How turbochargers work" section on www.howstuffworks.com



1996 Eclipse RS / 5-Speed / Body Components by Aerogear / Coated in Magnetic Red Metallic v2 - More at http://home.insightbb.com/~mgalyan/
Mark Galyan - Greenwood, Indiana

I've never read the book, but they were probably referring to a pop off valve and not a bov. A pop off valve does vent off copressed air from the charge pipe, but it's used as a back up device in conjunction with a wastegate. If your boost spikes for some reason the excess boost will vent off through the pop off valve. I don't know of anyone that is using one, but it wouldn't be a bad idea; all it takes is a tear in the vaccume line going to your wastegat to destroy your engine.

That does indeed sound like a good idea... I figured a device such as that would exist somewhere... you could actually make one if you wanted... something like a huge ball & spring type boost controller coming off your intake pipes.


1996 Eclipse RS / 5-Speed / Body Components by Aerogear / Coated in Magnetic Red Metallic v2 - More at http://home.insightbb.com/~mgalyan/
Mark Galyan - Greenwood, Indiana

I think that HKS makes one, but it's pretty expensive for what it is. It shouldn't be to hard to make. You would just have to make sure that the tension on the spring is just a little bit higher than your max boost.