HomemadeTurbo - DIY Turbo Forum - where to put the BOV?

Let’s take a quick look at the BOV (Blow Off Valve) and what it does for you and your car.

First, a quick history lesson.
Q. Where did the BOV come from, who could possibly have made something that sounds so cool?
A. Porsche, Duh. Developed for there race cars (early 80's i believe) it was patented as a Pop Off Valve. You may still hear some old timers refer to it as such.

There, now you have learned something today, and can wow your drunken friends at the bar.

Ok, let’s take a look at what this little gizmo does.
The BOV releases the pressurized air from the intake pipes when you lift your foot off the throttle; either to shift, or just to hear the cool sound.

Now, why do we need to do this? To taunt others in to racing? Just because it sounds cool? Possibly, who knows with those Germans, those may have been underlying reasons. Actually the intended purpose was actually for performance.

When you lift your foot the throttle plate closes, and that air bounces off the throttle plate and makes it way back towards the turbo. When that air gets back to the turbo it will try to spin it backwards (compressor surge).
Now when you reopen the throttle the turbo has been slowed, or even stalled, and needs to be reaccelerated. With a BOV in place, this air would never make its way back to the turbo, and the turbo would be able to continue spinning freely. Now when you get back on the throttle your turbo is still spinning from before, and will spool more quickly.

Lets take a look at BOV placement.
Some will argue that it should be near the throttle body, others say closer to the turbo.
Well, lets look where Porsche put it on their race cars.
Porsche made the BOV part of the compressor housing, and redirected the compressed air to the turbo inlet. Pretty slick, huh?
I am from the close to the turbo camp, and i will explain why. 1. The air doesn’t just bounce back off of the throttle plate, there are sharp bends in the pipe, and a giant obstruction called an intercooler in the way. 2. By placing it near the turbo you vent only the air that has made it back to the turbo, leaving the rest of the pipe with some pressurized air still in it.
3. Having the BOV after the intercooler vents air the ic has already cooled. Kind of a waste, dont ya think?

Placing the BOV near the throttle body seems to be out of ease. Just like the boost pressure reference nipple being on the turbo housing. (We all know pressure in the housing is not the pressure in the manifold) It was just easier, and less costly that way.

Now i am sure you have been reading this and saying to yourself "Hey, i thought the BOV was to keep the turbo from getting surge and damaging it." It does do that, but that was not its original intent.

And that my friends is the BOV and you.

write up from :zerodaze at civic.indybbs.net

hope this helps

matt