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Discussion Starter #1
This is another old write up, but this is more of a discussion piece. At the end I have included a response from an engineer friend of mine (who is way smarter than I). In the end we figured it probably doesn't matter enough for anyone to ever notice, but it's still an interesting topic to discuss over pints.

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 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) Maybe was just easier to assemble/work on, anor possibly less costly that way.


Response from Newman

I still disagree with placing it at the turbo. At least in the theoretical sense.

When you open the BOV, it doesn't just vent the air at the turbo, it also opens the charge pipe and intercooler up to atmosphere (there isn't some magic valve keeping the charge pipe pressurized), so the air in those pipes reverses direction, then when the BOV closes and you begin to recharge the pipe, it has to reverse direction again. This requires more energy than:

If the BOV is near the throttle body, the majority of the air continues to move in the same direction.

However, in the REAL world, it's probably splitting hairs.
 

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Classic Man
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been trying to tell people this for a long time.


they dont listen





"IT NEEDS TO BE NEAR THE THROTTLE BODY OR ELSE THE BOOST WILL BREAK YOUR THROTTLE PLATE!!!" hahaha
 

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Would it be worth trying to make something like the porsche system on a D? Typical BOV placement on the charge pipe after the intercooler close to the TB, then on a Greddy style BOV taking a hose and rerouting it into the intake pipe?
 

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I think the best location is close to the TB pretty much because of all what Newman said.

I think If you have it close to the TB, the extra pressure would vent at that spot but the air would keep its momentum from the turbo and throughout the pipes. After the air is vent this would cause the lower air pressure to be closer to the TB which would help the movement of the high pressure from turbo side to torwards the TB and it would "re-pressurize" the pipes faster. We all know that rule of physics.

If you have it close to the turbo the pressure that rebounds off the plate would need to make its way all the way back past the IC and almost to the turbo loosing all its momentum towards the intake and having to re-pressurize more area and reverse direction.
 

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How do the new Borg Warner snails compair to running the old style BOV? They use the built in bypass, and wastegate as optional upgrades to the EFR turbos. I know a shop owner out here that will run one on his rear wheel drive subie, it looks like it will eat a baby hippo. Supposedly it has almost instant response after a lift/shift situation.

I ask becasue they obviously like the closer to the turbo option.
 

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Do whatever works. Both methods have been proven to work, and work well. At a certain WHP or certain setups might be benficial to do one over the over...but for what we work here it you can really go either way. I chose nearer the TB because that's where the I/C piping had the flange. LOL!
 

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It doesnt need to be anywhere because it doesnt do shit other than eliminate noise.

Also, show me these Porsche race cars with BOVs. Ive seen several turbo Porsche racecars, never seen a BOV on any of them. The same goes for 90% of other, real purpose built racecars.
 

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Porsche race car at Pebble Beach. No not really.

owned, lol Thanks dude.



There no neat red circles.
 

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Classic Man
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It doesnt need to be anywhere because it doesnt do shit other than eliminate noise.

Also, show me these Porsche race cars with BOVs. Ive seen several turbo Porsche racecars, never seen a BOV on any of them. The same goes for 90% of other, real purpose built racecars.
keeping the compressor speed up during throttle closed applications does something!
 

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Classic Man
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Why would the throttle be closed?

in the real world people need to slow down, and then speed back up very quickly.

the throttle doesnt even need to be fully closed to make vac in the IM
 

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Discussion Starter #18 (Edited)
It doesnt need to be anywhere because it doesnt do shit other than eliminate noise.
Porshe developed it to stop the compressor from stalling between shifts.

It doesnt need to be anywhere because it doesnt do
Also, show me these Porsche race cars with BOVs. Ive seen several turbo Porsche racecars, never seen a BOV on any of them. The same goes for 90% of other, real purpose built racecars.
It was used in the 70's and trickled over to production in 79 on the 924. They would have looked like this on the production cars


Anywho, I think in the end Newman and I agreed that mounting it to the cold side intercooler tank made the most sense.
 
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