The BOV and you... a love story. er write up

NYSpeed Automotive
1

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.

And that my friends is the BOV and you.

2

Sounds good to me. I like your close to the turbo reasoning. Makes sense.

3

wat about knowing which BOV is right for you?

4

That depends on what type of air metering your vehicle is set up for. MAF based (draw through) systems need a recirulating BOV/BV where as a MAP based can use a vented BOV/BV. Obviously spring pressure and whatnot are needed to match your boost pressure so you won’t experience leaking (too soft a spring) or compressor surge (too strong a spring).

Let’s keep specific products out of this thread so it won’t turn into a debate and then subsequent drama. Theory only, please.

5

cool thanks, u got it

6

I will add drama. I fucking hate people who put BOVs on stock cars with recirc valves already in place for the sole purpose of the sound. FUCK YOU ASSHOLES. BOV=aluminum wing on mostly stock evo/VW/srt4/STi.

7

ahh yes, the BOV i don’t have…and me. sometime the day will come though, someday.

8

Good work mister daze. :slight_smile: I’m with you on the close to the turbo tip.

I was just re-reading this passage in one of my tuning books last night. KKK’s turbo for the Porsche 924 turbo in 1979 had this pop off valve and I bet all the ricers of yore went crazy. Porsche holds the patent but I’m guessing they aren’t seeing any more from it these days. :slight_smile:

As far as choosing a BOV goes, there are a ton of crappy ones out there. You should tune BOV spring pressure based on the amount of vacuum your car pulls. Only poorly designed valves such as the Greddy Type-S which leaks and/or opens during positive pressure require using higher spring pressures causing surge. A good BOV such as the Tial unit never opens under positive manifold pressure. It should only open when a certain amount of vacuum is reached as the throttle plate closes.

-Mike

9

:smash2:

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I can agree w you on BOV placement but as far as boost refernce. If this is for boost control it makes more sense for it to be near or on the turbo especially in a race prepped car as these are usually dialed in pretty well. The turbo is at it’s most efficient at a certain boost level. You are going to have pressure drop across the piping and innercooler so to keep the turbo at the efficient boost level you would need your reference at the turbo.

11

Very good read man. :tup:

12

having the BOV near the compressor makes sense. perhaps this is why I get a bit of compressor surge (one chirp) on my turboSHO. I have the BOV after the intercooler. That said the car has over 40K on the turbo and there is no shaft play or excessive oil leakage as of yet.

13

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.

14

bump of the year?

and in reference to splitting hairs, I doubt most cars arent tuned to enough to matter where the BOV is, convienence is probably the driving factor

15

This thread got me thinking about my car. If the BOV was on the other side of my intercooler and eliminated the chirp it could in theory help my spool time if I have to let off the gas for some reason durring a race. If I had read this thread when I was building my car in 06 I might have changed the design slightly. In theory that compressor surge is not to easy on the turbo.

16

sawzall and a few couplers could let you try it out…

17

before the intercooler or after is pretty much not an issue, both places work well and neither hold a performance advantage, if you have a maf car obviously you will want it somewhere close to the intake , besides that out of all the theories ive seen and read no one could prove any adverse affect either way

18

putting the bov just after the turbo makes it louder than up by the throttle plate. its the proper ricer position lol.

Brian

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What causes BOV flutter/chatter? Is this normal or avoidable? I need to preload the spring a certain amount to keep the valve closed at 12psi, but when I shift around 2psi, the vacuum has a tough time overcoming this spring force.

FYI, its a Turboxs type-H. I don’t want to debate the brand, but I’m aware that there are different designs and this might affect an answer.

20

i have the same bov with the same issue, i considered trying to narrow the vacume line to it thinking its just to sensative. but have not tried it .