faithless

Why are some cars equipped with a valve that "dumps" throttle body pressure and others (951) that recirculate it? What is the advantage (or disadvantage) of "dumping" v. recirculating?

I think the Audi A6 2.7 T has an atmospheric BOV and the Dodge Neon SRT as well. I know the 1.8 T Jetta doesn't have an atmospheric BOV, or at least I never heard it. Does it depend on the size of the turbo?

NZ951

There are maybe 10 distinct threads on this issue already...

faithless

I'm not asking what kind or how. I did a search; where are the threads?

emwporsche

Basically they say that recirculating is better because the air is reused (as oppossed to being lost when dumped) and I have also heard claims of it helping against rich spikes when you shift (that might be entirely hear-say)

awilson40

I did some data logging with my W/B with the stock bypass valve VS " dump to atmosphere"
valve and it was very interesting. TONS of oscillations with the VAF output when using the dump valve that is properly adjusted. Perhaps this wouldnt be an issue with a MAF.
The logs are at home, if anyone is interested i can post some screenshots.

RKD in OKC

Just a waste to throw away that metered air.

Seems to me that since the function of the BOV is to protect the Turbo from the pressure wave of closing the throttle, having boost bleed back to the turbo inlet would be just that much more protection. It seems it would also keep the available boost just a bit higher and the turbo spinning a bit more for when the throttle re-opens.

Or how about this one...
Bleeding boost pressure back to the compressor inlet when closing the throttle unloads the compressor side of the turbo so the turbo keeps spun up a bit even though there is reduced exhaust flow.

Is this an essay question or multiple choice?

faithless

There has to be some reason why some car manufaturers use them. Do you think that maybe the pressure is lower inside the engine bay that it would actually serve more efficiently to dump the air than force it back through the turbo?

I just have this feeling but I can't put it into words (lack of knowledge of physics), that the recirculating effect is not as efficient. For example, if the throttle was left closed and the car was coasting down a hill, I would imagine that the turbo would slow down dramatically, simply because the air inside has nowhere to go if its just being recirculated by the inertia of the impeller building pressure. On the other hand, if the system went from a high pressure point of the car such as where the intake should be, and dumped to a low pressure point of the car, the air flow should might maintain the revolutions of the impeller similarly to the recirculating setup, if not more efficiently because there is no restriction in air flow. Is this a resonable assumption? (ignoring all issues with a:f)

emwporsche

?

as I understand it your not pushing it back through the turbo but re-entering it into the intake side

if I had a choice I would go recurculating just because it's suppossed to keep a better a/f and your not losing metered air.

"There has to be some reason why some car manufaturers use them"
The grand nationals didn't even have BOV's, and then there is our amazing venturi system.

Andrew Wojteczko

My understanding is that on a mass air flow system, recirculating is best because the mass flow sensor has accounted for the volume of air that has entered the system, when you dump it, the car will run rich for a short period of time.

However, on a map system, it is always monitoring the pressure in the intake manifold, and will compensate regardless of what you do.

The idea of prespooling the turbo with the dump charge seems feasible, and that is why even with a map system (TEC3R), we still run a return on our racecar.

Andrew

Pavlo

If you search google you will find a lot of answers and threads about this.
From what I found, recirculating is better. The air is cooler, as it already went through an inter-cooler, when you get back on the throttle there is boost right away, and as Andrew Wojteczko said, maf thinks there is a X amount of air in the intake and will give equal amount of fuel.
One of my friends also said that Atmospheric BOV gets annoying (the pssshhhhh sound on shift).

Just a quick question: Why is MAF mounted before the turbo, not the intake manifold? Wouldn't that be more efficient, because if you have a leak, or say damaged the front, you will still run pretty good. In situation stated before but with MAF mounnted at the traditional place, a leak will cause the car run very rich and make in un-drivable (is that a word? ).
Sorry for stealing the thread for a bit.

TurboTommy

My take on it is that I wouldn't even know why you would use the dump version.
Like others have said, here, all the advantages come from the recirculation type.

The ricers like the dumping 'cause it makes that sound.
Personally, I think it sounds stupid.

Tomas L

A BOV will give you slightly lower air temp since it uses fresh air, not air that's been heated by the turbo (even if it has passed the IC the temp will be higher than ambient).
The pumping resistance may also be slightly lower with a BOV, keeping the turbo spinning easier.
I'm quite sure these differences are small and that the main reason people go for a BOV over a recirculating valve is the sound.
We who run air measuring systems (AFM or MAF) don't really have a choice since we can't vent out metered air.

Tomas

faithless

Tomas knows what im talking about. I didn't even think about the turbo reheating effect, interesting. I like the sound.

So are blow-off valves also recirculating valves, or are they two separate things?

Eyal 951

I'm not aware of any turbo cars that vent to the atmosphere. I'm nearly sure audi wouldn't do this, and the SRT-4? well, they could have done anything to that car and it wouldnt surprise me.
~Eyal

faithless

I've been slowly finding new sites. Its possible that the Audi A6 I heard passing by me was modified, and my friends SRT-4 as well. But I'm not crazy, I positively heard the sound.

I just read about BOV's that partially vent to atmosphere (enough to create a PFFFSH) but deliver most of the air back to the turbo inlet. I suppose one could modify their BOV to do that, as long as there is some sort of rubber tubing delivering the air back to the turbo inlet. I also read that sometimes MAF cars won't have major a:f issues, it depends heavily on how well tuned the car is. If the car is running really rich already, it could have problems.