Willub and company you definitly made a point there im going to have to go with your theory and agree w/ you..

Alot the 9 sec rx7's I know about have custom or race bov's..

 

https://www.rx7club.com/rotary-car-performance-77/bov-really-necessary-use-438481/

 

Maybe Its A External Wastegate?

 

his sentences make perfect sense if you know what he is talking about.

read more about turbocharging...the shock wave will hit the compressor wheel, not turbine.

listen to how fast they are shifting...average joe turbocharged car drivers are not shifting nearly that fast in that manner, therefore turbo life is more crucial then the performance gained.

not one car shown had a WG plumbed into the exhaust...for emission's or sound damping.

 

^^ yeah, that was an after thought that they are shifting so quick it wouldn't even matter.

 

Christian,
The surge isn't actually causing the compressor wheel to spin in the oposite direction, but instead the pressure waves are causing the wheel to slow down. So the turbo won't have to "respool" since it will still be spinning. If you check out the link Mahjik posted you'll see a chart that shows a car equiped with a BOV and one without. I hope this all makes sense and the chart helps a bit Most competition cars do not use any sort of BOV for the performance benefits, as stated many times.

-Dan

 

Compressor surge is hard on the thrust bearing mechanism because the pressure that the compressor is seeing is much greater than the pressure that the turbine is seeing, causing a thrust towards the compressor side.

Many OEM cars actually have good turbo size matching so that everytime the there is low load on the engine or the throttle plate is closed, it does not run the turbo in the surge part of the compressor map.

Running huge turbos like our GT42's on our 13B's is NOT a practical or good match, but it does yield great high end power. That being said, BOV's are very necessary when running large turbos, and that's why BOV's are open at idle.

Not running a BOV won't cause reverse spinning of the compressor wheel. It will slow the wheel down, but not like many people think would happen. The compressor wheel is not designed to spin when you put air reverse through the compressor outlet.

I think it is possible that not running a BOV would increase response, but I don't have any data that proves it one way or not, maybe someone else does. If this really bothers people, I can see what I can dig up.

 

don't know about everyone else but i can't sleep because i'm so distraught over compressor surge

 

Thats because it's not just Auto select. I remeber seeing that GTR before and being labeled as "Powercraft" R34 or "Powershark" I think. As far as that silver R34 goes being "the high power tuned car in Japan!" LMAO @ THAT! 800hp is nothing to them! Look at signals R34 pushing over 1200 to the wheels! We have achieved over 1300 RWHP in supra's over in the states which leaves no doubt in my mind the Japs have gotten to over 1600+ hp in any of Jap's top contenders. I'm pretty sure those wanguners have gotten some rediclious power!

 

hahahaha, me either....I had nightmares lastnight.

 

could they have meant 800hp is tops for that application? i could see 1200hp-1600hp in drag cars but for a road car?

 

I really really don't believe this is true. That sounds EXACTLY like a GReddy Type S / Type R.

 

Err..pretty sure an open BOV at idle would cause a mega-vacuum leak..

BOV's are closed at idle, and only open when your engine switches from boost to vacuum. Thus the reason for a vacuum line running to your BOV from a source of vacuum.

 

Nope. The BOV is actually venting at idle, because it allows air to bypass the turbo for smoother idle (so it doesn't have to draw air past the impeller). The idle fueling maps take this into account.
Right, so think about it: what is your intake tract producing at idle? It's certainly not producing any boost...

 

didn't see one in the engine bay...not that, that would be the end all debate here.

My BOV isn't open at idle...and doesn't open until around 18in-hg

Kento - How exactly is air bypassing the turbo? Are you saying the engine is sucking air from the BOV to feed idle?

 

Sorry, the stock one is what I'm referring to; I don't have any experience with aftermarket ones if that's what you're referring to.

Yes, the engine uses it for partial idle air feed. Check how the stock "air bypass valve" (basically the OE blow-off valve) is hooked up. There's the big hose feeding into the valve from the Y-pipe (after the turbos) and then the exit side into the airbox. The actuator line is hooked up to a nipple on the UIM. When you start boosting, positive pressure in the actuator line shuts the valve. When you let off the throttle and the throttle plates shut, a vacuum is created in the UIM, which opens the BOV. However, there's vacuum created during idle as well, so the valve is open. The stock specs for the BOV are fully open at 9.2 inHg, which is far less vacuum than a good-running stock engine produces at idle.

EDIT: Sorry, made a mistake, the idle fueling maps on the FD don't take the air feed from the air bypass valve into effect, because the throttle plates are shut, and the throttle body controls air feed at idle. I was thinking of another application...

 

^right, I think that is where the confusion was.

some cars will not run correctly if BOV (also called recirc valve) is vented to atmosphere...we all know this is not the case for FD's.

 

Kento, so if it's venting at idle, it's actually allowing the turbine to slow down even more, correct? Which would cause lag (negligible or not, but some lag in the absolute sense) when attempting to make boost, correct? And this is in contrast to no BOV, so there's no venting at idle, so while the wheel slows down, it does so passively, and as such not as much? So response is faster when boost is desired? Or am I way off?

~Ramy

PS: I find it fascinating that something as simple as a BOV is so poorly understood by most on the forum hehe.

 

Most GT-R blow offs are down under the bumper anyway...you wouldn't see it in an engine shot.

Sure sound's like it has one to me.

 

No, the turbo doesn't lose any spool speed from the BOV venting at idle because the exhaust energy is still spinning the exhaust turbine. Depending on the turbo setup, there will be some slight lag when you get back on the throttle, as it must pressurize the intake tract in order to build boost pressure again, but this is why the race vs. street use application must be taken into consideration with regards to turbo life vs. performance needs. And if the turbo setup is properly sized to the engine without requiring astronomical boost psi figures, lag time with a BOV is minimal anyway.

Without any BOV, I'd think the pressure waves that would result from backing off the throttle would play havoc with the compressor side, because a turbo depends on the airflow's centrifugal inertia to build pressure. However, I guess the response time when getting back on the throttle is worth it to those particular racecar applications.

 

Well, never thought my asking the question, "what BOV is it" would go in this direction. But, I'm glad to see that we have stumbled onto some rather interesting discussion! Any others with imput?

 

Hmm...so the BOV is venting the intake tract, and repressurizing THAT is what may cause that lag (in the case of a huge turbo generating lots of boost)?

And what exactly is causing premature turbo life, now that we've established that spinning in reverse isn't true. Is it that it's spinning nonetheless w/o a BOV, or at least spinning faster at idle w/o a BOV? So just more wear and tear?

 

Yes.
In other words, the pressure waves from compressor surge cause a ton of stress on the turbine shaft and associated bearings, especially ones that are superheated and are spinning at 125,000 rpm or thereabouts.

 

I thought the car being referred to was the stillway Rx-7?

 

Uh . . . . . no. He clearly stated he was asking about the GT-R..


And to the BOV comment. I am positive that the BOV is NOT open at idle. Now, the only cars I have experience with this are the 87-91 RX-7 Turbos, and SRT-4's. But...if you put your hand on the BOV at idle, it doesn't change anything. This is of course for aftermarket, or atmospheric-vented stock BOV's...but this whole discussion was around BIG turbos, thus..you wouldn't be using a stock-setup BOV...