EFI guys

[jbrown]

What are you guys doing for the BOV.. I want to put a pipe and filter right on the turbo.So that knocks out the factory spot for the BOV. I know there is somewhere else I can plumb in the BOV. Where should I put it.

Thanks John

[quattrorunner]

On the inter cooler?

[dos531]

The bypass valve is always mounted on the pressurized side of the turbo. You're thinking of the 964 turbo style recirc valve which looks like its blowing out of the intake pipe. In reality the air is traveling from the intercooler to the intake, so with the efi running speed density you can mount an atmospheric bov onto the intercooler, or pretty much any other pressurized source. Intercooler is easiest since the plumbing is already there.

[xbmwguy]

I personally like to mount them as close to throttle body as possible therefore keeping as much air in the system between shifts

[Tippy]

Originally Posted by xbmwguy

I personally like to mount them as close to throttle body as possible therefore keeping as much air in the system between shifts

That's a good idea.

[jbrown]

Thanks guys

[Speedy Squirrel]

Originally Posted by Tippy

That's a good idea.

That's a bad idea. The goal of the BOV is to get the compressor outlet pipe pressure down to atmospheric fast. You want to unload the compressor wheel as fast as possible so that it doesn't slow down.

The Borg use an integrated BOV right on the compressor cover. No pipes at all.




I stuck with the classic K27 for awhile and fab'ed a "faux" air meter housing to use all the stock stuff. Very stealth.

[Tippy]

The more I thought about this, the more I think it doesn't matter. Pressure is equal everywhere.

[Speedy Squirrel]

I think Mr. Bernoulli would disagree.

[Tippy]

Thinking out loud, when the BOV opens, pressure is reduced throughout from compressor wheel to throttle blade. When you go back to WOT, you are still having to fill the same volume of air back up if you bled pressure at the TB or volute, no?

[MikeD930]

I can see where Speedy is coming from...wherever the pressure is released 1st sees pressure drop instantly whereas pressure drops later further away from source of released pressure. But over time pressure equalizes. How much time I don't know (1ms or 100ms?) perhaps enough to make a difference in racing environment or even reliability of a turbo over a long haul.

That's true that same amount of air is lost regardless of where the pressure is released.

[boosted79]

FWIW the 935 twin turbos had their BOV's on the plenums on top of the ITB's after a few feet of plumbing and the IC's. Porsche put the BOV on the 930's right before the throttle plate. I suspect they had a reason for it. The area just before the throttle plate is the first to see the pressure spike on a throttle chop so that's where you want to put the BOV.

[Spenny_b]

Mine's on the cold pipe between IC and throttle body...venting to atmos at the moment, but not sure how annoying this will be when driving; may well switch to a recirculating type and dump into the inlet tract.

[GJF]

The whole point of the valve, is to keep the turbo spinning in between shifts so when throttle is re applied boost is readily available. Thus, the optimal location is as close to the compressor as possible. Not only does this help on boost recovery between shifts, it also helps protect the turbos thrust bearings. In all reality the valve purpose IS to aide in adding as much longevity to the turbo.

[krasuskyp]

SpennyB... I don't hear my TiAL vented to ATM at all, and it's right behind my head just after the firewall pointing down and to the left/backish @ 45*

prolly depends a lot on the valve too

[boosted79]

"Thus, the optimal location is as close to the compressor as possible.

No. You want it as close to the throttle as possible. You have a column of air flowing across the throttle plate when it's open. The throttle plate goes shut on a shift and BAM, that column of air slams into the throttle plate and has no where to go. If you put the BOV back at the compressor discharge that column of air between the BOV and throttle plate wants to reverse direction because you are lowering the pressure upstream of where the highest pressure point is, which is right at the throttle plate when it slams shut. With the BOV as close to the throttle plate as possible you are lowering the pressure right where the highest pressure point is so you keep the air column moving in the right direction.The purpose of the BOV is to prevent compressor surge on throttle closure and either location will accomplish that. But for optimal intake tract filling after a shift it needs to be at the throttle.

[GJF]

The intention and design of a Compressor by pass valve by design it's whole purpose is to protect the turbos thrust bearing (due to surge) and aide in boost recovery. the only way that can happen is to have it mounted as close to the turbo as possible. You can mount it anywhere in the charge line closer to the throttle body but mounting it at the throttle body removes 90% of thrust protection.. At the same time there is always a restriction of air mass due to the intercooler. Venting at the TB makes it harder to fill the area back up between shifts from the compressor to the TB. If the BOV were mounted at the turbo then the turbo would free spin without back pressure because it vents at the turbo and not through the plumbing (charge pipe, and the IC). When the TB plate slams shut in between shifts it acts as a storage for the air mass that was flowing before shifting. By the time the throttle is reapplied the compressor only has to add air mass to match what is left in the charge tube at the TB to recover. The efficiency of the IC is the limiting factor to the surge, placing the BOV at the TB with a poor flowing IC defeats the purpose.

[Spenny_b]

I know! Why not fit 2 x BOV's? One either side of the IC, then everybody's right

[grabs popcorn]

[/jesting]

[boosted79]

"but mounting it at the throttle body removes 90% of thrust protection."

C'mon, if that was true 930's would be infamous for destroying turbos due to surge. The stock 930 BOV is at the TB. Porsche put the BOVs on the intake plenums above the ITB's on the 934 Turbo RSR and the 935. On the 930/10 they were on the plumbing between the compressor and the intake plenum. On the 930/30 they were right at the inlet to the plenums. The fact is that either location will prevent compressor surge. If the BOV is at the compressor a pressure wave is set up right at the throttle plate when it shuts and it is traveling from the TB backwards to the BOV. The same thing happens when an intake valve shuts, that's why there is an optimum intake runner length for a specific rpm range. I suspect Porsche did testing with both locations and chose the TB location for a reason. But this debate has been going on for decades and will probably continue.

[Speedy Squirrel]

The below statement is correct. In reality any place on the pipe will perform this function, but the compressor end is the best place because it will benefit from the pressure release sooner.

The 935 used mechanically actuated pressure release valves, so the linkage was a factor. There were no magic calculations done to capture pressure waves because they are dissipated in the plenums above the ITB's in any event. Packaging often comes into play, even in racing. That certainly affected the 930 arrangement as well.

Originally Posted by GJF

The whole point of the valve, is to keep the turbo spinning in between shifts so when throttle is re applied boost is readily available. Thus, the optimal location is as close to the compressor as possible. Not only does this help on boost recovery between shifts, it also helps protect the turbos thrust bearings. In all reality the valve purpose IS to aide in adding as much longevity to the turbo.