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Does CBV = porsche's bypass valve?

hokman

hokman

New member
Joined
Dec 15, 2005
Location
Vancouver, BC
The first porsche turbo has a bypass valve that pre-spin the turbo by recirculating the boosted air to pre-turbo. So the air won't keep hitting the throttle wall and restricting the turbo from spooling.

Does Volvo's CBV = that function? My 240T has no implementation for anti-lag at all, should I install one?
 
I doubt very much that the first Porsche Turbo had a valve that was intended to "pre-spin" the turbo.
The idea of compressor bypass valves is much the same as blowoff valves; get rid of excessive boost when the throttle-body closes, so you don't get waves of "boost" heading back towards the turbo & smacking the carp out of your turbo impeller blades.

And yes, the Volvo CBV performs the same function.

This has absolutely nothing to do with anti-lag; anti-lag is something completely different, and usually involves a special ECU tune mode.
 
yes... anti-lag is a computer map that fuels like mad when you close the throttle and has an air bypass valve or ign cut... or something to get air in the exhaust to cause a burn in the pipe upstream of the turbo. this is why rally cars shoot flames on shifts.

see here... http://en.wikipedia.org/wiki/Anti-lag

you can do it on a street car, I know a guy who has it. You can't run a cat, it uses all kinds of fuel, it makes all kinds of noise, and it's really hard on your turbo... but the flames are cool.

the CBV is basically a BOV that is routed back into the intake stream ahead of the turbo, both because it's quieter that way AND because with a MAF system if you dump your BOV to the atmosphere you overfuel big time and get bangs and flames... which... you know... might be what you want...
 
on a manual shift car a CBV or BOV does minimise lag.
After the boost has been dumped during an upshift the turbo can spin easier, without a CBV or BOV the turbo would be slowed down a lot once the throttle is closed during a shift.

CBV/BOV equiped engine: once you apply the throttle again the CBV/BOV is closed and the turbo ,still spinning free (because of inertia, it is not slowed down by the pressurepeak in the intakechannel pre-throttlevalve) , will start boosting faster. = less lag
NO cbv/bov equiped engine : once you apply the throttle again the turbo has to pick up speed again before it can build up boost = lag

should you install a BOV or CBV? Yes, it makes a gasoline turbo-engine with a manual gearbox more driveable/enjoyable .
It is also better for the turbo seals.
 
I want the BOV or CBV for its ability to prolong turbo life in the event of backing off throttle. But I thought the difference in CBV and BOV is that CBV recirc and BOV doesn't? CBV also recirc at idle. What's the reason for that? Is it to relieve the pressure buildup at idle so the turbine spins freer and can spool up faster?
My goal is to get faster spool up, my 240 is automatic. I know some ppl say BOV is useless on AT, but I still want one for the benefits.
Did any 240T get a CBV stock at all? Manual versions?
 
hokman said:
I want the BOV or CBV for its ability to prolong turbo life in the event of backing off throttle. But I thought the difference in CBV and BOV is that CBV recirc and BOV doesn't? CBV also recirc at idle. What's the reason for that? Is it to relieve the pressure buildup at idle so the turbine spins freer and can spool up faster?
My goal is to get faster spool up, my 240 is automatic. I know some ppl say BOV is useless on AT, but I still want one for the benefits.Did any 240T get a CBV stock at all? Manual versions?
Click to expand...

Its not useless but a CBV is more usefull in the fact that it recirculates the air instead of wasting it to the atmosphere
 
neither will help your spool on an automatic, actually neither help your spool time on any turbo motor. They only help KEEP the turbo spooled when you snap the throttle closed, like if you're shifting. They also keep pressure spikes from jacking your turbo bearings and seals. You won't see any change in your acceleration times on an auto car, and a "reasonably" driven car will last forever without it, especially at stock boost levels. Lots of OEM setups did not have any type of BOV.

if you're cranking up your boost and concerned about turbo longevity, then do it. it's not going to hurt anything.

... and yes, CBV does put hot air back in the system, but it doesn't make your car fart fuel or make shrieking noises in traffic... that's why they did it that way. It's not ideal, but neither is a MAF system with a turbo setup. If what you want is to do it "right" then I send you sir off to the magical land of aftermarket EFI and a speed/density system.
 
There is some serious misconceptions about CBV and BOV's in here, particularly in terms of performance and turbo preservation...

The CBV started getting used originally to keep turbo cars quiet, and smooth out some tuning issues, BOV came about when someone probably popped off a hose from a CBV one day, and thought it sounded way cool.

If your running 20psi+ maybe venting air helps something, but 7psi of pressure travelling back though your system is never going to kill a turbo, sure it makes a nasty sound, but slow spool or blow out seals?, no chance.

I actually read that on an auto car, you could gain performance with no valve, less pressure loss between shifts, if your shifting fast enough on a manual car it's also possible, any type of valve seems more there to ease off pressure when your decelerating, why would you constantly want to dump off heaps of boost while accelerating?.
 
Old picture, but:
cbv.jpg

Porsche part number 993.110.337.50 called an abschaltventil according to the box. Works fine for me, I hate the BOV sound.
 
sounds like CBV is the way to go, but putting it on a 240 means getting special boost hoses all around which is a pain in the arse. BOV will be easy weld on but seems to be useless/noisy. And if nothing is fine, I might as well not bother then.
 
neptune said:
Old picture, but:
cbv.jpg

Porsche part number 993.110.337.50 called an abschaltventil according to the box. Works fine for me, I hate the BOV sound.
Click to expand...

This is porshce's version of CBV? looks like same concept as on 760's, but maybe better because it's by porsche :-P Did you weld a bung on the 90* pipe just after turbo? The thin hose is T'ed in to manifold vacuum right? Did this help spool up from idle at all or just at gear changes like people say?

This is excerpt from a porsche 911 history book:
"The KKK turbo system uses a relief valve from the pressure plenum chamber. Under closed or nearly-closed throttle conditions when the intake air stream slows and the compressor is in danger of stalling against stationary column of air, this valve vents to atmosphere. As a result, the compressor wheel is always spinning and when the throttle is opened, the time lag before it regains full operating revolutions (90000rpm) is reduced."

Porsche 930 turbo made 260-350hp in street form on k-jet/CIS, I can't see a reason to swap to EFI from a performance point of view.
 
hokman said:
sounds like CBV is the way to go, but putting it on a 240 means getting special boost hoses all around which is a pain in the arse. BOV will be easy weld on but seems to be useless/noisy. And if nothing is fine, I might as well not bother then.
Click to expand...

EHH Once again, bovs are not USELESS, they help with a little thing we like to call compressor surge
 
hokman said:
Under closed or nearly-closed throttle conditions when the intake air stream slows and the compressor is in danger of stalling against stationary column of air, this valve vents to atmosphere. As a result, the compressor wheel is always spinning and when the throttle is opened, the time lag before it regains full operating revolutions (90000rpm) is reduced."
Click to expand...

From modern turbo tech I have read, most of that is a little BS at best.

But yeah a CBV does serve a purpose, it's there to improve driveability, smooth out on and off throttle power delivery, stop surging etc.
 
hokman said:
This is porshce's version of CBV? looks like same concept as on 760's, but maybe better because it's by porsche :-P Did you weld a bung on the 90* pipe just after turbo? The thin hose is T'ed in to manifold vacuum right? Did this help spool up from idle at all or just at gear changes like people say?
Click to expand...

Its a porsche part yes. I went with it after reading Anthony Hyde's page years ago. I could just have easily used the SAAB one, they're the same externally. Or the forge version in a couple threads recently. I was originally going to use a Pierburg unit off a 740T, but the plastic Bosch valves were much much lighter, so I went with the Porsche version.
I had to have a bung welded on to the cast aluminum pipe between the turbo and the airbox. You can see it pretty well in that picture. After the turbo I used the pipe from a 740 with the external Pierburg CBV and garret turbo.
Yes, the hose goes directly to the manifold after the throttle body. I have since removed that little grey/red delay valve, didn't see it making a difference or understand why it was there.
Its been so long since I did this I don't even remember if it made a difference or not.
 
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