Hows your CBV (compressor bypass valve) ?
01-19-2002, 03:46 PM
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Hows your CBV (compressor bypass valve) ?
I have been filling my head with info these last few weeks from the many info boards and found this problem on my car. The CBV (compressor bypass valve, it bleeds back pressure back to the pre-turbo intake so as not to let the turbo spin against the back-pressure of the closed throttle) There is a great report on the valve located at: www.rsphysse.anu.edu.au/~amh110/Technical_pages/blow_off_valve.htm
Like I said, i tested mine and found it does nothing more than leak vacum
so I opted for a after-market unit. I'll post the results
after the install. Here is the unit I ordered: www.stratmosphere.com/forge_valves.htm
Kevin
Like I said, i tested mine and found it does nothing more than leak vacum
so I opted for a after-market unit. I'll post the resultsafter the install. Here is the unit I ordered: www.stratmosphere.com/forge_valves.htm
Kevin
01-19-2002, 06:19 PM
#2
Drifting
When I bought my car, the stock one was dead. I replaced it with the 993 part which is much more reliable, but is still not very optimal for performance (it's designed more for reduced sound than all-out performance). I, like you, also purchased the Forge unit and will also report back the results.
In the 944T world, it is common knowledge that the stock unit is junk and should be replaced right away with at least the 993 part, if not aftermarket, I don't know why in the 911T world, they haven't caught onto it yet
Farzaan
In the 944T world, it is common knowledge that the stock unit is junk and should be replaced right away with at least the 993 part, if not aftermarket, I don't know why in the 911T world, they haven't caught onto it yet
Farzaan
01-23-2002, 07:09 PM
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To check the valve I just pulled off the 2 hose clamps off, removed the valve and did a vacuum test on it and found it did not open, just leaked. 5-10 min job.
Kevin
Kevin
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01-30-2002, 11:40 AM
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Guys
Thought that I would share some info on this problem , we ( Forge) manufacture many valves for this type of application , the 007P valve is a direct replacement for the OEM Bosch valve found on the Porsche .
The Bosch is prone to failure on most applications, Porsche , Audi & VW , Cosworth , Lancia etc.
We are represented in the USA by <a href="http://www.stratmosphere.com" target="_blank">www.stratmosphere.com</a> , give these guys a call and they will be pleased to help
Thought that I would share some info on this problem , we ( Forge) manufacture many valves for this type of application , the 007P valve is a direct replacement for the OEM Bosch valve found on the Porsche .
The Bosch is prone to failure on most applications, Porsche , Audi & VW , Cosworth , Lancia etc.
We are represented in the USA by <a href="http://www.stratmosphere.com" target="_blank">www.stratmosphere.com</a> , give these guys a call and they will be pleased to help
01-30-2002, 01:57 PM
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What would be the symptoms (other than those described by unplugging hoses etc) of a valve that needs replacing? What would you notice during driving?
I know that when I'm accelerating hard, the power comes off quite violently as I put the clutch in... Probably nothing to do with this, right?
I know that when I'm accelerating hard, the power comes off quite violently as I put the clutch in... Probably nothing to do with this, right?
01-30-2002, 05:58 PM
#10
Drifting
[quote]Originally posted by greg1234:
<strong>What would be the symptoms (other than those described by unplugging hoses etc) of a valve that needs replacing? What would you notice during driving?
I know that when I'm accelerating hard, the power comes off quite violently as I put the clutch in... Probably nothing to do with this, right?</strong><hr></blockquote>
The violent "jerk" is due to the K-Jetronic and not the compressor bypass valve (aka: air cut off valve, blow off valve). Apparently it has something to do with the flapper on the air meter. There is no known cure to it besides going to a motronic system. Once you get used to the car, you can avoid it, but it does make for some dramatic moments while you still get used to it.
To tell if your CBV is faulty, there are some hard tests you can do:
1. Blow through the valve. If air gets through the valve, it's defective.
2. Blow into the vacuum fitting and/or hook up a vacuum pump. If you can blow through, the valve is faulty, if you can't pull vacuum, it's also faulty.
3. Check to see what vacuum level the valve opens. If by 14" of vacuum the valve has not opened, it's stuck shut.
In terms of soft tests, when you change gears, feel how the boost comes on, if the car falls on its face for a moment after a quick shift, the CBV is not working correctly. With a good CBV like the <a href="http://www.stratmosphere.com/forge_valves.htm" target="_blank">Forge</a> unit, you can actually hold boost during quick shifts so that when you engage the clutch in the next gear, you will have 5-7psi of residual boost left in the intake.
On cars with more open intakes, you can hear something that sounds like "cha-cha-chaaa" when you lift off the throttle, that is the noise of the compressed air trying to get out between the turbocharger and the throttle plate. It can also cause compressor surge and the destruction of the compressor wheel.
In testing I've done on my 911, the factory valve allowed boost pressure to spike to 15psi on lift off. With the 993 valve, it does not spike and only rises to about 11psi on lift off in certain situations. With the Forge valve, I get no pressure spike at all, the boost just falls during lift off, that's how it should be.
On my racecar, we knocked 0.75 seconds off the 1/4 mile time by venting the excellent CBV to atmosphere rather than back into the intake. If reducing the backpressure on a good valve made such a dramatic difference, imagine what you're losing with a faulty valve!
Hope this helps,
Farzaan
<strong>What would be the symptoms (other than those described by unplugging hoses etc) of a valve that needs replacing? What would you notice during driving?
I know that when I'm accelerating hard, the power comes off quite violently as I put the clutch in... Probably nothing to do with this, right?</strong><hr></blockquote>
The violent "jerk" is due to the K-Jetronic and not the compressor bypass valve (aka: air cut off valve, blow off valve). Apparently it has something to do with the flapper on the air meter. There is no known cure to it besides going to a motronic system. Once you get used to the car, you can avoid it, but it does make for some dramatic moments while you still get used to it.
To tell if your CBV is faulty, there are some hard tests you can do:
1. Blow through the valve. If air gets through the valve, it's defective.
2. Blow into the vacuum fitting and/or hook up a vacuum pump. If you can blow through, the valve is faulty, if you can't pull vacuum, it's also faulty.
3. Check to see what vacuum level the valve opens. If by 14" of vacuum the valve has not opened, it's stuck shut.
In terms of soft tests, when you change gears, feel how the boost comes on, if the car falls on its face for a moment after a quick shift, the CBV is not working correctly. With a good CBV like the <a href="http://www.stratmosphere.com/forge_valves.htm" target="_blank">Forge</a> unit, you can actually hold boost during quick shifts so that when you engage the clutch in the next gear, you will have 5-7psi of residual boost left in the intake.
On cars with more open intakes, you can hear something that sounds like "cha-cha-chaaa" when you lift off the throttle, that is the noise of the compressed air trying to get out between the turbocharger and the throttle plate. It can also cause compressor surge and the destruction of the compressor wheel.
In testing I've done on my 911, the factory valve allowed boost pressure to spike to 15psi on lift off. With the 993 valve, it does not spike and only rises to about 11psi on lift off in certain situations. With the Forge valve, I get no pressure spike at all, the boost just falls during lift off, that's how it should be.
On my racecar, we knocked 0.75 seconds off the 1/4 mile time by venting the excellent CBV to atmosphere rather than back into the intake. If reducing the backpressure on a good valve made such a dramatic difference, imagine what you're losing with a faulty valve!
Hope this helps,
Farzaan
01-30-2002, 06:45 PM
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[quote]Originally posted by fc-racer:
<strong>
With a good CBV like the <a href="http://www.stratmosphere.com/forge_valves.htm" target="_blank">Forge</a> unit</strong><hr></blockquote>
Hi Farzaan. I think I read that this valve (for the 1.8T motor) will also work in the 944 Turbo. Can you verify that the model 006 valve can be used to replace the 993 valve. Thanks.
<strong>
With a good CBV like the <a href="http://www.stratmosphere.com/forge_valves.htm" target="_blank">Forge</a> unit</strong><hr></blockquote>
Hi Farzaan. I think I read that this valve (for the 1.8T motor) will also work in the 944 Turbo. Can you verify that the model 006 valve can be used to replace the 993 valve. Thanks.
01-30-2002, 09:17 PM
#12
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[quote]Originally posted by Tom Pultz:
<strong>
Hi Farzaan. I think I read that this valve (for the 1.8T motor) will also work in the 944 Turbo. Can you verify that the model 006 valve can be used to replace the 993 valve. Thanks.</strong><hr></blockquote>
The valve on the C2T and 993T is the same as the 944T so I would assume that the 007P would also work on your car as it did on mine. The unit itself is slightly bigger (about 5% perhaps), but it fits exactly the same as stock.
I really like this unit, it's fully rebuildable, it'll be the last CBV you buy. Some comments on the board about the Forge units being unreliable are false, they were made by one individual. In conversations with some friends of mine that work on Subaru rally cars, they say the Forge units have never let them down and those guys pound the sh*t out of their cars, running anywhere from 15psi all the way up to 29psi depending on their task at hand.
Farzaan
<strong>
Hi Farzaan. I think I read that this valve (for the 1.8T motor) will also work in the 944 Turbo. Can you verify that the model 006 valve can be used to replace the 993 valve. Thanks.</strong><hr></blockquote>
The valve on the C2T and 993T is the same as the 944T so I would assume that the 007P would also work on your car as it did on mine. The unit itself is slightly bigger (about 5% perhaps), but it fits exactly the same as stock.
I really like this unit, it's fully rebuildable, it'll be the last CBV you buy. Some comments on the board about the Forge units being unreliable are false, they were made by one individual. In conversations with some friends of mine that work on Subaru rally cars, they say the Forge units have never let them down and those guys pound the sh*t out of their cars, running anywhere from 15psi all the way up to 29psi depending on their task at hand.
Farzaan
02-26-2002, 05:24 PM
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Sorry for digging up an old thread (just got my car etc...). I was unsure from the posts what exactly is correct behavior from the CBV.
Tonight I took my car out and floored it in second (40 MPH limit so 3rd was out of the question) to about 0.5bar. I then throttled back to about 50%. I get quite a violent shudder, revs and boost spike (to max boost) and then the car decelerates normally. If I throttle back completely i.e. take my foot off the accelerator then I get less violent behavior.
Is there anything I can conclude from this?
Is there anything I can conclude from this?
Tonight I took my car out and floored it in second (40 MPH limit so 3rd was out of the question) to about 0.5bar. I then throttled back to about 50%. I get quite a violent shudder, revs and boost spike (to max boost) and then the car decelerates normally. If I throttle back completely i.e. take my foot off the accelerator then I get less violent behavior.
Is there anything I can conclude from this?
Is there anything I can conclude from this?
02-26-2002, 06:46 PM
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It's simple - just don't throttle back. 
Actually James - as I understand it, the way to check if you have a problem (the rough and ready way) is to see what happens to boost you've built up as you accelerate hard through the gears. If the valve is working properly, you won't losr much boost during the gear change, and as you floor the accelerator in the next gear you should have a big chunk of boost there right away.
I hope I've understood correctly. Someone please correct me if not. As you can tell I'm somewhat "technically challenged".
Actually James - as I understand it, the way to check if you have a problem (the rough and ready way) is to see what happens to boost you've built up as you accelerate hard through the gears. If the valve is working properly, you won't losr much boost during the gear change, and as you floor the accelerator in the next gear you should have a big chunk of boost there right away.
I hope I've understood correctly. Someone please correct me if not. As you can tell I'm somewhat "technically challenged".
02-26-2002, 07:08 PM
#15
Drifting
[quote]Originally posted by JamesE:
<strong>Sorry for digging up an old thread (just got my car etc...). I was unsure from the posts what exactly is correct behavior from the CBV.
Tonight I took my car out and floored it in second (40 MPH limit so 3rd was out of the question) to about 0.5bar. I then throttled back to about 50%. I get quite a violent shudder, revs and boost spike (to max boost) and then the car decelerates normally. If I throttle back completely i.e. take my foot off the accelerator then I get less violent behavior.
Is there anything I can conclude from this?
Is there anything I can conclude from this?</strong><hr></blockquote>
You likely have an intake leak. When I had a leak, I was getting a wild surge when I'd lift partially off the throttle at high load settings. Check the o-ring on the intercooler neck (goes into the throttle body).
fc-racer
<strong>Sorry for digging up an old thread (just got my car etc...). I was unsure from the posts what exactly is correct behavior from the CBV.
Tonight I took my car out and floored it in second (40 MPH limit so 3rd was out of the question) to about 0.5bar. I then throttled back to about 50%. I get quite a violent shudder, revs and boost spike (to max boost) and then the car decelerates normally. If I throttle back completely i.e. take my foot off the accelerator then I get less violent behavior.
Is there anything I can conclude from this?
Is there anything I can conclude from this?</strong><hr></blockquote>
You likely have an intake leak. When I had a leak, I was getting a wild surge when I'd lift partially off the throttle at high load settings. Check the o-ring on the intercooler neck (goes into the throttle body).
fc-racer