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KB S/C losing boost as RPM's increase

 
Old 05-09-2019, 01:18 PM
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Cleonard
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Default KB S/C losing boost as RPM's increase

2004 Z06 w/ a Kenne Bell supercharger. Meth injection, 317 heads, BAP, and many other mods. I had an SLP underdrive pulley on it (pre-s/c days) and was making 10lbs of boost from off-idle to redline. I added the meth injection and decided to increase the boost by going back to a stock harmonic balancer. The underdrive pulley was 33% I believe, so I anticipated 15lbs of boost. However, something bizarre happened. Now I make about 12-13lbs of boost but it slowly tapers off until it hits 0lbs. I *think* it is RPM related. I went on the interstate and in 4th gear low RPM I can hold ~11lbs of boost for awhile, but if I downshift the boost drops. My first suspicion is belt slip. How can I verify if it slipping or not? I don't see any belt damage or hear belt whine. It could also be a boost leak, but being a PD blower there aren't many places it can leak boost. I have checked all of the vacuum lines which see boost and none are broke/disconnected. Lastly, it could be the bypass valve. I have heard they sometimes fail but I find it ironic it failed only after I changed my harmonic balancer.

Any other ideas or suggestions?

Colby

Last edited by Cleonard; 05-09-2019 at 01:34 PM.
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Old 05-09-2019, 02:05 PM
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Turpid porpoise
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Originally Posted by Cleonard View Post
2004 Z06 w/ a Kenne Bell supercharger. Meth injection, 317 heads, BAP, and many other mods. I had an SLP underdrive pulley on it (pre-s/c days) and was making 10lbs of boost from off-idle to redline. I added the meth injection and decided to increase the boost by going back to a stock harmonic balancer. The underdrive pulley was 33% I believe, so I anticipated 15lbs of boost. However, something bizarre happened. Now I make about 12-13lbs of boost but it slowly tapers off until it hits 0lbs. I *think* it is RPM related. I went on the interstate and in 4th gear low RPM I can hold ~11lbs of boost for awhile, but if I downshift the boost drops. My first suspicion is belt slip. How can I verify if it slipping or not? I don't see any belt damage or hear belt whine. It could also be a boost leak, but being a PD blower there aren't many places it can leak boost. I have checked all of the vacuum lines which see boost and none are broke/disconnected. Lastly, it could be the bypass valve. I have heard they sometimes fail but I find it ironic it failed only after I changed my harmonic balancer.

Any other ideas or suggestions?

Colby
Check for excessive amounts of black dust around your pulleys. Bypass valve could be the culprit as well but I don't know enough about PD blowers to make an opinion on that.
Did you change anything else at the same time as the pulley? What cam is in the car?
Would it seem less ironic that it failed as the pressure it had to control increased?

Have you gone through all of the items on KB's site regarding low boost?
http://kennebell.net/tech/supercharg...-causes-cures/

Last edited by Turpid porpoise; 05-09-2019 at 02:07 PM.
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Old 05-09-2019, 11:23 PM
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Z06 cam. I’ve read it’s a good cam for s/c cars. I have gone through everything on their site, but was not certain how to isolate one cause from another. I am leaning towards belt slippage. Today I drove the car and it was acting “different” in a better way. It was making more boost (14psi) and holding it. The supercharger was also noticeably louder. The only other thing I changed was the belt. I had to go longer due to the larger crankshaft pulley. I figured a belt would grip best when new, but maybe it just needed to settle in (or there was oil or something on the pulleys or belt). I’m going to run it some more tomorrow and will report back!

Colby
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Old 05-10-2019, 11:08 PM
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It is definitely belt slip. If I gently roll on the throttle I can hold boost longer than if I just romp on it. Now I just need to decide how to best deal with it. An 8 rib system would help but isnít cheap. I could go back to 243 heads from my current 317 heads and with the increase in c/r dial back the boost with a larger s/c pulley. This would also help with IATís. What about belt brands? Do cheaper ones slip more than expensive ones? What about the belt tensioner? Does it provide more pressure if itís pulled back more, say 3/4 back instead of 1/4? Or is the pressure the same regardless? I ask because I could probably go down one more belt size and crank it back further. Any other suggestions?

Colby
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Old 05-11-2019, 06:17 PM
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[QUOTE=Cleonard;1599381616]It is definitely belt slip. If I gently roll on the throttle I can hold boost longer than if I just romp on it. Now I just need to decide how to best deal with it. An 8 rib system would help but isnít cheap. I could go back to 243 heads from my current 317 heads and with the increase in c/r dial back the boost with a larger s/c pulley. This would also help with IATís. What about belt brands? Do cheaper ones slip more than expensive ones? What about the belt tensioner? Does it provide more pressure if itís pulled back more, say 3/4 back instead of 1/4? Or is the pressure the same regardless? I ask because I could probably go down one more belt size and crank it back further. Any other suggestions?

I am dealing with some belt slip right now, I just tightened the tensioner adjustment hopefully this will help some what, also using gates rpm series belt. At this point, I am not going to invest in a flip drive or 8 rib, will just have to deal with it I guess.
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Old 05-12-2019, 12:07 PM
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The tensioner needs to be @ 2/3rds of itís travel when static , you can adjust it with belt length, try that first

Last edited by LedfootLarry; 05-12-2019 at 12:08 PM.
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Old 05-12-2019, 02:27 PM
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If youíre sure it is belt slip and could go down a belt size that is absolutely the first thing I would do. Replace with a smaller belt and tighten the tensioner.
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