Loss of boost at high rpm?

JT15601

Registered User
So for some reason when I rev the car to 4800+ rpm while racing or driving hard under load you will hear the supercharger go from a whine to a shhhhh for a split second and boost will fall from ~13 psi to about 9psi and then start boosting back up to 13psi after the shift. Its not overdriven just a stock pulley and a stock belt and all that jazz. So (in my head) either its the belt slipping because the tensioner is pretty close to bottomed out or the supercharger bypass valve is faulty and letting boost slip by over 13psi. The little hose that goes to the plunger on the lever of the bypass valve is all pieced together because it broke so idk if that would effect it (I don't really know what its purpose is). But why would it drop straigh to 9psi and not fall slowly? Im a turbo diesel guy so bear with me, Any help is greatly appreciated!
 
My vote is belt slip. The vacuum line going to the bypass valve is there to open it up under light load so the supercharger isn't using power when it isn't needed. If the vacuum going to the bypass valve were off, the car would still run, but it would heat soak the intercooler and your gas mileage would go down, but it wouldn't affect the amount of boost it makes. If the bypass valve were leaking, it would leak boost all the time and your boost numbers would be slightly lower all across the rpms, not fine then suddenly dropping.

Since you said everything is stock, I would start by replacing the supercharger belt. Yours could be stretched from wear, which could cause the tensioner to bottom out, which would cause belt slip.
 
Ive been planning on buying that ~7% OD pulley and belt combo off spinning wheels but I was going to hold off because of this boost drop problem figuring it might get worse. But if its the belt slipping I'm assuming that set up would probably fix the problem. The belt looks a little worn anyways.
 
serpentine belts stretch quite a bit under load. If the tensioner is close to bottoming out then it will bottom out for sure and result in belt slipping.

The hose connected to the bypass valve provides the vacuum signal from the intake manifold. It requires vacuum to open the valve otherwise spring pressure holds it closed. If the quality of the hose is in question it should be replaced otherwise unexpected results may occur. Typically being poor engine performance due to vacuum leaks and difficulty building boost.

The change in sound that you hear indicates to me that the bypass valve has opened under boost bleeding off the pressure though belt slip may also create an issue for you.

Stock blower belt on a stock setup should last a good 2 to 3 years depending on your environment before it starts getting glazed and no longer performs optimally. When I was racing I replaced my blower belt every 2 seasons and inspected it before each race. That was 8 and 10 rib belts on an overdriven blower.
 
Sounds like a classic belt slip issue to me. Check condition of both belts for cracks, missing ribs or hard glossy surface on the ribs, and look to see how much travel is remaining on tensioner arm. If nearly stroked out, the belt is going to start jumping and slipping when tensioner arm runs out of travel to take up the slack from the stretched belt. Also keep in mind that it could be either or both belts that are slipping. The belt from crank pulley to jackshaft pulley is often overlooked.

If the hose completely fell off the bypass actuator it would default to the closed position which provides boost, and the amount of air leaking out of the hose wouldn't have a noticeable impact on boost level at WOT and would only be noticeable as a vacuum leak causing some issues at idle, so I really don't think that is the problem.

David
 
I'm still pulling 16-18 hg of vacuum at idle and the bypass valve is open at idle and flips closed when I rev it I've watched it while revving from the throttle body under the hood I just didn't know if it could leak under load while driving but I'm pretty sure its the belt for the sc becausethe tensioner is close to bottomed out. I'll check the other belt too

What if I put a slightly larger tensioner pulley on the sc drive to hold me off until I get a new belt and the od pulley. I have a bunch of parts lying around and probably a bigger tensioner
 
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I'm still pulling 16-18 hg of vacuum at idle and the bypass valve is open at idle and flips closed when I rev it I've watched it while revving from the throttle body under the hood I just didn't know if it could leak under load while driving but I'm pretty sure its the belt for the sc becausethe tensioner is close to bottomed out. I'll check the other belt too

What if I put a slightly larger tensioner pulley on the sc drive to hold me off until I get a new belt and the od pulley. I have a bunch of parts lying around and probably a bigger tensioner

Yes..a larger diameter tensioner pulley would help. If replacing the belts, I've found that the Gates Micro V brand works best, and Goodyear Gator back brand are the worst.

David
 
Yes..a larger diameter tensioner pulley would help. If replacing the belts, I've found that the Gates Micro V brand works best, and Goodyear Gator back brand are the worst.

David

I've noticed this first hand Gates belts only for me. Guess by ~3yrs anit too bad then time for me to get a new one, same ~10PSI max plus the powder. I know mine is stretched just curious if time frame was right.
 
I'm still pulling 16-18 hg of vacuum at idle and the bypass valve is open at idle and flips closed when I rev it I've watched it while revving from the throttle body under the hood I just didn't know if it could leak under load while driving but I'm pretty sure its the belt for the sc becausethe tensioner is close to bottomed out. I'll check the other belt too

What if I put a slightly larger tensioner pulley on the sc drive to hold me off until I get a new belt and the od pulley. I have a bunch of parts lying around and probably a bigger tensioner

You may very well have a small vacuum leak AND belt slip. a stock cam Sc should idle at 20hg when warmed up
 
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