Significant Boost Drop
 

Hey Guys,
My car is an 87 TII with an s5 turbo and manifold, hks exhaust, rtek 1.8, manual boost controller, and a greddy bov. The car can build 11psi by 3800 rpm but by the time it gets near redline im seeing 2-3 psi. I've checked for vacuum leaks and can't seem to find any. I tried tightening the diaphragm of the bov without any effect, could it be the wastegate?

Thanks in advance,
Derek

Sounds like a gnarly boost leak.

How much wear is there on the sleeve for the waste gate flapper? The fact you initially building good boost is a decent sign.

Spike I'll have to check, I wouldn't think there should be too much, the shop that rebuilt the turbo enlarged the wastegate and put a new flapper in (supposedly). I tried a new diaphragm in the blow off valve, and now its seeing more consistently 5psi at redline. I'm beginning to think that might be my culprit.

Im talking the sleeve that the flapper arm goes through into the hotside housing. Just porting the gate and putting a bigger flapper doesnt handle any wear in the sleeve itself. Also, the actual spring in the stock wastegate diaphragm is only 5.5psi. Even running a MBC if you have enough backpressure in the hotside housing it can push open the flapper because there is more pressure in the housing than the stock spring can handle.