Losing boost

[bosco659]

Had my MKIII on the dyno this w/e for tuning and found that I am losing boost. Car boosts great to 14.5 psi at about 3200 rpm but slowly loses boost to redline, actually it drops off pretty quickly after hitting 14.5. Boost will drop to about 12 psi by the time I hit redline.

No hose leaks. Greddy Profec B boost control. CT26 turbo. Tested wastegate actuator and the diaphragm does not leak. Thinking it may be the spring that is getting weak and cthe actuator cant hold the wastegate closed once the exhaust starts to flow.

It was suggested that I try to shim the wastegate with washers to preload the spring a bit and that might help hold the boost. Any thoughts? If you agree to shimming the wastegate, how many washers do you think I should use? Actual recommended thickness of shims would be helpful as all washers are not the same thickness.

Thanks

Vic

[Jeeves]

I wish you'd have listed your mods.
Your problem is actually much more simple. The CT26 is not a LARGE turbo and can only supply X flow at higher RPM's. As it sits the stock fuel cut off is at 12psi (so I hope you're running 550's or such w/a decent fuel pump upgrade). This is actually the limit for several reasons, the top two being "that's where it runs out of fuel" and "that's where the turbo goes out of efficency".

At anything greater than 12psi, the turbo isn't pushing anything but hot air and you see very little gains out of it above 12psi.

For all of this, I'm assuming you're using a stock CT26. Upgraded CT's will flow differently. For more boost (remember, efficency is key here) I'd suggest looking at upgrading the turbo or addressing other potential bottlenecks.

[bosco659]

My bad, here's the upgrades:

CT26 54 trim
Greddy Profec B
Lexus AFM
RC 550 injectors
Spearco intercooler
2.5" intercooler pipes (other than thru the fender)
Greddy type S BOV
SAFCI
Full 3" turbo elbow
3" exhaust

Didn't have the boost drop before but just noticed it on the gauge at the end of last season. Confirmed the boost drop on the dyno. Just didn't think it dropped that much.

Any additional comments are welcome.

Thanks

[Jeeves]

Hmm, I don't really know anything about the 54 trim, and I can't seem to find a compressor map, but most people seem to be putting it into the 400-450 range.

You might still be running into flow issues, I don't recall the factory wastegate to have any real losing tendencies.

[bosco659]

Did a proper boost leak test with compressed air and found several leaks in the intake piping, the biggest of which was at the BOV. Everything is tightened up and I have regained my 3 psi I was quite surprised how tight I had to go to get a couple of the connections to eliminate all the leaks.

Moral of the thread - check for boost leaks with compressed air, not only by visual inspection.