I've got a C30-94P with a 85mm pulley, TDR kit, and I'm seeing boost fall off at higher RPMs... anyone have some clues as to where to start looking for where it's going?

Doing some pulls with the tuner, we saw our highest peak of 169.5 kPa (~10psi of boost) at 5723 RPM. Boost is consistently peaking around 5700 RPM and falling off.

Just wondering where to look. Belt tension issue? Chinese BOV issue? Remote mounted IACV? Throw it away and get a K20? any help is greatly appreciated!

 

Pressurize the system and check for leaks. My guess would be slip though; either belt (look for dust) or internal drive.

 

In my experience, high rpm boost issues has always been belt slip (TDR C30-94 for me). I set the base tension to where I can only twist the long section of the belt 90 degrees, and also keep a can of tite-grip belt spray in our fluids box at the track. Seems to work well enough, but I'm cautious about how much tension I put on that rotrex pulley. I switched to a turbosmart BOV with 25mm inlet/outlet pretty early on, but that was mainly for peace of mind.

For what it's worth, the one time I had a silicon charge hose not tightened down quite enough, it pushed itself off the metal pipe completely. And when my first rotrex failed internally, it was very obvious because it was unbearably loud at idle.

What's your engine setup? My non-P -94 makes about 15psi at 7k rpm with a 90mm pulley. Stock NA8 engine.

 

'96 block, Manley rods, Wiseco pistons, BP4W head shaved for a 9:1 total ratio. Junk2 intake with plenum spacer, Mustang throttlebody, TDR kit with the 94P and 85mm pulley. I've been putting her together for a while and finally hit the driveable stage of things. I've just been quietly taking my time lol

 

I'm not a rotrex guy so I can't comment on the belt slip side of things, but I will say I have had a Chinese BOV leak on me at only 10 psi on my turbo car. I ended up switching to a Turbosmart and never had any issues again. I second what Harry said and say pressurize the system. That will tell you if its the BOV, some coupler, etc. If nothing leaks at 12-15psi then its probably belt slip I would assume.

 

I’m also of the opinion that it’s most likely belt slip, however also check your inlet path for restrictions.
when I previously ran an inlet restrictor on my rotrex setup to drag down the power for classing reasons, the result was exactly the same as the symptoms you are describing.

 

Update: i tightened up the belt tension and gained another 2psi. i think that pretty much confirms a belt slip issue. I'm still not seeing the amount of boost i think I should be seeing, so my plan is to search for 6 rib options. if anyone readily knows where such an option exists, lemme know. if it's a 150mm overlay I'd be shooting for a 95mm s/c pulley instead of my current 130mm/85mm situation.

 

Pretty sure ATI has a 6 rib crank pulley. The obvious issue then is the PS pulley; they are pinned on the shaft so they are not "replaceable". There was a guy in the UK offering 6-rib PS pumps as an option. Alternatively you can try one of the "high-grip" belts out there and see if it solves the issue

 

getting 6 rib on the crank doesn't seem like it'll be difficult to find, and i don't have A/C or PS to worry about. what's proving tricky is finding a 6 rib rotrex pulley source.

 

Mine (KW kit) originally came with a 6-rib pulley that I have always been considering the norm. From memory, TDR supplied their kit with a 6 rib pulley as well.

 

https://www.rotrexshop.com has all the blower pullies you would need.

most are 8 rib but that’s fine, the belt just sits on the 6 it needs.
fwiw, my 4 rib belt sits in the middle of the 8 ribs on the blower pulley and works just fine, never moves out of centre ribs.