Power falls off in high rpm range, what's the problem?
 

Went to dyno today to tune HKS turbo & Blitz camshafts and noticed some weird unsolved problem which I thought was fixed previously, horsepower power flattens out quickly by 5000rpm and started to dip down right around 6500rpm. We have tried various Mivec setup, while boost is 27psi tapering to 23psi, and A/F 11.9 to 11.1. Tune is good and spools quickly, but for some reason high end power just falls flat to the ground. Looking back at my old tune, this problem has started during the time I was running the FP Green. Originally it pulls all the way up to 7500rpm, but months later when I went back for a retune, the power flattens out at 5000rpm and falls down by 6500rpm. At that time we found there was an exhaust leak and manifold leak which were fixed. I thought that were the issues so when we installed the HKS turbo and made sure there was no leak, we thought the problem went away.

What could be causes for this? I have read a clogged cat would be one scenario but I'm running test pipe. And the fact that my boost is not being withheld doesn't sound like a clogged exhaust syntom? Could it be a pest or something clogging the exhaust? Mind you my NurSpec DT isn't a straight through style system so I can't see whether it is being clogged up or not...

Could it be SST TCU issue? I remember seeing some high horsepower MR which has power dipping issue in the high range as well. But that doesn't explain why I didn't have the problem originally, it happened after a while with the turbo upgrade.

Can you post a dyno graph? Are you tapering boost on purpose or is this part of the issue?

I don't think its a clogged exhaust issue.

Here's the dyno, it's hard to see as the runs overlaps but you get the idea how the power stops increasing around 5500rpm and drops quickly at 6500rpm. Seems like there is some kind of restriction going on.

Boost is tapered on purpose, but it made no difference to the power tip at 6500rpm even if we increased the boost.

And this happened not just with the new setup, I had a similar issue with FP Green as well, initally it would peak out at 7500rpm, but months later the same power loss problem occured.

http://i18.photobucket.com/albums/b1...5/L1010650.jpg

Did you say you had the problem before the new cams were installed? If so it you can rule out the new turbo and cams, and that would lead you back to your clogged exhaust theory. It shouldn't be to time consuming to just remove the cat back and do a couple of passes with an open down pipe.

Tried cleaning the maf sensor but made no difference.

I wanted to do all the simple diagnose first before taking off the varis diffuser to remove the exhaust...

What else could you guys think off?

check all the hoses and clamps, you may have a huge boost leak?

I suppose a huge boost leak would affect drivability? here's my boost graph which shows stable boost from 27psi to 23psi:


http://i18.photobucket.com/albums/b1...5/L1010651.jpg

It seems like a boost leak to me. I would check around the blow off valve just to be sure. This way you just eliminate any possibility of it being that or any of the boost pipes.

Same or different wastegate actuator for the FP GREEN and HKS turbo?

What psi is the wastegate actuator rated at?

Short of a boost leak, I guess it WGA is causing the problem. Maybe it is unable to hold the boost?

Will look into boost leak, but I thought the car wouldn't run properly if there is a leak.

Different wga, the FP green I had JUN wga, HKS turbo came with its own high pressure wga which is able to hold boost. The problem started to occur even when I had the Green, there is no problem in tuning or holding boost in high rpm range, it just not translating to power where it should be.

My friend had the same problem on his car and it turned out the pre-load on the WGA wasn't tight enough.

What turbo/wga is your friend running? Pretty sure mine is tight otherwise it wouldn't be able to hold boost, but I will have to look into that as well.

Like i said before its probably not your exhaust unless it sounds different to you. I just don't see how you could clog a 3" pipe. Don't waste your time removing your diffuser.

I had a massive boost leak through a torn coupler that would not leak until 18lbs of boost. So the car seamed to run fine it just felt low on power. I actually had to do the leak down test twice since I only did the boost leak test to 15lbs the first time around.

Try a boost leak test. Its a simple procedure.

I'm on side with you it's not the exhaust, it did sounded louder after the install of new camshafts which is normal.

Will give boost leak test a try.

Any updates on this? I'm really interested in using the turbo.