Hi guys, I have recently experienced intermittent power delivery above 6,000 rpm which occurs simultaneously with a hissing sound. I noticed the problem before converting to a full GReddy VMIC setup and decided to see if I can found anything suspicious during the process. The sound is very noticeable during the pull, and it is followed by the wave-like power curve. Here is a dyno graph and a video demonstrating this problem.

Link to the video: At this point the car is running a rebuilt engine, J-spec sequential turbos, fluidyne radiator, blitz smic, supra fuel pump, 1300cc secondary injector, 3-inch downpipe, and a RB Dual-tip catback. with Power FC tuned to about 14 psi. I initially thought this might be the air pump. But removing it made no difference. So I went on with the VMIC build.

This graph is obtained with the car running GReddy VMIC kit, Setrab 25-row dual oil cooler. I changed the old spark plugs and fixed a downpipe leak. The boost is creeping to 15 psi. Boost and AFR are consistent on both runs so I don't think it is an intake leak. I was advised to tune down the boost because it might be the stock wastegate. So here is my result with 12.5 psi.

As you can see, the issue is less noticeable at lower boost. But the power at higher rpm is still unsteady. The squealing sound can be heard roughly when the secondary turbo comes in but it is almost gone. I am doubting my water pump or the stock ignition coils. My power looks low for the boost level too. I hope someone can point me in the right direction. Thanks a lot!

 

My guess would be an air leak, maybe the solenoid for the charge relief or charge control valve is fluctuating open and closed.

I would start with getting a vacuum T and a long vacuum hose. Splice the T into one of the lines - probably start with charge relief, and have the other end going to your boost gauge. This way you can easily see what is going on, if you see the boost gauge moving up and down in time with the noise, there's your problem.

Since the noise and problem is more pronounced at higher boost this leads me to think a solenoid is trying to hold the boost back, gets forced open, pops closed, gets forced open again, etc. Some of the solenoids can't handle higher than stock boost pressures well and can leak.

Dale

 

Thanks Dale! I haven't fully considered the air leak because the boost was quite steady. I will do what you suggest and see if I can find anything.

 

On the side note, has anyone experienced boost creep with just minor intake&exhaust mod on stock or J-spec turbos? If so, at what psi does it usually occur? I wonder if it's my wastegate fluctuating. But I have also seen many running about 12~14 psi with similar setup and make higher power without any problem.

 

Each car is different, I had boost creep on my car with just a downpipe and cat-back many moons ago.

The trick with the vacuum T will work for ANY part of the turbo control system, just T it in and see what happens on your boost gauge.

Dale

 

Looks like ignition break up to me. Shop around for a used HKS Twin Power. The fact that the problem reduces with boost level. Here is a thread I created on the subject. https://www.rx7club.com/3rd-generati...rison-1103125/


Here is the before and after dyno comparison, similar power oscillation starting around 6K