Vehicle has had tune up, CAI, full exhaust, headers, replaced/upgraded computer, new MAF sensor, and some more I cant remember. I've had the car for many years and it has always plateau'd or lost power around 3500 rpm, to the point where I had a manual valve body installed so I can make the car shift early. I can shift at 4000 rpms in first-second, but if I try to shift where the powerband should be (4400+) it falls completely flat on its *** and I get passed by honda civics and ford escapes. It still has plenty of torque down low, and the motor does rev all the way up to 5 or 6k smoothly without jitters/jolts, but it loses so much power by then it feels so slow I don't let it go that high anymore. I'd love some help figuring this out as I've been told it could be fuel pump but usually that sputters(mine doesnt sputter) and one mechanic told me it's probably just losing compression.

 

You said full exhaust, does that include cat?

 

That could be all kinds of things. Without any other info I would start by verifying some vitals like ignition timing (which is a likely culprit), fuel pressure and check for exhaust restrictions like a plugged catalytic converter like chaz mentioned (another very likely culprit). Low compression is possible I suppose, I doubt that is your problem but its nice to know and easy enough to check.

 

 

I agree that timing could be a problem, but not just ignition timing. Valve timing could affect that, and a stretched chain and/or worn sprocket could alter valve timing. Fortunately, a relatively easy way to determine if that is a problem would be to check the base ignition timing. If it has moved significantly, the timing set may have jumped. If not, set the base ignition timing to spec and move along, eliminating two possibilities.

Weak valve springs can also cause those symptoms. And by "weak" I'm not referring to the originally weak 85 lb stock springs, but springs which have lost pressure after years of use.

Of course, there are numerous other possibilities.