I have used search and cant find out what I need so here goes:

I have an '85 Z28 modified tpi system. I converted from MAF to MAP as well, installing new harness and ECM with burnt custom chip. My IAC valve is from my 85 system. I am having trouble with fuel at 4500rpm and above. It flat lines there and also will crap out and stutter before I get to 4500. I replaced IAC but my question is: Should I use an IAC valve from a '90 MAP system or is the original one ok? Maybe I just have a bad vacuum leak I havent found yet. And, I am to assume the 90 valve would go in the same place on the TB? Thanks for your help...

 

I may be wrong but I dont think the IAC should have anything to do with your problem there. it really only controls idle and prevents deceleration stalling. You have a new harness or modified the 85 one? I would say if all is from a speed density system get the newer one, if they are in fact different IAC's. Otherwise I would stay with the 85, the pintle may be different too and the seat untder the TB may not match up correctly

 

Agreed. Unless it falls loose and gets sucked into a valve, the IAC is completely insignificant above 2,000 RPM.

You're getting "flat line" in the power curve above 4,500 RPM? Welcome to the world of TPI and stock valve springs. One or the other (or both) is near the limits of ability at that RPM and above.

The TPI intake is designed to enhance induction at lower RPMs. You can modify it to some extent to improve its upper RPM capability, by there are limits.

As for the stock, 85# seat presure valve springs, they will usually allow some valve float right around 4,500-4,800 RPM. The '85 TPI has a slightly better cam profile than some of the later TPI engines, so you would likely notice the problem at a slightly lower RPM. If they are original valve springs, they might not even have 85# of closed seat pressure any more, since they are almost old enough to vote.

 

Hmmm. I have a 90-92 harness from painless. I dont have original valve springs. My engine was modified. I have Z28 ex. valve springs @ 1.25 dia 105lbs on seat. I also have an Elgin cam. Could this be a problem because I still have the stock fuel pump? I am changing that out soon. Based on the posts I had read about IAC, I was sure that my crapping out problem was because of the IAC. oh well, the original was no dam good anyway but I wasnt having idle or stalling problems.

 

By the way, this flat lining has been a problem from the start. I having been modifying the tpi a little at a time since I had the original MAF system. Now that its done its still there. When I had MAF I thought the problem was because I still had the 19lb injectors. Once I changed them to 30lb to handle the hp the problem didnt go away.

 

Feed 400 hungry cubic inches through a TPI setup is probably one of your main problems.