Recently I finished swapping in a GEN3 5.7 from a 2002 Camaro Z28 into an 88 300zx. I'm having an issue with power loss since the swap, The camaro donor prior to being torn down for the swap, had dyno'ed at 312whp.

Since completing the swap the Z is putting down 250-260hp to the tires.

During the swap I used sanderson 1 5/8 tight fit headers to ensure clearance for everything, I also installed comp cam dual valvesprings to prevent valve float because I had noticed one of the valves had kissed a piston and left a half moon mark on the top of it. The heads were checked by a machine shop, and then milled .030 to bring the compression up a bit. Using .030 shorter pushrods than stock my compression during cranking is in the 260psi range. The intake is a 4inch elbow leading to a 4inch K&N filter. The exhaust consists of dual 3 inch leading to glasspacks and collecting with a Y pipe to a 3inch axleback with muffler at the bumper. AFRs look ok and there doesn't seem to be any detonation. Oil and coolant temps look ok, the motor seems to rev smoothly and doesn't seem to hesitate. But It doesn't seem like its got what it should, and it doesn't put down close to what it did in the camaro.

 

Those 1 5/8 headers could be a restriction. What is your wot timing at and afr on a wideband? Are you tuning this yourself?

 

I am, WOT timing is 18degrees and AFRs range from 11.9-12.4 depending on RPM. I didn't think the headers would do it, I had a friend with 1 5/8 JBA headers on his LS2 GTO and he put down 360 on the same dyno. And at least compared to stock they look like they should be able to keep up, I'm sure they aren't the best for performance, but they should be able to handle what the motor is for now.

 

18° sounds like too little

 

I had it that low since I didn't want it to detonate on crappy CA 91 gas, I was going to try pushing it a little further depending on what it made, but I feel like it wouldn't lose 50+hp from a few degrees of timing. I could be wrong for sure but It just seems like a lot of loss for that.

I'm running summit racing glasspacks could those stall airflow enough to lose that kind of power?