I went to automotive tech school in Chicago a few years ago and there was a class there called Hot Rod U. One class we'd build a carbureted chevy 350, put it in a T-Bucket chassis, run it on the chassis dyno and upgrade parts and keep running it with each new part. Simply a trial by error demonstration of what could be done to an engine to make HP. The other class was street legal performance. We had several Mustangs and F-bodies there. The Mustangs responded great to the mods, whereas the F-bodies didn't, especially the LS1's. After we'd put every bolt-on on, exhaust, chip, intake, ignition, throttle body, it would gain maybe 15HP at the wheels. Mustangs were building close to 30-40 HP! The only mod they responded great to was the nitrous. So, just because its been modded, doesn't mean the car responds well to the new parts. Just my experience with old, worn-out cars.