Alright here goes... I have a SBC that has been converted to EFI ( XFI ) . When I got this thing running it was hitting on all 8 holes. After a few restarts it started to sound like a stock motor with a huge cam in it. Really lazy and sounds as if it missing on a couple of cylinders... took the heads of checked the valve seats... checked plugs and they were ok as well. This thing revs up just fine and takes throttle like a champ. All cylinders check within 10 -15 psi of each other. I'm thinkin either it has a bad injector or maybe a small vacuum leak.