In my opinion the 4.0 was a poorly thought out engine. The only thing I can think is it was all to do with emissions. There is a calculator on jeepstrokers.com that is pretty handy for working up combinations. I found that with a stock 4.0, zero decking and switch to a 0.040" head gasket gets you 9.3 static compression, 8.0 dynamic compression ratio and 0.040" quench. It should run fine on 87 octane. I'm curious how much power those 2 things would help. Anyone have a horsepower calculator that is some what accurate?