TrimDrip

I have looked online and have come to the conclusion that a explorer engine with gt40p heads has a 9:1 compression ratio. I am just not sure why. Explorer engines with the 3 bar heads have a 9:1 compression ratio also. So what is the difference in the shortblock? The heads have a 5cc of difference or so in the combustion chambers. So why would the compression ratio be the same?

AdderMk2

piston changes most likely

IIRC, a set of 96-97.5 exploder heads will net you ~8.8" CR on a stock mustang 5.0HO shortblock, where the 97.5+ gt40P heads will run you around 9.4-9.5:1

TrimDrip

You're probably right on the pistons change. I did look for some replacement pistons online to see if there was a difference. I gave up on that search pretty quick.

mjr46

dish in the pistons iirc

TrimDrip

thanks guys

EYE8RICE

Actually it's because the GT40P heads have a 59cc combustion chamber instead of the E7's 63-64cc combustion chamber. On my 306 after having my deck milled .010", adding +3cc flat tops, and gt40p's it yielded me a 10.2:1 CR.

mjr46

true but reread his post, that does not answer his question, two different chamber heads on the same bottom end would yield different cr's , so to remain at 9:1 on the p head explorer motors the piston would have a slight dish to compensate for the smaller chamber on the head

EYE8RICE

whoop, I did indeed misread... I agree, a slight dish it is.

<-me