what gain or loss to expect

[ddettloff]

I currently have a 318 with 1968 stock heads , just headers a eddy 600 with eddy intake.
I have a set of 72 360 heads I want to mill .40 and do slight port work and matching on.
My question is will this help performance or just create a lower compression dog.

 

[AJ/FormS]

What are you using that engine for? Street, cruiser, track car, etc? What cam size? What cuurent Scr? A loss of a lil cr at 5000 to 6500 in a racecar is less of an issue, than the same loss at idle to 3000 rpm in a streeter, and a cruiser mightn't give a hoot. The racecar can use the port size, but the streeter may not like the loss of port-velocity, and the cruiser mightn't give a hoot.

 

[ddettloff]

The scr was at 130 psi the cam is a stock mopar 340 ,I basically just play on the street but go to track a couple times per year.
The car is a 4 speed and fun to Drive just looking to see if these will help or hurt performance .

 

[496 polara]

Sounds like an awful lot of work for minimal gain.You may be better served doing the milling,a good valve job,and some bowl work on your 318 heads.1.88/1.60 valves added a plus.

 

[AJ/FormS]

At just 130 cylinder pressure, you really can't afford to lose any pressure.
Ima guessing you'd like a little more giddy-up, below 3500rpm?
496 in post#4 has a plan.
But really getting the pressure closer to 165 will really improve your giddy-up.This would require one of two things; 1) a smaller total chamber volume, or 2) an earlier closing intake valve.
The earlier closing valve will sacrifice top-end/midrange probably faster then it would build off-the-line torque. so that's not the first choice.
Reducing the total chamber volume ain't cheap.This is best achieved with bringing the pistons closer to the top of the decks. However, heads with smaller or closed chambers can do this too. But it seems to me your 68 heads are already at about 60/62cc. So we are back to pistons.
You can disguise the lack of giddy-up with a higher stall TC, or rear gears, but these mostly help with getting off-the-line.
Another option would be to lighten up the car, and is always my #1 go-to. They say loosing 10# is like gaining 1 hp at the big-end. So if your car can loose 200 pounds, that's 20 hp, and about a cam size in a teener.