Hi, differences between flattop and dishtop pistons?
I have a 305 with a 2 stage cam and a few other mods but i am having bad performance so i will probably rebuild the low end soon. I think it is a loss of compression at the rings. Im thinking flattop may be better due to maybe more compression but i read something about flat spots on accelleration. Also, i got a set of 400 heads which someone gave me, would they be worth installing when i do a rebuild?

NO, do not use the 400 heads. why do you think they were given to you in the first place? b/c they are not worth anything. leave the 400 heads alone, they were smog motor heads. they have real big chambers and small valves. the chambers are also way bigger than you bore.
use flat top pistons to bump your compression up and get a good set of 305 heads. they have a smaller combustion chamber to help with compression ratio. 1.94 intake, 1.50 exhuast should be plenty big for a 305. if you can do some porting your self, just try to smooth the runners up so the air/fuel mixture will flow smoothly. if you open them up to much, the velocity will decrease and you'll lose low end torque.

305s mostly came with dished pistons. The only ones that didn't were 83-up L-69 HO carbureted motors used in some F and G body cars- they were flattops with 4 valve reliefs and were factory rated at 9.5:1. Everything else was dished: big dishes on pre-83 305s and 83-up LG-4 and other low output 305s (factory compression around 8.5:1). Small dishes on some later model fuel injected 305s (87-up) that were factory rated at 9.2:1 compression.

95% of the time your 305 will have dished pistons of one type or another. The ones with flattops were a small minority of the overall number of 305s built.

Agreed on the 400 heads- they have muc larger 76cc chambers and will absolutely put your compression in the toilet on a little 305.

Factory 305 heads were all around 58cc chambers and came with either (small) 1.74/1.50 valves or (slightly bigger) 1.84/1.50 valves. The later heads with the bigger valves also had better chambers than the earlier small valve heads, in most cases.


[This message has been edited by Damon (edited October 19, 2005).]

some 400 had 80cc chambers, go with flattops.

Thank you for the help everyone, i think i will definitely redo the heads and go flattop. I appreciate the help, this site has been very helpful.