I am trying to select pistons for the rebuild on my 91 305TPI. I was looking at the KB hypereutectics with the flat top and either a 2 or a 4 valve relieves. However, when I checked the compression ration with my stock 55cc combustion chamber heads, it comes up to be 10.4:1. Is this compression ratio too high to run pump gas in with the cast iron heads. I don't want to run into detonation problems and trash the newly rebuilt motor.
I also called summit racing and they recommended the dished hypereutectic pistons from KB. They are supposed to put my compression ratio at 9.5:1, which is slightly higher than stock. What I don't understand is how it is that a dished piston gives a higher compression than a flat top piston?
Any explanations and recommendations would be greatly appreciated.
Thanks

 

Flat tops can give lower compression if they are shorter, as in less height from the pin to the top than the dish piston has.

I like to run just under 10 to 1 in most street engines because I drive a lot and I never know when I'm gonna get a bad batch of gas in any given town. 9.5 to 1 is great but 9.8 or 10 to 1 is better for me. Above that, you have to know your gasoline. You should have ESC (knock control) with TPI so most likely you'll lose power but not grenade the engine.

I also don't like hypereutectic pistons. I helped destroy an engine (sbc) with those because one cylinder leaned out more than the others. The piston disintegrated and destroyed everything but the intake and one head. Never again for me. Cast or Forged only, please, and I prefer forged.

 

Are you accounting for the depth of the piston below the head, and head gasket thickness?
Check your head cc.