im finaly getting all the parts for my engine and i need some pistons. Im gunna get some nice forged ones and i was wondering if it would be better for get ones that have a 10:1 or 11:1 compression ratio with the 64cc heads im using. The 10:1 are only about 50 dollars cheaper. Whats the best way to go and how much of a hp difference will there be between the 10:1 and the 11:1? thanks guys

 

What engine? What heads? What's the goal?

"Rule of thumb" is, about 4% of HP per point of compression. However, you go chasing that last %, and you could find yourself with holes in the pistons, hammered-out rod bearings, or just a generally sluggish-running motor because you had to back the timing off so far to keep it from pinging.

11:1 on pump gas is pretty much the ABSOLUTE UPPER LIMIT; and that's with the most cam you can realistically run. More than likely, that's not going to be a good idea. Especially if it involves domed pistons.... pretty much the worst-case scenario.

 

its a 350 boared 30 over(2 bolt main) and the heads are aluminum with a 64 cc combution chamber and 190cc intake ports. Im gunna use it as a daily driver but but i want it to be reliable and fast

 

It probably wants to be in the low-mid 10s then; 10.3-10.5:1 would be about ideal.

Flat tops, A ZERO-DECKED BLOCK (very important!!!! not just slap whatever into whatever block and call it good), and 64cc heads with a sheet or 2 of copy paper shaved off, would be just about perfect.

 

is 10.63 still a little to high?

 

Probably... ESPECIALLY if the pistons have domes.

 

If i could do it over agine I would go with the 10:1 or 10.5:1. I went with 9.6:1 with my engine because I got the right deal on them. Buy my friends are running 10:1-11.5:1 and they run awsome with premum pump gas, But the 11.5:1 Is in a manul if that makes any diffence.

 

I've done 11.25:1 with iron heads and pump gas, and 10:1 with the exact same combo and only changed the heads - same cam, exhaust, intake, etc.

It was much 'snappier' and more responsive with the higher compression (although that could also be because the 10:1 iron heads had 200cc ports vs 165cc ports), but required constant attention to tuning to keep it from pinging. The new 10:1 setup is much less finicky.

For what its worth though, 11.25:1 with 350ci and an XE274 cam worked great on pump premium when the tune was right... torquiest and most respontive motor I've ever played with! Its still very good with the 10:1 setup, but not quite as much.