heads and high compression pistons

[LETZRIDE]

Starting to do the research for the set of heads I will be going with...not sure if I want to go to a local shop or not, but I want to know a couple of things.

Can we run 11:1 CR on 91 octane gas, or will I damage the engine?

2ndly, is it possible to get that high a compression ratio through just milling down the heads, or do I need to get a higher compression set of pistons to get it up there?

3rdly, how much of the engine do I need to take apart to swap the pistons if I need to go that route? Would I be able to achieve the swap with just the heads off?

[bigbrian442]

Re: heads and high compression pistons

1 To high for cast iron heads with 91 octane fuel. With aluminum heads it might work.

2 Yes, but to mill the heads that much you would have sealing problems with the head gaskets. It would be a better idea to use higher compression pistons.

3 You would need to remove the engine from the car and completely disassemble it. The bore would need to at least be honed for the new pistons / rings and the oil pan needs to come off.

[LETZRIDE]

Re: heads and high compression pistons

so what is the max CR I can safely run without having to go with different pistons...I can't pull the engine to disassemble it...I don't have the room or the engine stand for it...

[bigbrian442]

Re: heads and high compression pistons

For a pump gas motor with cast iron heads you want to stay under 10:1. 9.7-9.8 is safe. Even with that much compression your pistons should be forged.

[LETZRIDE]

Re: heads and high compression pistons

I thought we were 9:5:1 now...9.7-9.8:1 is not a significant difference over stock at all...

[bigbrian442]

Re: heads and high compression pistons

9.4 according to the Tech data base. LS6 motors are 10.5:1 with aluminum heads and GM recommends 92 octane. You can go about 1 point higher with aluminum heads and still have about the same resistance to detonation because the aluminum dissipates heat much faster. The hotter LS motors running on pump gas are around 10.7- 10.8 tops. Anything more and they get into trouble. Other things effect how much compression you can safely run like valve overlap. With high compression you run a cam with more overlap to kill the cylinder pressure at low rpm.

[Meatyshells]

Re: heads and high compression pistons

10.0:1 is perfectly safe

[cam98aro]

Re: heads and high compression pistons

1 To high for cast iron heads with 91 octane fuel. With aluminum heads it might work.

that totally depends on what cam he is using.. if it is a stock cam, then no it cant be done, if it is a wild cam then it will work fine

the overlap of the cam will bleed cylinder pressures down making you able to run lower octane gas

for instance, i can run 89 octane gas with iron heads and 11.5:1 compression with no detonation

[LETZRIDE]

Re: heads and high compression pistons

ok I have the abbott N/A cam...210/220....not sure how much overlap that really is though....

[cam98aro]

Re: heads and high compression pistons

ok I have the abbott N/A cam...210/220....not sure how much overlap that really is though....

not much, that seems like a rather mild cam

[LETZRIDE]

Re: heads and high compression pistons

damn...and I was thinking it was one of the more aggressive cams for our cars...

210/220 .512/.512 112 lsa