11.9:1 compression too high?
 

that's the compression i was running on my 355 lt1 motor.
location: sea level.
gas: 93 octane
stock ported aluminum heads and flat stop pistons.
cam goes up to 6500rpm.

well, the piston rings didnt set in, so the motor has to be torn down to replace them. since that'll be done, should i lower my compression?

i've heard stories on here about how with the right parts and tuning, you can achieve 13:1 compression. well, i'm pretty sure that i couldnt afford to pay some hot-shot tuner to do this, and i'm also sure that i dont know what the "right" parts means (pretty vague term to me).

so, what should i lower my compression to to prevent detonation.


shokor.

My flat top Speed Pro forged pistons compression ratio is in excess of 11:1 with stock ported heads and ZZ9 cam.

The 355 is now setting in my 1993 YJ wrangler Jeep. I am at sea level too and running 93 octane gas.:cool:

It's a badass Jeep with that LT1.:D

you can go much higher than 13.0cr you dont have to get a hot shot tuner just go get 8cc dome pistions. should get you at or a little higher then 13.0 depending on deck height, bore size etc.

I've had 11.9:1 compression on the stock bottom end for a long time without any problems. I have a cam that goes all the way to 6900rpm and use 93 octane, but I'm not at sea level. There are people that've run higher than that on the street so it is possible but not always recommendable.

dynamic compression
 

The thing you should be looking at more so than the static compression ratio is your dynamic compression ratio. If your came closes the intake late enough it may just bleed off more compression than you think. Heres a link to my spreadsheet engine build sheet I can't remember what version I have of it up, but you can enter all your data at the top of the page in yellow. I think there is a place for intake valve close in deg @.005". Enter all the data and it will spit out the static and dynamic compression ratio at the bottom.

A safe dynamic ratio is around 7.5:1. With my current cam I'll be makeing static compression of 12.5 but a dynamic of 7.6. The factory LT4 non-hot cam has a late intake closeing event. At least mine does. Measured it to be 81 deg.

Also make sure your quench height is in a good range .035 to .040 quench should help as well.

Re: dynamic compression
 

That's a good post but I think that with a reverse cooled aluminum head motor with EFI like the LT1 you can go a bit higher on the DCR. 8:1 or even as high as 8.5:1 is feasible with a carefully built well tuned motor, at least IMHO. Mine is ~6.5:1 but it's a blower car.

Rich Krause

I said it was "safe" I didn't say it was the limit. :D

The dcr in my vette is pushing the limits of a cast iron headed big block at 8.9, or something like that can't remember the number off hand.

Hey guys , i'm planning a pretty agressive heads and cam swap soon. Which compression/cc size should i run. It will be with a stock bottom end

That will be with 93 octane gas and good tune by Rapid Motorsports

Just don't let her get lean:D

The cam specs are needed to answer this question in all but the most general way.

Rich Krause