FNQWK

i'm at 11:1 now and am thinkin on doing heads. car is a lethal cammed z makin 540ish.
also how much hp/tq is gained from 11:1to 12:1?

40YRW8

An exact figure on compression ratio or hp gain is a pretty iffy call since the tune will have a big factor in the limits. Two things that help you tollerate more compression are the altitude you drive/live at and the cam timing. The cam bleeds off low-rpm cylinder pressure which allows a little more compression. How much? Pretty tough to pin it down. What I would do is go for 11.5 and tune it there. Much higher and the benefits of the increased compression don't stack up against the probability of detonating and killing the motor. Can it be done? Yes and no. Some guys have done it and some have had problems. Remember that detonation can sometimes be the real reason behind broken pistons and broken valves, not just parts failure.
Gary

6Speeder

What's your dynamic compression ratio? With the QM600 cam I am running 12:1 static compression and a DCR of 8.9 on 91 octane (but at 5-7,000 ft altitude) with no KR at all (24 degrees of timing).

I believe the DCR of the Lethal cam will be higher due to the lower intake duration and you won't be able to increase the static compression as much without having problems. Get with your cam guy. He'll know how much compression it'll tolerate.

gtodoug

Great responses thus far. It will really depend on the cam timing . Larger more longer duration cams with more overlap will tolerate a higher compression ratio generally because they bleed off compression. What is really important is to keep dynamic compresson ratio in mind. If you can get in the upper 8's that seems to be about the limit. I ran 8.9/12.3(dyn/static) compression on a built 408 and it was fine on 93. Another motor ran 9.3/11.5 and pinged and was very sensitive to weather. Get your dynamic compression between 8.5-8.9 and whatever the static compression is wont really matter.

LSs1Power

What is the maximum timing you are running to reach that 540rwhp on 91oct? Also what were the temps you dyno'd in?

rsmith1944

I have .030 milled from my heads on my C6 Z06 that brings static compression ratio to 11.6 and makes the lethal cam to 8.83:1 dynamic compression ratio considered to be touchy on timing with 91 octaine fuel by my cam tuner. The Lethal would probably be real good with un milled heads in my opinion. The best thing to do is get with your cam specialist and have him set you up with a cam that sets your DCR in the sweet spot for what you desire. I have learned the hard way that throwing parts in an engine might not work together as I hoped and ended up with over 9.++ DCR & had lots of spark retard & had to take timing way down to have a less than happy motor. I have a cam coming that will put me at 8.63 DCR with my .030 mill & should tune well with 91-93 octane.

FNQWK

thanks for all the responses. the car was dyno'd in about 65* i believe timing is at about 28* i do have a forged motor with aftermarket pistons, so i'm sure that has to be taken into account of dcr. this compression talk is very confusing to me

PRE-Z06

the DCR(dynamic compression ratio) is the actual amount the air is being compressed, based on when the intake valve closes on the compression stroke, starting the actual compression of the air...the sooner you do it(ie smaller intake lobe, tighter lsa, more advance ground in) the more the air gets compressed regardless of SCR(static compression ratio) although milling of course effects both...general rule of thumb is 15/15 for every point increase of SCR, DCR will be different