I am probably ging to have a higher then normal compression on my s/c engine I am going to be somewhere betwee 10.5 and 11.1 I was wondering on a motor with forged crank LT1 rods and forged pistons how high i can go on my effective compression ratio and not be seriously risking the the motor.....with pump gas that is 93 octane.

I was told I can go up to 18.1 effective compression ratio but nothing more then that?

thanks
Dustin

 

I don't think there is a consensus on how to define or calculate "effective compression ratio". But in practical terms you should not plan to run more than 6-8lbs of boost. Water/alcohol injection will be a must, IMHO.

Rich

 

there are many calculators on the internet that compute the effective compression ratio of a blown motor. Are you saying none of them are right?? or at least close

 

I am saying that there is no common definition of the term "effective compression ratio". Unless they define what they purport to calculate there is no way to determine if they are right, or wrong. And what's more, there is no way to use the number they generate.

Think about 18:1 as an "effective compression ratio". What does that mean?

Rich

 

18:1 on a blower motor = not a good idea IMO. Ideally you want a lower than stock CR for a blower application to make it effective.

Personnally I am running 9:1 CR in my 383, D1SC, w/ 12#'s. When my 383 was N/A, my CR was 11:1 (still have the pistons for sale if anyone is interested).

 

Maybe i have been lucky...but my 396ci is 10.5 to 1 with a D1SC @12psi, i use Cam2 and have the FAST ECU set up. I have had this combo for 6 years now and have not had a single engine related problem. It made 672 rwhp , and with traction issues it has been 10.48@137mph many times (1.60 60ft).

I am not saying it is safe, but i havent had any problems with mine, and besides one thing i do KNOW for a fact is that my car still makes very good low end power (before building boost) since i still have higher compression. An 8 or 9 to 1 comp eng would be a pig until is starts building boost IMO. Also another thing is that the camshaft in my car in NOT a blower cam...so i am not sure how that would figure into the equation.........

rkrause, i always respect and appreciate your opinions and responses, you are one of the most knowledgable and helpfull people on the ENTIRE internet. If my set up has worked for as long as it has (6 years and about 12k miles, and i beat the hell out of it at times, and its a 6 speed) does that mean that maybe it isnt that bad to do after all (meaning higher comp w a blower)????? Please share your thoughts.. and thanks again.

james

 

I like the ability to use pump gas. By effective CR, I think he means "Final" CR.
Final CR = [ ( Max. Boost / 14.7) + 1 ] x CR. Since altitude plays a part,
corrected CR = FCR - [ (altitude/1000) x .2 ]

 

james: I would be curious about your cam specs. I would guess you are bleeding off some compression with a late IVC and some of the boost with a relatively large amount of overlap. You also must have some hella good pistons

Rich

 

Hey Krause...i will attempt to find my cam card and post the specs, i am curious to how they will compare to what you have said..i will look for them tomorrow and post up as soon as i find them.....................