8:1 or 9:1 comp
8:1 or 9:1 comp F'd up a piston and a sping in the heaad.Right now I have 8:1 compression with d1 procharger 15 lbs of boost and 75 shot nitrous, gettin higher flowing heads and the same piston,? or should I go 9:1 compression with stronger heads (internals)?
I would say 9.0:1 if not a little bit higher for a blower car. Just beef up the internals and you should be good to go. On 8.0:1 you'd have to run alot more boost to make the power you could on a 9.0:1 CR engine. Just my thoughts though.
One extra lb of boost will more than make up the 1 point cr difference. Plus the 8/1 lets you run a lot more than 1 extra lb at the same timing as 9/1. Only reason to bump the compression up is fuel economy or off boost throttle response. CR usually amounts to about 3-4% per point power difference. Boost is 7% minus whatever it takes to drive the blower, so at lest 5-6% per lb. Obviously these are general guidelines that can vary considerably.
Originally Posted by gametech
One extra lb of boost will more than make up the 1 point cr difference. Plus the 8/1 lets you run a lot more than 1 extra lb at the same timing as 9/1. Only reason to bump the compression up is fuel economy or off boost throttle response. CR usually amounts to about 3-4% per point power difference. Boost is 7% minus whatever it takes to drive the blower, so at lest 5-6% per lb. Obviously these are general guidelines that can vary considerably.
the only reason to run 9:1 is if you know you wont ever have a bigger blower than a D1sc.... i plan on running a larger one down the road so i went with 8:1
i should have some track times this weekend...
I vote 8:1.
I run 8:1 @ 17lbs on the street on pump gas. I have done this for quite a while and it is very reliable. The nice part is that I can turn it up further (ie bigger blower) if I ever chose to and not have a problem.
I originally ran 8.75:1 (something like that) when I was ironing out the setup. I changed heads and went to 8:1 compression. There was no difference in SOTP low end. FWIW
I run 8:1 @ 17lbs on the street on pump gas. I have done this for quite a while and it is very reliable. The nice part is that I can turn it up further (ie bigger blower) if I ever chose to and not have a problem.
I originally ran 8.75:1 (something like that) when I was ironing out the setup. I changed heads and went to 8:1 compression. There was no difference in SOTP low end. FWIW
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Originally Posted by Pro Stock John
Keep what you have unless you are wealthy. Blower + nitrous = an aggressive combo.
We build our ATI ProCharger street/strip engines for 9.0/1 to 9.5/1, and run them at 14 to 16 psi on 93 octane without any problems. The higher compression ratio will give you good fuel economy if you are still on the streets with your car. The 700+ rwhp D-1SC equipped Corvette we recently finished still gets 28 mpg, and this customer recently drove this car from Chicago to Virginia on a vacation with his daughter. Bob
TECH Apprentice
Joined: Jul 2004
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It’s not really that simple… like with an NA combination you want to make sure that your setup is well matched. Just like NA you’d probably want to run a more mild cam with less compression and what will ultimately end up “right” will depend on your intended powerband, how the car will be used, fuel octane, the rest of the engine parts that you’re matching to… It is possible to end up with a combination at 9:1, with a lower DCR than at 8:1 and have the 9:1 combination be less detonation sensitive then the lower compression setup.
