How much compression for 355 on 91 octane
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Registered User
Joined: Jan 2000
Posts: 731
From: Calgary, Ab, Canada
How much compression for 355 on 91 octane
Hey guys,
How much compression would you run on an lt1 motor with maybe a cc503 cam, 355" and perhaps minor work to the heads for use with 91 octane fuel.
I remember in the past reading that guys are doing 12:1 on 93, but fuel here isn't all that great, 91 is as good as it gets.
Keep in mind I might want to spray upto 125 shot some day. 11.3:1 or so?
Thanks.
How much compression would you run on an lt1 motor with maybe a cc503 cam, 355" and perhaps minor work to the heads for use with 91 octane fuel.
I remember in the past reading that guys are doing 12:1 on 93, but fuel here isn't all that great, 91 is as good as it gets.
Keep in mind I might want to spray upto 125 shot some day. 11.3:1 or so?
Thanks.
Registered User
Joined: Jun 2006
Posts: 196
From: Birmingham, AL
Google this: dynamic compression ratio calculator
They are easy to find. You'll need the IVC spec on your cam and various other dimensional specs from your engine combination. On regular pump gas I wouldn't push much past 8:1. You can conservatively go to about 8.5:1 dynamic compression on these motors. If you do like some geniuses recommend and push the DCR envelope you could well end up with something that you have to pull timing from and dump extra fuel into to quell detonation.
Details like setting up your quench clearance on the tight side ( .040" to .045") and blending the sharp edges on your piston domes help noticeably on octane requirements. Injectors with a balanced flow rate helps avoid a lean cylinder that rattles. The timing and fuel maps in your tune can also cause or resolve detonation issues if you're reasonable on DCR.
For a street car I want to build it once, have everything work out and forget about it. Staying conservative on the DCR might "cost" you a small amount of power compared to the max you could run. Problem is the only way to find out what the max is involves going past it and pulling heads or other parts to fix a too high compression ratio. Stay safe on this, you won't know the difference. If you go too high here you can get to a place where only pulling the heads or changing the pistons to drop the compression will let your run properly on pump gas. Guaranteed
My guess is if you get up around 8.5:1 DCR you might have to upgrade your fuel to midgrade or premium. No one can give you an exact number as there are too many variable for even a good computer program to be 100% accurate. Some people running the same cam might chime in but remember to ask if they can run low octane pump gas without significant knock retard.
Michael
They are easy to find. You'll need the IVC spec on your cam and various other dimensional specs from your engine combination. On regular pump gas I wouldn't push much past 8:1. You can conservatively go to about 8.5:1 dynamic compression on these motors. If you do like some geniuses recommend and push the DCR envelope you could well end up with something that you have to pull timing from and dump extra fuel into to quell detonation.
Details like setting up your quench clearance on the tight side ( .040" to .045") and blending the sharp edges on your piston domes help noticeably on octane requirements. Injectors with a balanced flow rate helps avoid a lean cylinder that rattles. The timing and fuel maps in your tune can also cause or resolve detonation issues if you're reasonable on DCR.
For a street car I want to build it once, have everything work out and forget about it. Staying conservative on the DCR might "cost" you a small amount of power compared to the max you could run. Problem is the only way to find out what the max is involves going past it and pulling heads or other parts to fix a too high compression ratio. Stay safe on this, you won't know the difference. If you go too high here you can get to a place where only pulling the heads or changing the pistons to drop the compression will let your run properly on pump gas. Guaranteed
My guess is if you get up around 8.5:1 DCR you might have to upgrade your fuel to midgrade or premium. No one can give you an exact number as there are too many variable for even a good computer program to be 100% accurate. Some people running the same cam might chime in but remember to ask if they can run low octane pump gas without significant knock retard.
Michael
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