11:1 compression ratio fuel?

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Joep

Joep

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Hey guys ive got a 440 in my 71 demon. According to the little sheet i have the compression ratio is 11:1. I don't really know that for a fact because i havent actually done the measurements and stuff but what i am curious about is what type of fuel should i run? I have read people being able to run 93 octane but i know its right on the edge. What type of fuel should i run?
 
Heads? Cam? Timing? Profile of the cam has every thing to do with what you can get away with compression wise...
 
Aluminum heads are said to allow running one ratio point higher, so an 11:1 CR would react like a 10:1 because of the better heat transfer. Also, dynamic CR counts too.
 
Heads : ported 906s
Cam: crower .509 hydraulic lifters
Pistons: forged speed pro .030 over
Timing was at 15 initially but I backed it down to 12 because it was stumbling and sputtering a little at 3000rpm and it sounded better that way with 93 octane
 
INK is right on the money. we need more details to determine what you need.
11 to 1 and a poor chamber design, poor cam selection and zero quench you may need 110 octane.
 
If it's 11:1 with that cam I think it might be ok on pump 93 as long as you can tune it. You'll want to look at the total timing and make sure it stays under 35°. You'll also want to make sure that it doesn't come in too quick, or start coming in too early. Then you can make sure the carb is tuned right, not just at idle, but jetting and power valve especially. It can be done, but you have to know how to tune it. Taking initial advance out is probably not the best idea... I'd guess it would like 16-18° initial, and the rest centrifigual, starting at 2K and all in by 3500.
 
how far in the hole were the pistons? what are the spec's on the cam? what did the heads CC at? depending on which speed pro's are they? was that initial timing?
 
I run 93 with 11.25 it has about 200 psi cranking . I would mix race gas at the track. What would help is a light car, hi stall, heat crossovers blocked, fresh/ram air, timing backed down a bit, and cold plugs.
 
I don't know a lot about the engine internally I'm just going what my uncle wrote down on the card.
 
well i would take a compression test, that will help right around 180 or below you should have no problem running 93 with a good tune, over 180 to 200 or 205 its harder. watch the plugs for black specs indication knock.
 
moper said:
If it's 11:1 with that cam I think it might be ok on pump 93 as long as you can tune it. You'll want to look at the total timing and make sure it stays under 35°. You'll also want to make sure that it doesn't come in too quick, or start coming in too early. Then you can make sure the carb is tuned right, not just at idle, but jetting and power valve especially. It can be done, but you have to know how to tune it. Taking initial advance out is probably not the best idea... I'd guess it would like 16-18° initial, and the rest centrifigual, starting at 2K and all in by 3500.
Click to expand...

Yes...

Not enough initial timing is the first thing I'd do. Probably closer to 20* The stumble was likely because it was so fat at idle to run that when it transitioned, you nailed a lean spot that was needed to try and clean up the dirty bottom end. Fix the mechanical advance and go from there.

That compression and cam may be able to run a 93 pump gas. It will be razor thin on the tune up to run it.
 
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