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Author Topic:   Comp Ratio
theslowguy
Dirt Full Roller

Total posts: 30
posted October 09, 2005 05:09 AM  
I was wondering if 12.5:1 is too much for 91 octane. Running aluminum heads and hypertechtic pistons. Talked to the Dart tech guy and he reccomended 64cc heads for my 406 and that comes to 12.5:1. This just seems rather high for 91 octane. Any info will be greatly appreciated. Thanks.

eenfield
Dirt Maniac

Total posts: 116
posted October 09, 2005 07:38 AM  
With carefull tuning you MAY be able to get away with it in a drag car... No way in a dirt track car. Too much sustained load, heat and RPM.

J3
Dirt Maniac

Total posts: 188
posted October 10, 2005 03:36 PM  
About the best you can get by with 91 octane is 11-1 CR. Any higher than that, and you need higher octane.


theslowguy
Dirt Full Roller

Total posts: 30
posted October 12, 2005 06:34 AM  
With aluminum heads i've heard that you can run a full point higher comp than with cast. Is there any truth to that? Is 11:1 the most you can run with cast heads or the max for aluminum? This is on a street car.

nine finger
Dirt Forum Racer

Total posts: 76
posted October 12, 2005 12:28 PM  
NO !!!!!!!!!!!

Normal Sullivan
Dirt Full Roller

Total posts: 39
posted October 12, 2005 11:01 PM  
NO WHAT?

theslowguy
Dirt Full Roller

Total posts: 30
posted October 14, 2005 11:05 AM  
Ummmmm, ok then.

DJR22
Dirt Maniac

Total posts: 111
posted October 14, 2005 12:39 PM  
I've read many times that you can run lower grade gas with higher comp ratios on aluminum heads because the better heat dissipation reduces the amount of pre-ignition. This may only apply to street/strip applications though where the load on the motor is not sustained so long.

------------------
-stand on it-

A. Bighurry
Dirt Full Roller

Total posts: 35
posted October 14, 2005 10:37 PM  
On a street car with aluminum heads you might get away with it. The combustion chamber won't hold as much heat as it would with cast iron. You can also back the timing off if needed. With mufflers you should be able to hear detonation under load. Another alternative is to see if any gas stations in your area sell the E85 fuel (85% ethanol, 15% pump gas). It has a 110 octane rating and costs around $2.00 a gallon. You'll need a fuel system specifically set up for it (bigger pump, lines, and calibrated carburetor) and you'll use more fuel, but it's pretty neat stuff. Supply and demand will cause petroleum fuel costs to rise in the future, so setting up with this now will save money in the long run. And it's something different.

nine finger
Dirt Forum Racer

Total posts: 76
posted October 15, 2005 08:12 AM  
i meant no as in its gonna blow,if you can keep the head gaskets on it long enough

regal 22
Dirt Full Roller

Total posts: 28
posted October 17, 2005 06:14 PM  
what octane will work for 12 to 1

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