octane

with a 8.7comp, 1-1 blower drive,25degs total timing,what is the minium octane fuel that should be used

hard to say . how big is the blower. and how big is the engine. how much boost????????

hi pat, the blower is 4-71 with 7lbs boost on 0.080 over bored 225slantsix

you would need to know the cid of the blower is will help and you can go from there .i have a chart at my shop . you mite need to run c16 fuel or better. you need to know the true size of the engine now .i do not know the bore and stroke of your 6. but this may help .7854xno of cylinders x stroke xborexbore this will tell the size it is now add 0.080 to the stock bore and go from there

i think cid work out to 235 with a 0.080 over bore

Interpolating from the attached BDS chart, your effective compression ratio would be about 12.7 - 12.8, on the ragged edge for fresh 93 octane pump gas.

A tight squish, polished chambers and a long cam might get you by in my opinion. Pat has more experience than I have, so follow his lead.

http://www.blowerdriveservice.com/techcharts.php

thanks for the link,so at 13/1comp would a mix of supreme and race to get 96octane work?

try VP racing fuel try c110 but i wound run race fuels so you know you have the true octane pump gas can very and you do not want to smoke it. i have played with cutting fuels. air plane fuel 100LL low lead is what i ran in my cars. if there is airport by you try some i had good luck with it and it saids it has some lead in it not bad .

thanks, i did run a 2/1 mix of av gas and supreme in my 4-53 blown engine with no trouble,the 4-71blown engine i used 1/1 race fuel and supreme but i had a lot of other problems, so i am not sure what to use next season.

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Originally posted by terrylittlejohn
thanks, i did run a 2/1 mix of av gas and supreme in my 4-53 blown engine with no trouble,the 4-71blown engine i used 1/1 race fuel and supreme but i had a lot of other problems, so i am not sure what to use next season.

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why not run full races fuel your engine?? in boost it needs it why screw a round and burn it down you have over 12to 1 and the blower makes heat so i can see why you would not run 110 race fuel in it pump gas octane is not a shure thing it could be 94 or less i would bet on less . and i would not bet on this for a blower engine:whacked:

hi pat, i might have to run straight race fuel but up here it cost 9.00$ per gallon so most of the racer try a blend of race fuel or avgas with suprene to kept cost down. I know its could be a gamble with parts,but i did find a good mix with my smaller blown engine just hoping to do the same with the larger blower.

try running just avgas it has been cheaper hear than races fuels

yes it is cheaper here to,probably the best bet for budget racer

yes:)

Interesting subject.
Here's a thread I stole from the Cobra web site:

If your engine is properly tuned for use on pump gas, and you switch to TRUE aviation fuel, you can be nearly assured that you will detonate and potentially cause serious harm to the engine!

Please allow me to explain in detail why this is a problem:

An internal combustion engine's fuel requirement is determined by its Brake Specific Fuel Consumption (BSFC). By definition, the BSFC is the amount of fuel required per horsepower to properly run an engine. As a rough rule of thumb, most moderately built naturally aspirated street engines have a BSFC of about .5. This means that the engine requires .5 lbs/hr of fuel per horsepower. So if you have a 400 horsepower engine, the formula would be as follows: 400 x .5 = 200 lbs/hr fuel requirement. Notice that this is figured as a weight of fuel and NOT as a volume of fuel!!

The specific gravity of any element is determined in relationship to the weight of 1 gallon of pure water which is about 8.33 lbs. “Pump gas” has a specific gravity of about .72. To determine its weight, do the following: 8.33 x .72 = 5.99 lbs per gallon. (Technically, gasoline is rated at 6.009 lbs @ 72 degrees Fahrenheit).

The specific gravity of “Av-Gas” is much lighter than pump gas! It usually has a specific gravity of about .61 to .64 and weighs around 5.3 lbs per gallon. So you can see that the same volume of Av-Gas weighs dramatically LESS than “Pump-Gas!!” Remember, your engine needs a certain WEIGHT of fuel, NOT a volume of fuel. If you switch from “Pump-Gas” to “Av-Gas” and you do not change the jetting or injector size to compensate for the fact that the weight of fuel being ingested by the engine has been substantially reduced, YOU WILL RUN THE ENGINE LEAN!!

Let’s go back over that formula from the beginning of this Nerd-Like reply:

A 400 horsepower engine running on gasoline has a .5 BSFC.
400 x .5 = 200 lbs/hr fuel requirement
If “Pump-Gas” weighs 6 lbs per gallon, the engine needs:
200 / 6 = 33.33 gallons per hour.
But if you are running “Av-Gas” that only weighs 5.3 lbs per gallon and you do not make any changes to the jets or injectors, you will continue to receive the same VOLUME of fuel but will run lean determined by the following:
33.33 x 5.3 = 176.6 lbs of fuel

Now we go back to the 400 horsepower engine with the .5 BSFC requirement. It needs 200 lbs/hr, but it is only receiving 177 lbs/hr. So your actual BSFC ends up being:
177 / 400 = .44 BSFC

So now your engine that requires a .5 BSFC is only getting a .44 BSFC. This is very dangerous! I have heard claims by some that when they put “Av-Gas” in their engine, it makes more power—and this is very possible; A lean engine WILL make more power because “Heat is Horsepower.” But just remember:

TOO LEAN, TOO LONG, TOO HOT, TOO BAD!!!

Sorry if this reply is more detailed than necessary. It’s just that I have had an extensive amount of experience with this subject and I felt that this would be an opportunity to hopefully clear the air of some of the many myths, half truths, and sometimes just plain old wrong information. I hope that you realize with the extensive detail, you will see that I intend to EDUCATE rather than “brag” to people that I don’t know.

have used it in a 548 10 to 1 blown 10 of boost 548 14to8 bigblock a 468 12to1 462 11 to1 yes you need to tune it but they are not pump gas engine. on pump gas they would of bunt down for shure on pump gas. if it not more then 10to1 you dont need much octane makes the burn slow and makes heat the more octane the more you can squeeze it so i think he will be ok i have done it many times but reall race fuel would be better so yes it will need to be tune in

I agree Pat, it should be OK when properly tuned. But, keep an eye out for detonation.
I remember a guy from the old neighborhood (east side of Detroit) that had an early Max Wedge Dodge. I seem to remember it ran 13.5:1 compression from the factory! He used to mix half AV gas & half Sunoco 260.. Ran great on that mixture.

no need to appoligize ,any imfo is good. so with avgas i will have to jet up to be on the rich side.is there any info on the benifits of the lead in avgas that helps reduse detonation or increase the octane number wether mixed or strait?

Good one Dyno. We love to ignore that 'theory' stuff about fuel charge DENSITY and volumetric effeciency but you explained it well.

Actually, I can't take the credit for that thread. I found it on the Club Cobra site. But, it was some very good reading.

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Originally posted by shevy not heme
Good one Dyno. We love to ignore that 'theory' stuff about fuel charge DENSITY and volumetric effeciency but you explained it well.

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may be you but not me .this was the first thing they hammer in are heads at mercury mercruiser school and that pump gas was not the true octane all the time. they where not going to buy any more engine for boats that hammer them self to death. have done many thing and tried them .on this post i will not give out bad info evey thing i post on i have done it many times .YES he will need to tune it in and if not done rigth it could blow it up i would trust avgas in a12 to1 before pump gas for the rigth octane rating

ON A 12.0-1 MOTOR NEEDS 110 OCTANE FUEL, AND NO ONE TALKED ABOUT A TIMING RETARD CONTROL FOR THE BOOST?

every engines is diferent .msd goses in to this if the pistons are dome or flat top how are the head chambers and more on big block i like to start at 36 and 10 on crank and 26 in the dist =36 like to see it all in at 2500 rpm that is the starting point i spend some time on the tops of the pistons i get them smooth like a old bar of soap and get all the junk out of the chambers of the heads and make shure the head gaskets fire ring is got hanging in the bore and cut the deck to O or the pistons down a bit but not to much if the head gaskets are 0.040 run it at0.005 or so