octane

[terrylittlejohn]

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

[pat mccarthy]

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

[terrylittlejohn]

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

[pat mccarthy]

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

[terrylittlejohn]

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

[techinspector1]

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

[terrylittlejohn]

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

[pat mccarthy]

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 .

[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.

[pat mccarthy]

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.

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

[terrylittlejohn]

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.

[pat mccarthy]

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

[terrylittlejohn]

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

[pat mccarthy]

yes

[DynoDon]

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.