Lowering Gas Octane

[Vila]

OK, I have two gas stations in my area that sell Ethanol free gas, but they only sell 91 or 92 octane. They are not real close, but near enough that if I fill up on those occasion when I am in the area I can keep the cars running on it.

My issue is I would like to use it in my 1933 Chevrolet, but I should be using lower octane gas in this car. I don't want to do any Voodoo chemistry, but are there any professional petroleum engineers on the forum who can tell me what I could put in gas to LOWER the octane rating?

It works great all my other cars.

For example my 2002 BMW Z3 is supposed to use 92 octane and I get 2 miles per gallon better gas mileage with the Ethanol free gas. I have always checked to gas mileage for this car with each fill up by pressing the trip odometer just after I fill up and reading the miles at the next fill up and dividing by the gallons I put in to drive those miles. The cost of the Ethanol free gas is more, but the car runs better and with the extra miles I get per gallon the cost per mile works out to the same cost.

Visit my website at: Bob's Vintage Cars

Vila

1933 Chevrolet Master Coupe

1962 Triumph TR4

1984 BMW 633 CSi

[Rusty_OToole]

This has been discussed before. Others have had success in adding heavy ends to the gasoline. By heavy ends, I mean kerosene, stove oil, diesel fuel or furnace oil.

They report easier starting, more power, cooler running, and less vapor lock problems. 10% to 25%. Diesel fuel up to 10% will not cause smoking but might make the exhaust smell "funny". Kerosene or stove oil are better especially if you are going to use 25%.

As a general rule the octane should look like the compression ratio. Today's performance cars with 9:1 compression need 90 octane. Your old Chev with 5:1 compression needs 50 octane. This is not an exact ratio, but is fairly close. There are other factors that enter into it like combustion chamber design, engine speed, ignition timing etc. The main point is the old, low compression engines run better on lower octane fuel.

If you do a search you should find several in depth discussions.

PS a member who used a 1932 Buick as a tour car in the seventies, reported that he drove thousands of miles on 75% gas, 25% kerosene. The car ran better, smoother, cooler, and got better mileage. I believe the Buick had 4.5:1 compression.

Edited May 18, 2013 by Rusty_OToole (see edit history)

[Jim Bollman]

Maybe I have just been lucky but I have been running the 90+ octane Ethanol free gas in my 50 flathead Ford and it runs great without any doctoring. I dump about 4oz of Marvel Mystery oil in when I think about it and that is it.

I aways thought using higher octane than you needed was just wasting money not that it would not run ok.

[Marty Roth]

I agree with the addition of Diesel at 10 to 20%, and have found that it helps, especially with vapor lock and high altitudes (we live slightly below Sea Level here in New Orleans)

[DavidMc]

I aways thought using higher octane than you needed was just wasting money not that it would not run ok.

This comment by Jim Bollman has always been my understanding, I believe octane rating is irrelevant to the performance of very low compression cars. I have tried the high octane fuels to see if they are less prone to vapour lock but I have never detected any difference in power or economy between the various fuels or for that matter fuel with up to 20% kerosene or diesel in my 1902's Packard.

[DavidMc]

That should read 1920's Packard!

[Guest Bob Call]

Octane is related to compression in that increased octane prevents fuel detonation, or knock, which occurs prior to the ignition spark because of the high compression, like a diesel. Increasing the octane the fuel ignites with the spark and burns rather than explodes. Using a fuel with a higher octane than your compression requires only results in your spending more for fuel than necessary. If you don't have a vapor lock problem there is no need to do anything with the fuel. If you are using ethanol free gasoline and have a vapor lock problem you may want to try mixing in some diesel or kerosene, both more expensive than gasoline, to lower the boiling point.