Fuel Question For Duke

[Brad H.]

Duke,
Not long ago there was a very good series of threads concerning fuels and lead additives/octane ratings. In these discussions, I believe it was mentioned that avgas mixed with premium grade pump gas was an optimum fuel for older performance engines. I hanger my 69 at an airport so avgas is readily available. My engine starts, idles, and runs remarkably better when avgas is added to the tank. My question is this: Is it actually best to mix avgas and pump gas or would using just avgas actually be best with the exception of price? I don't drive the car much, so cost isn't that big a deal especially considering how much better the engine runs. I would really appreciate your take on whether to run avgas or to mix it with pump gas, disregarding pollution issues due to lead, etc.
Thanks,
Brad

[Rich W.]

Re: Fuel Question For Duke

Brad I am curious about this one also..I don't know much about avgas but have heard that it has in some parts of the country ethanol in it..I run turbo blue in mine its 7.50$ per gallon..but like you said it does run much better than the other gas..I put about 5 gallons in per tank and run the rest 93 octane I get from a farmers type gas station that doesn't have ethanol..only 10 cents more per gallon..I thought the avgas was a good high octane fuel..can't wait for the answer on this one! Come on Duke!

[Michael W.]

Re: Fuel Question For Duke

.can't wait for the answer on this one! Come on Duke!

Me too.

You might want to search the archives on this subject, it's been beaten to death multiple times.

[Rich W.]

Re: Fuel Question For Duke

No dought Mike..I haven't seen much on avgas..just all the other stuff that gets a little repetitive!

[Clem Z.]

Re: Fuel Question For Duke

Duke,
Not long ago there was a very good series of threads concerning fuels and lead additives/octane ratings. In these discussions, I believe it was mentioned that avgas mixed with premium grade pump gas was an optimum fuel for older performance engines. I hanger my 69 at an airport so avgas is readily available. My engine starts, idles, and runs remarkably better when avgas is added to the tank. My question is this: Is it actually best to mix avgas and pump gas or would using just avgas actually be best with the exception of price? I don't drive the car much, so cost isn't that big a deal especially considering how much better the engine runs. I would really appreciate your take on whether to run avgas or to mix it with pump gas, disregarding pollution issues due to lead, etc.
Thanks,
Brad

mix it as 100LL avgas is 106 octane the way they rate car gas so i would mix it 50/50.

[Duke W.]

Re: Fuel Question For Duke

Avgas has NO ETHANOL, and the 100 Aviation Octane Rating of 100LL avgas is about equivalent to 104-105 PON mogas. It can be mixed with mogas in any proportion up to 100 percent in vintage Corvettes to quell detonation.

Since avgas also has higher vapor pressure than mogas, it can also cure or mitigate vapor lock, which is a problem on many vintage cars.

I have one friend who gets all the free avgas he wants, because whenever avgas is removed from an aircraft for whatever reason the same fuel cannot be reinstalled in any airplane. It has to be disposed of in some other way.

So if you can get friendly with a local aircraft service center, you might be able to get some free fuel.

Duke

[Michael W.]

Re: Fuel Question For Duke

Duke has restated the leading attributes of Avgas- higher octane rating for detonation issues and higher vapour pressure for cars afflicted with vapour lock problems.

I'm just wondering which of these two problems the OP has with his car. If it's 'neither' then the Avgas is pretty much a waste of money as would be 'race gas'.

[Duke W.]

Re: Fuel Question For Duke

mix it as 100LL avgas is 106 octane the way they rate car gas so i would mix it 50/50.

Not quite. The PON - "pump octane number" - what is currently posted on the gas station pump is the arithmetic average of the Motor and Research octane numbers, which typically vary by 8-10. This is called the fuel's "sensitivity".

RON was used in the sixties, PON has been in use since the seventies.

The Aviation Octane Rating Method uses a different test procedure than the Motor and Research methods for mogas, but the number is usually very close to MON.

So assuming a sensitivity of 8, 100LL avgas is about equal to 100 MON, 108 RON, and 104 PON, which is why any detonation in an OE engine can usually be tamed with no more than a 25 percent blend of avgas in pump premium.

If you feel you need more octane than available at the pump, you need to understand the different octane rating methods, how to approximately convert one to another, and what method is used to rate the special gasoline you are buying. It is is often not explicitly stated, so find out!

Duke

[Clem Z.]

Re: Fuel Question For Duke

Not quite. The PON - "pump octane number" - what is currently posted on the gas station pump is the arithmetic average of the Motor and Research octane numbers, which typically vary by 8-10. This is called the fuel's "sensitivity".

RON was used in the sixties, PON has been in use since the seventies.

The Aviation Octane Rating Method uses a different test procedure than the Motor and Research methods for mogas, but the number is usually very close to MON.

So assuming a sensitivity of 8, 100LL avgas is about equal to 100 MON, 108 RON, and 104 PON, which is why any detonation in an OE engine can usually be tamed with no more than a 25 percent blend of avgas in pump premium.

If you feel you need more octane than available at the pump, you need to understand the different octane rating methods, how to approximately convert one to another, and what method is used to rate the special gasoline you are buying. It is is often not explicitly stated, so find out!

Duke

before the availability of race gas most race engine builders used avgas for dyno testing because since it was govt speced it was more consistent than available pump gas sunoco 260

[Brad H.]

Re: Fuel Question For Duke

Duke,
I really appreciate the information. I don't have detonation problems, but the engine just starts, idles and runs signifigantly better when avgas is mixed with pump gas. I was just curious if cost was not an issue, if avgas would be the optimum fuel to use at 100%...just your opinion. I believe I understand from your post that any mix ratio works as well as avgas at 100%. I know you hate rehashing this stuff, but your expertise is invaluable and greatly appreciated from those of us who have not been doing this as long as most of you have.
Thanks again!
Brad

[Domenic T.]

Re: Fuel Question For Duke

Brad,
I have used avgas since 1971 and found that the 50/50 mix is the best.

I have used it 100% and actually think it runs better mixed.

DOM

[Donald T.]

Re: Fuel Question For Duke

Duke,
I really appreciate the information. I don't have detonation problems, but the engine just starts, idles and runs signifigantly better when avgas is mixed with pump gas. I was just curious if cost was not an issue, if avgas would be the optimum fuel to use at 100%...just your opinion. I believe I understand from your post that any mix ratio works as well as avgas at 100%. I know you hate rehashing this stuff, but your expertise is invaluable and greatly appreciated from those of us who have not been doing this as long as most of you have.
Thanks again!
Brad

Brad,

If you are not experiencing detonation on pump gas then more octane will not provide any benefit other than making your wallet ligher. Octane is nothing more than the fuel's resistance to detonation.

[Michael W.]

Re: Fuel Question For Duke

avgas would be the optimum fuel to use at 100%...just your opinion. I believe I understand from your post that any mix ratio works as well as avgas at 100%.

One downside of using leaded fuel is dealing with the formation of deposits in the combustion chamber, on valves and especially spark plug electrodes. Spark plug life is dramatically shortened as well.

The 'cost' of using avgas is not just for fuel itself.

[Clem Z.]

Re: Fuel Question For Duke

Brad,
I have used avgas since 1971 and found that the 50/50 mix is the best.

I have used it 100% and actually think it runs better mixed.

DOM

to use 100% 100 LL and to take it to the advantage it may give you need to retune your engine.

[Domenic T.]

Re: Fuel Question For Duke

Clem,
Funny you say that because I turned my distributor every change of fuel.

I tightened my distributor just enough that it would turn with grease on the gasket.

I would drive a few blocks and listen for the ping or lack of then adjust the distributor a few % .

There were differences in the 92 octane from station to station and I found that 92 at one station was not the same at another and again I would pop the hood and reach out from the door and turn the distributor.

As you may remember, the old timers would say to forget using a timing light, just get it to ping under load, then retard it enough to eliminate the ping and you are optimum.

DOM