QUESTION ON AVIATION GAS COMPATIBILITY for Duke and all

[Brad H.]

Guys,
I know the avgas discussion has gone on for years and this question has probably come up before and if so, I apologize. In the aviation community, it is fairly common belief that avgas has a different make-up than auto fuel for more than the obvious reasons of octane. I have heard forever that avgas is not compatible with auto fuel components such as carburetor and fuel pump systems due to avgas attacking rubber components in these parts. I have even seen a demo on automotive rubber fuel components soaked in avgas and swelling up acutely.

I have read on this site many times of avgas being commonly used in corvettes so apparently it does not pose a problem. Anyone have any ideas about this. If its a "myth" I would really like to find out before I use it.
Thanks,
Brad Hood

[Ronald L.]

Re: QUESTION ON AVIATION GAS COMPATIBILITY for Duke and all

The sweet smell of those benzene rings, they burn so well. Back in the day before they were carcinogenic we washed stuff down with the pure stuff.

In the old days the av gas just had a higher portion of cyclic hydrocarbons that when burned released more energy, you'd need to check the composition of today's fuels.

Alcohol when it was first introduced into fuels a short 28 years ago destroyed everything including swelling of seals that were never intended to be immersed in ROH mixtures.

[Mike M.]

Re: QUESTION ON AVIATION GAS COMPATIBILITY for Duke and all

i've been running avgas in my autocross vette, a 61 with LT5 driveline. no problems whatsoever in 7 years of its exclusive use on tracks. mike

[Duke W.]

Re: QUESTION ON AVIATION GAS COMPATIBILITY for Duke and all

More myths and misinformation. As stated by Ron, there were some issues with fuel system elastomers when ethanol was first introduced in gasoline, which required upgrades in materials. That was 30 years ago, circa 1980.

100 LL avgas has no ethanol - just hydrocarbons with a little TEL and the usual stabilizers and detergents.

It will cause no harm to a fuel system that is in good condition including elastomers that are not beyond their useful life. They don't last forever, but the world is full of guys who put something different in their fuel tank, then a 30 year old fuel hose breaks a week later and they blame on whatever they added to their fuel. Yeah, right!

The primary benefit of aromatics, like benzene, is octane rating. They didn't significantly increase the energy content of the fuel or change the nominal A/F ratios for various operating conditions. Both avgas and mogas have an average C/H ratio of 8/18.

Aromatics have been mostly eliminated from mogas because many are considered to be carcinogenic.

Duke

[Ronald L.]

Re: QUESTION ON AVIATION GAS COMPATIBILITY for Duke and all

Duke - you are going to push me to go get that Ind Chem class book in the basement!

[Duke W.]

Re: QUESTION ON AVIATION GAS COMPATIBILITY for Duke and all

If you disagress with me and have a source - be my guest.

But I will probably counter your argument with data from Obert's IC engine textbook that lists energy values for various fuels available in the fifties and sixties, and both avgas and mogas from that era had substantial aromatic content.

Duke

[Brad H.]

Re: QUESTION ON AVIATION GAS COMPATIBILITY for Duke and all

Thanks guys,
I assumed it was myth and appreciate Duke's commentary on the subject. I feel better about blending it now. I was by no means offering this question as information as I suspected it to be myth. By the way Duke, I really enjoyed your article in the Restorer I just recieved in the mail.
Brad

[Ronald L.]

Re: QUESTION ON AVIATION GAS COMPATIBILITY for Duke and all

Duke, not on either point, what was, was and still is, octane, second when you calculate the energy to break the bonds at the molecular level and turn BTX into Co2 and H2o, those molecules fall short. The book will tell me what I forgot which is the why to both points, I remember doing these calc's more that 30 years ago. Probably a lack of hydrogen atoms on the ring...

[Duke W.]

Re: QUESTION ON AVIATION GAS COMPATIBILITY for Duke and all

The heating value of the fuel is net of the energy required to break the original bonds of the specfic molecule.

From Taylor (rather than Obert) the lower heating value of benzene is 17,270 BTU/lb, iso-octane is 19,080 and gasoline is listed at 18,900.

Duke

[Duke W.]

Re: QUESTION ON AVIATION GAS COMPATIBILITY for Duke and all

Thanks guys,
By the way Duke, I really enjoyed your article in the Restorer I just recieved in the mail.
Brad

Thanks. There have been several comments, both here and via e-mail that the readers liked the article.

The real question is does everyone understand it. I expected some questions, but so far there have been none.

My results blew out some commonly accepted "truths" going back decades, so I expected some questions.

Duke

[Joe L.]

Re: QUESTION ON AVIATION GAS COMPATIBILITY for Duke and all

Thanks guys,
I assumed it was myth and appreciate Duke's commentary on the subject. I feel better about blending it now. I was by no means offering this question as information as I suspected it to be myth. By the way Duke, I really enjoyed your article in the Restorer I just recieved in the mail.
Brad

Brad------


There are only two reasons to use avgas in a street car. First, it can provide more octane than you can get in generally available gasoline on the street. That's only a benefit if you need more octane than is currently available in commonly available gasoline. Most Corvettes don't need more octane, including most of those built prior to 1971. Any built from 1971 onward should not require more octane.

The second reason is to provide exhaust valve seat protection. The vast majority of Corvettes do not need any such protection. 1971 and later absolutely do not require it. 1970 and earlier really only need it if the engine is operated at high power levels for extended periods of time. Virtually no Corvette engine falls into this category.

So, avgas will provide the above benefits only if one requires those benefits and few Corvettes do. On the negative side of the coin, avgas causes many detriments. The scavengers which must be used in leaded fuel are corrosive. They will, over time, cause damage to most of the parts with which they come in contact. Leaded fuel will also increase substantially combustion chamber deposits. That, in turn, leads to ever-increasing octane requirements.

I will repeat something I've mentioned many times before: the late John Lingenfelter once said that unleaded gas was the best thing that ever happened for performance. He was right.

By the way, leaded fuel might disappear from the avgas market before too long. There are "environmental pressures" being brought to bear. More ominous is the fact that there is currently only one manufacturer in the world that continues to produce tetraethyl lead. For a number of reasons, including the extremely small demand and the extreme toxicity of the chemical, they "want out".

[Brad H.]

Re: QUESTION ON AVIATION GAS COMPATIBILITY for Duke and all

Thanks Joe,
I am using a fairly modified 350. Stock looking from the outside, it has been built to produce 400hp and does, at least on the engine dyno. I occasionally get what I believe is spark knock under rapid heavy acceleration. I was considering using an avgas/mogas blend to see if that would help. I rarely hot-rod the engine, but it would be interesting to me to see if the avgas made a difference here.
You are right that avgas is under attack and is likely to be gone in the future. There are several likely candidates producing alternative fuels for the aviation market. GAMI in Oklahoma has produced an alternative and is currently under heavy testing and will be evaluated by the FAA soon. So far it is able to sustain the same margins and operating properties as avgas without the lead. Aircraft engine manufacturers are currently producing FADEC power management systems for new engines and retrofit systems for existing engines to offset the detrimental effects of no lead fuel by computer control of mixture and timing. The cost of these systems though, could really deal a fatal blow to general aviation.
Thanks for your continued help and advice, I really appreciate it!
Brad

[Don H.]

Re: QUESTION ON AVIATION GAS COMPATIBILITY for Duke and all

Another benefit of Avgas is the life span, pump gas will go bad much faster. I have ran it in my 1960 FI for over 30 years and the fuel bowl stays "clean as a whistle." Also fuel percolation (sp?) when re-starting in hot weather (common FI problem) is much more managable - ask John D. Don H.

[Clem Z.]

Re: QUESTION ON AVIATION GAS COMPATIBILITY for Duke and all

before the advent of "race gas" we always used avgas for dyno testing because it was controlled by fed regs and you knew that your back to back tests results would not be screwed up by a difference in the gasoline.

[Valeria H.]

Re: QUESTION ON AVIATION GAS COMPATIBILITY for Duke and all

Another benefit of Avgas is the life span, pump gas will go bad much faster. I have ran it in my 1960 FI for over 30 years and the fuel bowl stays "clean as a whistle." Also fuel percolation (sp?) when re-starting in hot weather (common FI problem) is much more managable - ask John D. Don H.

That is one undenable advantage to avgas (which I Love whole heartably). Even MW had to admit that ethanol gasoline mixtures are really not optimal for early FI engines!

Only time will tell the truth about the old vintage cars and the effects of ethanol on the engine, etc. I can only hope to be there when they state the extreme negative effects of ethanol on our old and beloved engines!

I am staying on the conservative side of this debate and of course throwing my money away towards avgas.

The discussion about this is not a done deal or over. Time is truly the test. We shall see who is right in the end. I'm a waitin!
Valeria