Aviation "Racing" fuel dangers to the engine?

[Michael K.]

I have a '64 365hp with carb. Locally I can purchase 91 octane ethanol free gasoline $4.00/gal., 106 octane Avaiation fuel which has some lead in it $5.89/gal. or as one station advertises 115 octane "Racing" fuel $9.89/gal (ouch!). Will the aviation fuel pros outway the cons? I have been running the non-ethanol mixed with the avaition which probably brings the octane to the 100 mark, is ethanol free and has some lead. I also add Marvel Mystery to every 2-3 tanks of gas. She has AC-Delco R44 plugs. 1) Does this mix cause the engine to run hotter and cause damage to the valves or possibly burn holes in the pistons? I have not had any cooling system overheating problems. 2) Will I get equal performance (starting ease, smooth running) and and engine protection by saving a few bucks and running straight 91 non-ethanol as the ethanol/aviation mix? 3) Would running straight aviation be worth the extra $1.89/gal providing improved performance and engine protection?
What's with the racing fuel? I see people pumping that into motorcycles and muscle cars.
Thanks very much. Mike

[Michael W.]

Re: Aviation "Racing" fuel dangers to the engine?

What's with the racing fuel? I see people pumping that into motorcycles and muscle cars.
Thanks very much. Mike

I'd say 95% of people don't understand fuel basics and have bought into all the myths and misconceptions. Maybe I'm wrong, it might be 99% and not 95%.

There's no need or advantage to running avgas, racing gas, or dumping miracle in a bottle additives in any vehicle that runs as per design on straight pump gas. Tell us what's wrong with the way your car runs and someone here can help you fix it. It should run perfectly on straight 91, ethanol mix or not.

[Gene M.]

Re: Aviation "Racing" fuel dangers to the engine?

Mike,
Wow 106 octane Aviation fuel, the airport near me only has 100 octane and the #@*$ politicans in NY cause us to pay $5.99 a gallon.

Corn juice has no place in a chrome bumper vette........

[Larry E.]

Re: Aviation "Racing" fuel dangers to the engine?

Our local airport sells 100LL octane low lead. Cars run perfect on it. But the main reason to use av gas is not the octane but
the long lasting ability not to sour. It never turns bad(Within Reason) I'm told. Larry

[David K.]

[James B.]

Re: Aviation "Racing" fuel dangers to the engine?

Tell us what's wrong with the way your car runs and someone here can help you fix it. It should run perfectly on straight 91, ethanol mix or not.

Michael W- help me understand your statement. I have a 64 and 67, both with original 11:1 compression. I'm not too worried about valve beat in from unleaded fuel but I have always been told that you will have detonation (you can't always hear it so car may appear to run great) if you don't have higher octane than our local 93. There is a very well regarded engine builder in our area ( built a 427/425 for me several years ago) who refuses to built a motor with anything higher than 9.75:1 CR. I drove the 67 to B Gold last month (300+ miles). It ran fine on pump gas but again, I am not sure I would be aware of detonation. Is there some resource you can direct me to that can give more detail/ test results etc. I'm going to start a frame off on the 67 next month and will go through the engine as a part of that. I would love to go back with domed pistons and retain 11:1 but have assumed I would have to do flat tops. Regards, Jim B

[Jim T.]

Re: Aviation "Racing" fuel dangers to the engine?

My 1968 with the original engine the factory installed, a 327/350 that has not been changed and having 11:1 still using original L79 cam. All I use for fuel is what is available at the local gas pump, 93 no lead and the pump has a sticker stating all gas contains ethanol. The 68 runs really great and has over 96k on it now. I work on the local airport and have never used 100LL aviation fuel. Talk to the refueling FBO personnel every work day.

[Ron G.]

Re: Aviation "Racing" fuel dangers to the engine?

I am no expert on fuel type, but the reason I use racing fuel even for just start up purposes is that I am of the understanding that whatever is remaining evaporates and does not clog or build up in the lines and carb. Can someone with more knowledge on this confirm. Thanks.

[Michael W.]

Re: Aviation "Racing" fuel dangers to the engine?

Michael W- help me understand your statement. I have a 64 and 67, both with original 11:1 compression. I'm not too worried about valve beat in from unleaded fuel but I have always been told that you will have detonation (you can't always hear it so car may appear to run great) if you don't have higher octane than our local 93. There is a very well regarded engine builder in our area ( built a 427/425 for me several years ago) who refuses to built a motor with anything higher than 9.75:1 CR. I drove the 67 to B Gold last month (300+ miles). It ran fine on pump gas but again, I am not sure I would be aware of detonation. Is there some resource you can direct me to that can give more detail/ test results etc. I'm going to start a frame off on the 67 next month and will go through the engine as a part of that. I would love to go back with domed pistons and retain 11:1 but have assumed I would have to do flat tops. Regards, Jim B

Jim B-

Jim T's answer below is just one of many 'first person' testimonials on this site indicating that pump gas is just fine.

My 1968 with the original engine the factory installed, a 327/350 that has not been changed and having 11:1 still using original L79 cam. All I use for fuel is what is available at the local gas pump, 93 no lead and the pump has a sticker stating all gas contains ethanol. The 68 runs really great and has over 96k on it now. I work on the local airport and have never used 100LL aviation fuel. Talk to the refueling FBO personnel every work day.

I am no expert on fuel type, but the reason I use racing fuel even for just start up purposes is that I am of the understanding that whatever is remaining evaporates and does not clog or build up in the lines and carb. Can someone with more knowledge on this confirm. Thanks.

My car sits untouched over the long Canadian winter with straight pump gas in it. No 'stabil' or similar. Starts first crank in the spring.

[Bob J.]

Re: Aviation "Racing" fuel dangers to the engine?

Mike,
Wow 106 octane Aviation fuel, the airport near me only has 100 octane and the #@*$ politicans in NY cause us to pay $5.99 a gallon.

Corn juice has no place in a chrome bumper vette........

Ditto......only 100LL in FLA also.

[Bruce B.]

Re: Aviation "Racing" fuel dangers to the engine?

My 62 340HP with 11.25 : 1 compression runs great on Shell 93 0ctane with ethanol.
After 20,000 + miles no problems.
I also use the Shell 93 in my 57 FI car with a 4360 fuel unit, it starts and runs fine cold, always starts when it is hot and runs perfectly.
I did break in the engine with 100LL but since I drive the car a lot the Shell is available when I need it.
I never use any additives, lead, octane boost are all a waste of $$$$ in my opinion.

[Pat M.]

Re: Aviation "Racing" fuel dangers to the engine?

My 70's original 350/300 engine was "rebuilt" with all of its original parts, and it also runs perfectly on 93 octane 10% ethanol gas available at all of my local stations.

[Joe C.]

Re: Aviation "Racing" fuel dangers to the engine?

Michael W- help me understand your statement. I have a 64 and 67, both with original 11:1 compression. I'm not too worried about valve beat in from unleaded fuel but I have always been told that you will have detonation (you can't always hear it so car may appear to run great) if you don't have higher octane than our local 93. There is a very well regarded engine builder in our area ( built a 427/425 for me several years ago) who refuses to built a motor with anything higher than 9.75:1 CR. I drove the 67 to B Gold last month (300+ miles). It ran fine on pump gas but again, I am not sure I would be aware of detonation. Is there some resource you can direct me to that can give more detail/ test results etc. I'm going to start a frame off on the 67 next month and will go through the engine as a part of that. I would love to go back with domed pistons and retain 11:1 but have assumed I would have to do flat tops. Regards, Jim B

This is one of those topics that galvanizes people in opposite directions, and no amount of reasoned argument will make a lick of difference. In any case, here are some facts:

1. If you have reasonably quiet exhaust, and you can't hear detonation (considering you have average hearing acuity), then any inaudible detonation, if present, will absolutely do no harm, whatsoever, to your pistons and/or rings.
2. If you want to make absolutely sure that the engine is not severely detonating in a situation where the exhaust is very loud, then a chassis dyno will tell the tale. Advancing the spark timing to the point of power drop-off (indicative of detonation), and then retarding until power increases will yield optimal engine performance.
3. If an engine "runs fine" in no way indicates that it's running optimally.
4. All engines are built with varying tolerances and thus varying SCR, and although they will all "run fine" with the same tune, they will all respond differently to tailored tuning.
5. The engine builder whom you mention should go back to fixing vacuum cleaners. Steer clear of him.

[James B.]

Re: Aviation "Racing" fuel dangers to the engine?

You may be right but I question a few examples of anecdotal info. How many people really drive these cars, take them to redline and drive more than 200 miles per year. Also, how many people with "original engines" really have the CR they think they do. I am sure many now have composite and not steel gaskets and have flat top pistons through older rebuilds. I just don't want to take any chances when I rebuild L79 & or L84 and have been told for 40+ years, I've got to have lower CR with 60s electronics.

[Joe C.]

Re: Aviation "Racing" fuel dangers to the engine?

You may be right but I question a few examples of anecdotal info. How many people really drive these cars, take them to redline and drive more than 200 miles per year. Also, how many people with "original engines" really have the CR they think they do. I am sure many now have composite and not steel gaskets and have flat top pistons through older rebuilds. I just don't want to take any chances when I rebuild L79 & or L84 and have been told for 40+ years, I've got to have lower CR with 60s electronics.

I have an engine with honest, measured 11.65:1 average (plus/minus .07 across all 8) static compression with a cam whose intake valves close @ 72 degrees ABDC (about 10 degrees EARLIER than those of the 30-30 cam) run pump 93 octane with a very aggressive spark advance program (20 degrees initial, 16 degrees centrifugal, all in by 2200 RPM, plus 14 degrees vacuum advance), with no detonation. I routinely take it to 7250 and occasionally to 7500 RPM. Although the power has dropped off from the peak @ 6500, I occasionally make a boo boo and take it to 8000 or so. The rev limiter is set to 8250.