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aviation fuel


impacsys

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Aviation fuel is the worst gas you could burn in your car.

When your car was made, leaded gas was not invented. Typical gasoline was next to kerosene or lamp oil, and had an octane rating of about 50 or 55. Compare this to 87 octane for today's cheapest regular. Some owners of cars like yours, mix kerosene or diesel fuel in the gas to lower the octane especially in hot weather.

If your engine was in good shape and tuned up correctly it should start right up and run as smooth as silk. You need to figure out what is wrong and fix it. I would start with a compression test and if compression is within spec, go on to do a complete tuneup.

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Aviation fuel is the worst gas you could burn in your car.

When your car was made, leaded gas was not invented. Typical gasoline was next to kerosene or lamp oil, and had an octane rating of about 50 or 55. Compare this to 87 octane for today's cheapest regular. Some owners of cars like yours, mix kerosene or diesel fuel in the gas to lower the octane especially in hot weather.

If your engine was in good shape and tuned up correctly it should start right up and run as smooth as silk. You need to figure out what is wrong and fix it. I would start with a compression test and if compression is within spec, go on to do a complete tuneup.

Just curious, why is aviation fuel the worst you can put in your car ?

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Any car will run best on the fuel it was designed for. High test gas may be great for a high performance motor but it would ruin a diesel engine in minutes.

Cars of the twenties were designed to run on low octane fuel. Probably the closest thing today, would be low octane tractor gas. High octane gas would be just the wrong thing to use, and aviation gas is the highest of high octane leaded gas.

A twenties engine would run on it, but it would run better on low octane.

I have heard of using up to 25% kerosene. This was from the owner of an early 30s Buick with 4.5:1 compression. He used it as a tour car and even towed a trailer with it.

Others have cured vapor lock and overheating by adding 10% diesel fuel to regular gas in hot weather. They report no adverse effects and no smoke, just a slight diesel smell to the exhaust. This was on cars with low compression engines built in the twenties, thirties and forties.

As a general rule, your octane should look like your compression ratio. If you have a modern car with 8.7:1 compression use 87 octane. If you have a high performance car with 9.2:1 compression, use 92 octane premium.

Some super high performance cars of the fifties, sixties and seventies had 10:1 compression or even 11:1. They were made when super premium pump gas of 100 to 103 octane was available. These could benefit from aviation gas. Some of them will not run on today's 92 octane premium unless you add octane booster.

In the thirties and forties you could buy Union "66" and "76" octane gas. Cars back then typically had 6.6:1 compression.

Going back to the model T and model A, you had compression ratios under 5:1 and unleaded gas of about 50 octane.

Naturally this is not a hard and fast rule, just a guide. There are other factors that come into it like engine design, size and weight of the vehicle, and how it is being driven.

But as a rule your engine will start easier, run smoother, develop more power and run cooler on the fuel it was designed for.

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There are restoration and maintainence shops using NOTHING BUT Av-Gas (LL-100). I know of multiple. I'm experimenting with it in several cars. No carb changes, no problems thus far. Excellent starting.

Yes, it has twice the lead content of our last leaded gas but not enough to harm. The BIG advantage: approximately TWO YEAR life in the tank before it starts to deteriorate. Ethanol starts changing in a week.

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To add to Rusty's excellent post:

Octane ratings refer to the time required to light and burn the fuel; the higher the octane, the SLOWER the fuel burns.

As Rusty mentioned, modern performance cars (Pontiac RA IV GTO's, Challenger hemi's, etc., with 10.5:1 or more compression ratio AND distributor timing of maybe 15~20 degrees BTDC) CAN benefit from the 100 octane aviation fuel.

Lower compression engines will NOT have sufficient compression or timing to burn all of the fuel. Thus, not only do you spend more money at the pump; some, possibly a lot, of the fuel goes unburned out the tailpipe.

87 octane is probably still too high, but that is the lowest available locally.

Starting and running will be better on the lower octane on older, low-compression engines.

Jon.

Edited by carbking (see edit history)
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Av gas is also formulated to run at altitude.

Don't ask me why this would have any effect at sea level, but the race engine builders don't recommend it.

One of the desirable qualities of Avgas is its resistance to fuel boiling (vapour lock).

With an increase in altitude, as atmospheric pressure drops, one of the undesirable properties of gasoline is its tendency to form vapour locks; not exactly what you need for an aircraft engine.

The sringent requirements of Avgas 100LL include its resistance to vapour lock, for an automotive engine this is not a bad thing particularly when you are at the traffic lights on a hot summers day.

As others have noted there are pros and cons for its general use in automobiles; my personal experiences having used it in my 1925 Maxwell were all good, these included the ease of starting, increased ability to climb modest hills and generally smoother running. I should add that on the occassions it was used I didnt have to pay for it.

One of the things not mentioned so far is its ability to rapidly destroy catalytic converters, so if if you have a later model vehicle so equipped do not in any event use Avgas.

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Guest Xlarashun

Avgas is also Very 'Dry'. Way back when we used to run it on the street, our mix was 20 gal 115/145 avgas, 1/2 gal of diesel and 5 gal of premium. dyno run on a mild 440, 10.25:1, 509 crane, alum single plane w/ 850 Thermo quad, worked 906 heads and headers (open) gave us 495hp/580ft/lb on premium pump gas. With our mix fuel, the numbers were 556hp/645ft/lb. And that was back in the mid 80's! Stuffed that in a Stripped out and lightened '76 New Yorker, ( it only weighed 3100lbs!) with 3:91s and would scare the CRAP out of most of the Mustangs and Belly-Buttons around here!

Edited by Xlarashun (see edit history)
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Guest Bob Call

In the US the feds frown on using aviation gasoline in motor vehicles. One, it is leaded and leaded gasoline is not permitted in vehicles used on public roads and highways. Two, federal and state motor fuel taxes have not been paid on av gas. And, finally, not because of the law, but, you are spending money for octane that is not needed.

Where I live in NE Oklahoma there are numerous stations that sell ethanol free 87 octane regular. However, ethanol free is not available in bigger population centers. I work part time in Houston, Harris County, Texas, and ethanol free is not available in Harris and adjoining counties. Houston is the fourth most populous metro in the country.

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can i/should i run aviation fuel in my 26 chrysler? it starts only with starter fueled then doesn't run real well thereafter. someone suggested that modern auto fuel doesn't burn as well as fuel back in the 20's, thus aviation fuel suggestion.

Oil companies have gone to a lot of trouble to make modern fuels burn better. So whoever mentioned that to you doesn't know what they are talking about. Is there any advantage in running avgas in a 26 Chrysler? You may get better starting and some avgas has a higher calorific value (as well as higher octane) but I doubt the engine would be able take advantage of that. Plus it will stand longer before deteriorating.

The disadvantage? It costs double the amount of regular pump gas.

My father ran avgas for many years in his Morris Major, because he worked in a maintenance facility and fuel drained from an aircraft should not be returned to an aircraft. So why not use it in the Morris? The only advantage was it was free.

Your problem seems to be more related to engine turning rather than fuel type.

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They have gone to a lot of trouble to make them burn well in modern cars with fuel injection, overhead cams and high compression. Not so good for old sidevalve engines with carburetors. Among other problems, modern gas is more prone to vapor lock, does not have lead to protect the valves and goes bad much faster than gas of 50 years ago.

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  • 3 weeks later...

As stated before, higher octane fuel burns more slowly. In flat-head engines it is a long way from the plug to the other side of the combustion chamber, so the flame front may not reach the other side and as stated above, the fuel won't all burn. The result is that it is still burning in the exhaust manifold and you are wasting some of the calorific value, as well as over-cooking the exhaust system. I have heard people say the manifold on their flat-head was glowing in the dark when stopped after burning modern fuel. Remember that in modern engines, the plug is usually near the middle of the chamber and the chamber is small compared to our large bore, low compression flat-head engines. The combustion chamber is also shaped to optimize the way the flame front travels across it.

The other thing is the engine will knock (pre-ignite) like crazy on high octane fuel and that is sure to cost you a piston, eventually, if you don't retard the spark. You might also need to use different heat-range plugs for it to work well.

One good thing about modern gas (we have unleaded 90 and 95 legally, though a couple of companies sell 98, so far with no ethanol) is that after three years in the tank the high octane aromatics have evaporated and what is left is perfect for my 1930 Dodge Brothers 8. It loves it. Once fuel hit the carb it started immediately after three-plus years sitting.

So, fix your car, probably the electrics, before fooling around with av-gas! When my 1939 Studebaker would not start or run properly, I eventually found it had only two volts at the coil.

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I have been taking a hideous from the restoration, thus not checking on this thread. I wish I could get pinged when someone replies.

Anyway, I bought some racing fuel, 116 oct leaded and a new set of plugs. It does start better and runs a little better, not much.

Talking with a few brethren, leaded fuel helps with valves and valve seats ??

At any rate, next step is points and distributor rotor. Where can they be sourced?

I am also using a modern Zenith carb. I am thinking this is contributing to my problem.

If anyone can call with 2 cents, please do…… Carl 713-882-2322

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The new set of plugs are probably what has helped the starting and the slightly better running IN SPITE OF THE AV-GAS!

Have you done a compression test yet? With an updraft carb, the vacuum created by the engine MUST be sufficient to pull the fuel "uphill" from the carb to the engine.

Are you using the choke? Unless the carburetor is WAY too rich, cold and often even hot starts using an updraft carburetor should be done with full choke.

Have you checked the firing voltages at the plugs? If I had a dollar for every "carburetor problem" that we diagnosed as electrical by simply running a battery jumper cable from the ground terminal of the battery to a wire-brushed engine head bolt, I could buy one of the Hawaiian Islands and retire!

Yes, the leaded fuel WILL help with valves and seats. And if you drive the car 40,000 miles a year or pull a 40-foot gooseneck trailer with the car, then expect to do a valve job annually. Do you do either of these?

As to the modern Zenith carb; there are at least a dozen different models. We have been selling them for 40 years, and have sold thousands of them to enthusiasts that have had excellent results (as long as the proper Zenith is used).

Many electrical parts may be obtained from Then & Now Automotive Parts in South Weymouth (Boston) MA. I am sure there are probably other sources as well.

But seriously, before spending a bunch more money guessing; do some testing. Start with compression. If you have a dead cylinder or very low compression NOTHING else you can buy is going to solve the issue.

Jon.

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Jon, Thx for the info..... I did a compression check last week. All Cylinders measure 75-80 psi. That ptoblem eliminated.

Not driving 40,000 miles nor pulling a trailer. I guess I can go back to modern reg fuel ;)

Yes, using a choke. It must me fully closed before any start. Sometimes when running after warmup it still benifits from choke while accelerating.

How do I check firing voltages? With a meter? If so type and setting and proper result range.

Attached is a pic of my Zeinth carb. Does it look proper? I have adjusted the choke cable since the photo. As mentioned, while accelerating it needs choke. I guess that means to much air. I thought the model has an accelerator pump but maybe not. Something else, the carb mounting bolts are 90% to the manifold mounting botls requiring an adaptor plate. I suspect this has more of a chance to vacum leaks. What carb model would you recommend?

I shall check woth 'Then and Now' for points, etc.

Thx again and look forward to your reply.

Carl

post-56009-14314241433_thumb.jpg

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The carb pictured is one of the universal Zeniths, cannot tell from the picture which one.

You should be using part number 14992. The part number should be stamped in a tiny semi-circle on one of the carburetor castings.

The other end of the carb (not the pictured end) should have an adjustable main metering jet. (with a "T" handle). If choking helps acceleration, you may be lean. Try turning the T-handle counter-clockwise 1/4 turn, and see if it helps cruise and/or acceleration.

Or call me at 573-392-7378 (9-4 Mon-Tues central time).

The picture shows a really nice appearing engine. Sometimes, in the zest of making things beautiful, enthusiasts forget that paint is not a great carrier of electricity. Is your battery ground cable making a really good connection to the engine block?

Jon.

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I question whether lead will help much to save the valves in such an early engine. In a more modern high compression engine, that did not come with hardened valves and seats, under hard use a critical degree of heat is reached that is more than the metal can stand up to.

Older engines came with hardened valves and seats, and do not have enough compression to get that hot under normal circumstances.

Others have pointed out that using too high octane gas, slows down the burn and the exhaust gasses go out the valve still burning, creating excess heat on the exhaust valve and exhaust manifold. In this case, high octane leaded gas could actually be harder on the valves than cooler burning, low octane fuel.

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In a more modern high compression engine, that did not come with hardened valves and seats, under hard use a critical degree of heat is reached that is more than the metal can stand up to.

Because of the high temperatures, the valves micro-weld themselves to the softer seat and pick up a speck of seat metal. Tiny nodules build up on the valve face. Valves rotate in use and the nodules grind the seat away. The valves rotate because under compression the helical valve springs impart torsion on the valve along the line of the spring winding. Tetra-ethyl lead prevents the micro-welding and the build-up of nodules on the valve face and hence valve seat recession. Hardened valve seats prevent it in the absence of lead.

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  • 2 months later...
  • 1 month later...

Update......

I did a complete tune up, plugs, points, condenser, cap, rotor, coil and plug wire. also rebuilt the zeinth carb. it run marvy! starts quick runs great!!

Also running regular fuel.

Thanks everyone for all of your input and help!!!

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