Hello guys! I had read not to long ago,, the certain engine manufactures did not recommend using avgas?? I've been using for many years,, and I don't think i've ever hurt anything. Perhaps they are concerned about guys not richening the mixture? My technique is open both hi and lo mix screws a bit,, so it's running very rich,, and back them back in,, the run a clean 2 cycle. I know there's guys that will break-out in a cold sweat if they have to adjust the mixture,,LOL (you'd be surprised of the guys running these engines that are really afraid of them!) My reasons for using avgas is that your getting a MUCH better quality of fuel,,(today's car gas is really junk),, and you have longer term freshness with avgas,,3 years I'm told. (they say about a month with car gas,,but I'm inclined to think ever less),,not to mention the extra omff ya get,,and longer run time on a tank full! So,, my question is,, after all these years,, (IMAA # 120),, with both DA and 3W engines,, is there anybody else that uses avgas?

You are correct in stating avgas is a higher-quality fuel. However, higher octane gas does not have more power. The higher octane number means the fuel charge burns slower and thus is less prone to detonation (pre-ignition). Gas with lower octane ratings burns faster (the mixture ignites more readily AND the flame front advances faster).

People run higher octane gas in cars having high compression ratios to minimize detonation. An example would be race cars, or turbocharged cars when the boost is increased.

Model engines have relatively low compression ratios. Running avgas won't give you an increase in power, but you CAN run a slightly leaner setting without detonating. However this can lead to higher cylinder temperatures, and this is the reason manufacturers don't recommend using high-octane gas. Make sure you have plenty of airflow for cooling!

Brett

There was a long discussion on this subject not that long ago in this forum. There have been quite a few people that have tried 100LL, myself included, that haven't seen the benefit. To the contrary, the lead build up on the plugs is quite detrimental regardless of mixture settings. In some climates there has been a small benefit to using 100 octane racing gas (not low lead aviation) to help maintain a higher CHT in extremely cold weather or for delaying detonation in extremely hot weather. Significant power gains have not been noted for the most part.

Essentially it comes down to how much you want to spend on gas and plugs. Like spending a lot more for exotic fuels and replace/clean plugs a lot more often, then the 100LL is just the ticket. If all you want to do is spend a lot more, then the 100 octane racing fuel is for you. There does not seem to be any reason to use anything different from what you can obtain from any one of the pumps at the local gas stations.

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Avgas is not a higher quality fuel. It IS a different quality fuel.

Would you run Diesel fuel in your gas fueled automobile? Everyone knows that Diesel engines produce more torque and are much more thrifty to operate that gasoline fueled engines. Same thing.

If you have a high time engine in which you have ran Avgas, look at the piston surface and face through the exhaust port. See any pitting? I'll be surprized if you do not.

High octane gas burns slower and is less resistant to igniting earlier than when the spark plug fires. Its reluctance to burn, in some engines, means that the fuel/air charge is still burning when the piston opens the exhaust port, rendering abnormally high exhaust gasses, hence, the pitting that I previously mentioned. There is no technical advantage in burning Avgas in utility engines. Engines that typically possess very low compression ratios.


Ed Cregger

Av gas is has a high octane, slightly more BTU per hour, and has less volitiles. It is more highly refined, however that has nothing to do with power. It is more refined to keep out both the lighter and heavier volitiles out. The lighter ones cause the fuel to boil at altitude and also promotes carb icing. The heavier ones promote sooting and thus can foul the plugs in those full rich take offs. If you ever see an STC'd airplane that runs on Mogas look at the exhaust stacks, there will be more soot in the pipe and the fuse around the exhaust. Since the fuel has less light volitiles, and less additives (other than the lead) it will store longer.

However, none of this helps small two stroke gas engines. The lead will foul the glow plug, and with the low compression ratio will not burn off and may even cause rings to stick. I once belonged to a full scale areoclub with about half dozen planes and they used the Avgas in their lawn mower. In a couple of years the valves stuck real bad, and even after that was fixed it had poor compression. I'm guessing stuck rings though I don't really know.

Actually it doesn't burn slower, just takes a higher temp to ignite. Well maybe the flame front accelerates a bit slower at first as it is harder to ignite, but ultimately it catches up. The flame front has about the same speed. Diesel fuel really does burn slower but it wound detonate like crazy. But it takes a much lower temperature to ignite. This makes it great for a Diesel engine as it doesn't need as much compression to ignite. But becaue it burns slower the engine will not rev as high. Diesel also has a higher BTU content, that along with high compression ratios contribute to a Diesel engines high torque.

Hey!! Thanks guys!! This is sure something to think about,, if I'm not really gonna gain much,, then there's no sense in doing it,, and with what BTerry had to say about having more cooling air flow,,is gonna swing my idea about this! Just what grade of gas you guys use? Would there be any benefit to using that gas stabilizer stuff? (to keep the gas fresh longer) Thanks again guys for the comments!

Stabilizer does help in long term storage.......would not store my boat without it.........but the best way to keep gas fresh is a air tight container!!!!!!! Air Transfer is the enemy, spend a little extra time and money on a good container and check the cap seals often.

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Most gasolines already have stabilizers of one sort or another in them. I guess if one was really forced to store for long periods without a choice, additional stabilizers might be necessary. I don't use them myself. I really try to pay close attention to how much fuel I am going to use and just pour the rest into the lawn mower, if any is left over.


Ed Cregger

This subject has been beat up almost as much as "2-stroke oils". The discussion has talked about fuel odor, perfomance, availability, etc. Anyway, good points Sport_Pilot! I'd like to add a bit to your discussion. Regular auto unleaded fuels and 100LL really don't ignite at much different temperatures. Auto gas has a flashpoint below -49degrees F and 100LL flashes at about -35 degrees F. Yes, not the (-) before that temperature! Likewise auto gas auto ignites at about 536 deg. F and I don't have a reliable source for the 100LL auto ign. point. At any rate, gasoline is gasoline and both combust very easily! Sport_Pilot, you described the burn characteristics very well. Its right at the point of ignition where octane has the most effect. From there on, the effect deminishes somewhat. You mentioned diesel fuel .... a way different animal than any gasoline with flash points over 125 deg. F and an auto ignite point over 489 deg. MSDS and other performance info is readly available on the Internet for anybody that is interested. And for those really interested in history, read about the "Ethyl Corporation".

I'm no petro engineer and I'm sure one would set me straight but I did drive a fuel tanker truck many years ago in a prev. life and had to get very familiar with what I hauled. Even a tiny bit of gasoline mixed with diesel can ruin the whole batch .... makes it not pass a flash test. And if you ever contaminate gasoline with diesel fuel then try to run that gas in any automotive application, you will be amazed how it will make the engine knock and make it sound like it is about to self destruct. As a pilot, we are often warned of the dangers of mixing just a little Jet A with 100LL in a recip. engine. Can be fatal!

Anyway, regarding using 100LL in model engines, other than a sweeter smell, there really isn't any good reason to use the stuff. It costs more with few rewards. If a guy has a tank of the stuff nearby, I guess it would be OK. I have 110 racing gas (leaded) in my garage all the time, and it smells and behaves much the same as avgas but I choose to get my model gas at the nearby gas pump ... at half the price. I probably would use 100 unleaded racing gas if I had that on hand.... just for the better smell.

I don't think the differance is very large, as you noted the auto ignition differance in Diesel fuel and auto fuel is less than 50 degrees, yet the diffeance is huge. Just a little Diesel fuel in a car knocks like crasy and the car fuel won't even cause a Diesel engine to cough.

We beat this to death on rec.models.rc.air between 1996 and 2003 or so. It is a shame that most folks are not aware of the fact that those old archives are still available for downloading.

I can't remember the gentleman's name, but we had a design engineer for two-strokes that retired from a large outboard motor manufacturer that was on quite regular. He could tell you the true skinny on such matters, when he was in the mood. His opinions were based upon facts with lots of empirical testing using the absolute best equipment and labs in the world. I wish he would come on board here and spread his wisdom and experience.


Ed Cregger

Hi,

I just wanted to comment on the fact that some model aircraft engines will work better on high octane fuel. When I run Zenoah it seems like there is no problem using the norwegian standard 95 octane unleaded, however 3w seems to work much better at a higher octane. At least 98 unleaded (highest we get in a pump) or even better aspen 99 octane evironmental friendly fuel design for small 2 stroke motors like chain saws, go-carts etc.

Therefore I'd like to try the 3w's on avgas.