This thought just hit me:Has anyone ever tried mixing a ratio of avgas in with the gas oil mix for conversion wacker engines? Would this help to alleviate the harder starting charactaristics of the stock ignitions? Should I put my head back in the clouds? What thoughts are out there?

Avgas contains lead and higher octane and will run much hotter that using unleaded reg. Will not give you better performance

I feel I must address a common misconception regarding octane. A high octane rating is a measure of the fuel's ability to resist knock/ preignition/ detonation. That having been said, a higher octane rating will burn slower than a lower rating, and this is accomplished by having a lower volitility than the lower octane rating.

So,

High octane = lower volitility and higher knock/ preignition/ detonation threshold.
Low octane = higher volitility and lower knock/ preignition/ detonation threshold.

AvGas is 100 octane, which would be fine in a modern engine, but since it has trace ammounts of lead, will foul the plug.

the blue 100 ( so called low lead )
has 5 times the lead of former 85 av gas
and it gums up plugs rings and all
but it smells good
nothing to gain in our small engines

100LL should not cause plugs or rings to gum up. It works great in airplanes. The mix ratio of fuel to oil may need to be adjusted to compensate for different burning techniques. I have ran 100LL in a stock 7.5 mercury outboard motor with mixed fuel/oil and it performed much better than the regular pump gas. It ran smoother and would get up on step faster. I have also ran race gas (120 - 130 octane) on a stock 1999 RM125. The difference in performance was outstanding. Again, the performance and smoothness increased compared to running pump gas (87 or 93 octane). The thing that most effects the performance, smoothness, and how clean the engine runs is the fuel mixture. Rich=Cold=unburned fuel, also when the mixture is rich, there is more oil getting into the engine. Lean=Hot=complete fuel burn, but less oil to the engine. It is a balancing act between fuel burn temperature, oil ratio, and fuel mixture. As far as why the engines run better with the higher octane....my thinking is that the high octane shoves the piston down while the lower octane kicks the piston down. The shove (high octane) provides better combustion harmonics that translate into power and smoothness. The application determines whether the extra performance is worth the extra cost. My 2 cents.

it might be the case in high compression high rpm engines, but ask any aviation mech. wat it does to 65-75-100 hp older av engines Lyc. and Cont.
I tried it in a 41 cc us engine it ran smoother but gummed right up
I had the gas from my other hobby.

I can only assume that those that are against using Av-gas havent used it much and are repeating what they've read elsewhere. I've used Av-gas for years in everything from my 83' 750cc yamaha (7 yrs now) to my push mowers and weadtrimmer and saw motors. There may be a number of reasons why Av-gas runs better in all my engines than pump gas but in the end they do run better and I've seen no ill effects over the years, yes I have motors than have seen nothing but Av-gas their whole life and they run perfect year after year. No fouled plugs or sticky this or that. I must say not to run Av-gas in a modern auto engine however, the lead doesnt play nice with modern emmision stuff on the engines.

Av-gas seems to burn cleaner, produce slightly more power and run cooler than nasty pump gas and heres most likely why:
The FAA has set standards for Av-fuels that must be maintained by law. You can bet the fuel will be a fresher, cleaner high quality fuel. Pump gas on the other hand just needs to burn....no one like the FAA regulates the quality of pump gas to the same high standard. Sure, someone is looking to make sure the gas is gas but no laws require auto gas to be high quality.

Those who run pump gas, thats fine. But to those wondering about Av-gas, go right ahead, I wont run nothing else if I can help it. Piper

higher octane would make magneto ignition HARDER to start, not easier. (higher octane=slower burning gas, so you need more heat) the sole purpose of octane is to slow down the flame, giving an advantage to high compression engines that would otherwise pre-ignite the gas.

My reason for wanting to run avgas is that I am an Aircraft Maint. Engineer which is the equivalant to the A+P Mech in the US. Working in the industry I do, the fuel is more plentiful than the money (which isn't that plentiful). I figured that if I could get the fuel and run it, why not? I spoke to others at work who are endorsed on everything from Cessna's to wide bodies and they figured that it would work. I just wanted to find out if it would start better.
I also wonder why, if it burns slower than 87, why do race car drivers throw in extra octane boost for their races? Guaranteed it makes em' go faster or why put out the extra dough? Points to ponder.
Excellent input though!

Higher octane will let you take advantage of a more timing advance and a more radical timing curve. It burns slower so spark earlier equals longer power stroke.

Higher octane denotes a fuel's ability to resist knock or preignition at higher peak combustion temp and pressure - pure and simple. It has little to do with burn rate or flame front velocity.

Higher octane fuels allow the use of higher Compression Ratio precisely because of it's ability to withstand higher peak cyl temp and pressure before knocking. The power gain comes from the higher thermodynamic efficiency derived more complete expansion of the combustion gases - inherent to high CR. Additionally, higher octane may allow the ignition timing to be advanced further, thus creating even higher peak cyl temp & pressure. Most modern EFI engine have knocking sensor(s) that allows the engine to run at the hairy edge of impending knock - maximizing power and efficiency.

With a lower CR engine (like our weedies), you can advance the timing all you want and never get close to the peak combustion temp and pressure rating of the high octane fuel. Beyond a certain point, advancing the ignition timing further will only do more negative work, which manifest itself in higher engine running temp. As long as you don't over advance the timing, running higher octane fuel in an engine which cannot take advantage of it will do it absolutely no harm (the lead content is another story). It will be like flying a Mach 3 capable fighter jet only at Mach 2. Nothing wrong with it but it's kind of a waste.

All this is, of course, all theoretical. In reality, much of it depends upon a myriad of operating conditions - atmospheric, intake and exhaust configurations, cooling loads, and even carbon deposits can affect the onset of knock and alter an engine's octane requirement. Feel free to experiment. You might just find a particular engine setup on a particularly hot and humid summer day doing much better with heavy avy fuel.

I think Volfy is in the right way.

Resume: The engine needs an octane ratio. If you use a lower one, you wil destroy the engine (a hole in the piston is very common). If you use and higher one, you will not get more power but it will not harm. There is a posible benefit: if the aditive to get more octane ratio is the "plumb tetraethil" (In spanish: Tetraetileno de plomo), as this is a good lubricant, the engine runs smoother.

What we have to understand is that wwe don't have tod think about octane, but "efective compresion", which is not the same as compresion ratio.
Efective compresion is just what it means, in the combustion camber. Enginers design the engines with a concrete efective compresion, based, over all, on the compresion ratio, and the diferents types de gas available. The more the efective compresion, the biggfer the power obtained, but there are limits: preignition.

Once the engine is designed, the gas octane is fixed from manufacturer, in which it will work well.

So, if the engine is designed for 90 octane, you wil not get more perfomance with higher octane ratio, only spend more money. But if you use 85 octane, it is posible there will be preignition problems.

Once the octane ratio is selected, you can "play" with the ingnition point, advancing it at full throtle, untill it preignite in orden to get top power. There is a problem advancing timming: as you advance the ignition point, it is more dificult to start, because the spark fires before it is necesary.

To solve this problem, you need and electronic ignition module with auto advance, or a linkage with throtle arm wchich moves the sensor, or a retard starter, as the CH Electronics do.
Resume: The engine needs an octane ratio. If you use a lower one, you wil destroy the engine (a hole in the piston is very common). If you use and higher one, you will not get more power but it will not harm. There is a posible benefit: if the aditive to get more octane ratio is the "plumb tetraethil" (In spanish: Tetraetileno de plomo), as this is a good lubricant, the engine runs smoother. If the aditive is the Methil ter butil ether (MTBE) or similar there is no advantage.

All the above are de basis, in acordance with our "little" engines. In other engines like motorbykes, cars, trucks, boats, what happens is they are more sofisticated, and incorporate electronic ignition, depresion retarded ignition, preingition and predetonation sensors, etc, which set the timining point. The only way to get more power in theese engines with higher octane, is if such one was using a lower one it needs, and the preignition sensor was retarding constinuously the ingnition advance to avoid preignition.

I hope this helps you.

Bottom line soulman...av-gas will work for ya. Its more consistent than than pump gas and it sounds like you get it the same way I do, what they sump out of the tanks daily to check for water and dirt. Piper

Hey I got a question for ya, how you gonna buy that Av-Gas ?? used to you could pull out a 5-gallon can or a gallon can and buy some. Not Now. The law states the ramp rats cannot put fuel in a container it has to go in a wing or a fuel tank and nix the idea if you think "Hey my Uncle, dad, brother, sister, mom owns an airplane or helicopter and we can get fuel anytime we want." Airport security catches you hauling fuel off the property in a container there is a 99% chance the feds will pay you a little visit and they have a low idiot tolerance level these days. Av-Gas is great but not worth the hassle to buy.
Just my .02 worth

jds

I'm guessing that the sump gas is no longer considered avgas, but more like toxic waste. as far as the FAA is concerned, it is never going to see an airplane again anyway.

I think Mr Piper Cub is on the same wavelength as me. I have used Avgas, Sunoco 110 race fuel
(turbo blue) , CAM 2, not to mention several others in everything from my echo trimmer, scooters, go and go karts to my Troybilt lawn tractor for years- I run a high-compression big block chevrolet jet boat and always have gas on hand.

I have noticed NO adverse effects from running it in anything I have- and my wife doesn't even complain about how it smells when burned- in fact, she said the lawn mower smelled good when running. When I use the gas in my two strokes I use Amsoil synthetic two cycle racing lubricant anywhere from 40:1 to about 80:1 depending on what I put it in. (I don't trust 100:1 ratio with anything) No gum- No crap- very clean- very smooth- no problems. I really like the Amsoil- it is good stuff.

Now, as far as pure peak power developed, high octane will not deliver it unless the engine is designed for it with very high compression and the carb. metered correctly for the fuel. It absolutely WILL NOT give increased horsepower in a small two cycle over pump gas- they are designed for pump gas - bottom line.

Will it work? yes. Will it hurt anything? no- the slower combustion is actually easier on the engine. Will it give more power? No- expect it to drop of by 3% to 10% (seat of pants guestimate from my observations)
HBZ

Cap10b, your unable to get av-gas? Sorry to hear that if your wanting to buy some. But I dont think this thread was started asking for legel advice as to using Av-gas but how well it ran in other small engines. I for one dont go to legel disscussion sites and ask how they build planes so I wouldnt belive legel advice given on a model airplane site. Thanks all the same. piper

I'm curious myself how you buy it. it isn't like this is some mystery top secret fluid people! it is just high octane gas! besides, this site is for all talk about model airplanes, and i'm fairly sure fuel is one of them...

anyway, back to the topic... how do you buy it? is the sump gas legal for none flight use?

Oh, I really tried to stay out of this one...

Anyway, due to security restrictions, you can't buy avgas at a big (controlled) airport. However, last I checked, you can buy it at the smaller country airports with no problem. Generally I show up with my hot rod boat, and they wave me on through any gates there might be, and fill my boat and my cans up.

Now....

a. You will pay much more than for pump gas.
b. You will gain nothing in power.
c. You will subject your engine to increased deposits and carbon build up.
d. There will probably be no significant change in operating temperature.
e. The engine will probably be harder to start, especially in colder weather.
f. Your spark plugs will likely foul due to various deposits from the fuel.
g. Yes, the quality control of the fuel is much better, but with all the negatives for model use... who cares!!??

Hope this helps,
Sorry if I've been "short",
AV8TOR

LOL@av8tor1977, Well there seems to be two camps on the subject. Heres my ABC's:
a. True, its higher priced but I get sump gas for free
b. most of my engines show a slight increase in power but only slightly I'll give you. Most of the pump gas around here has that rotten smell to it, like its very old, this might be why its down on power.
c. many years, mant types of motors. still waiting on the increased build up.
d. maybe not cooler but it seems to be so, no temp gauge to back it up.
e. Nothing has started sweeter for me than a motor on Av-gas! Start my 83' 750 yamaha on a cool day with no choke and ride off then. Pump gas on the same day, forget going in a hurry. Full choke, run for five min's all the while backing the choke down slowly untill it would idle, smoke a cigerette and by then it could be rode off on. My small motors show better cold starts too.
f. Again, still waiting......no fouling yet..oh the yamha, 3yrs on av-gas with the same plugs in it, never had to take'em out, about 8k-9k miles in that time. drove it like a car when the weather was good.
g. in the end, we all need to try new things if only to prove something to ourselves. I've had no ill afeccts from running Av-gas only the opposite. others seem to have had bad affects or are meerly stating what should or should not work " on paper". Heck if you can get some for free and want to give it a go, put it in your trimmer for a summer of yard work before you trust it in your favorite plane, but dont take anyones word on it without testing for yourself, if your that sort. Piper