There are probably many different thoughts, opinions on the pros and cons about this topic. It may have already been discussed even so sorry if it has. What I would like to know really is will there be any bad effects to my engine if I were to run Avgas two stroke fuel? I work with aircraft so it is readily available for me for free, I have been running my trimmer on it and seeing as though it is 130 octane I would think that it would make a great performance fuel for my boat engine. The only real thing I could think of is the mixture may need to be tuned to the higher octane fuel or will I need to completely change my timing ?

Hay mate have a chat to marc or scott .

on that. theres be posts befor on it . and i also lurned somthing on that subject i never new befor i heared it from them . you actuly want to go the other way around:silly: ... and lower the octain . try it you will be suprised i use to run premium or utimit 98 octain . but i now run the lowest pup fuel avalable for me it 91 octain. or 89 ... the lowert the better.
but there will be somone out there that can realy expain it to you.
scott has even told me from posts on here of using colman camp fuel or running as low as 68 octain . i think not shore exatly. but im shore you will be suprised as i was . lower the better on low compression motors like we run.....cooler running and other benifits.

also im run my motors as rich as 15:1 and use castrol a747 2stroke oil......runns good allday..

Here's another post on this subject: www.modelgasboats.com/index.ph...atid=14&id=1574

Chad,

Just keep in mind we are pretty much ONLY using the really low octane fuels in our Zenoahs, of which most are modified having an even LOWER compression ratio than stock specs.
( Note: Those who CLAIM to have raised compression on modified Zenoah engines are making FALSE claims ) You CAN NOT raise the height of exhaust port without lowering compression. Lowering cylinder tightens up the squish clearance and would raise compression if exhaust was NOT raised as well.

In an engine that has LOW compression, you need faster burning fuels.
This is also a benefit when total area of combustion chamber is quite small. The flame front distributes must faster in a small area than a larger one.
Result is a very complete burn of fuel getting maximum BTU potential from fuel burnt.
Unburnt fuel that leaves the combustion chamber burning up in the pipe contributes nothing to the power and in reality this burn off of residual fuel also helps burn up the fresh air/fuel drawn into header section that would otherwise be your BOOST charge from the pipes returning pulse.

Hey OZ! once more scott is right.cut the cyl,tighten the squish.But if you raise Exh height you will lower C.R. High octane works good on high comp engines,base this on your air craft exp....you know high alt. super charged engine.just remember high octane,slower burn. And to Scott or Marc-has any one made a small alum insert that would bolt into comb chamber to raise C.R.??? you know just to reduce head vol? Just a thought.

I'm not really sure what you mean here? Really isn't much room in there after having dropped the cylinder...isn't much BEFORE you drop it in fact...we are talking about 30-35 thou squish stock and 10-15 after dropping. Maybe you can explain it some other way if I'm not getting what you are trying to say.

HMETALH wrote:
Myself and a fellow Zenoah tuner played with this idea 10-12 years ago with the G230 series engines. We called it the Sneeky head

Basically requiring the complete machining out of the squish area and most of the chamber, tho retaining the S/P threading.
A dummy half head that looked like a big washer with compound angles was pressed into top of the cast cylinder.
Sure we raised compression and got more torque as a reward, but top end HP really did not change much. The HP was more directly controlled by port sizes and amount of exhaust timing you could get. So in the end not worth the trouble

* An issue that also plagued us was that the expansion coefficient of a machined aluminum head button was a LOT different than the cast aluminum cylinder, so keeping the sneeky head in place was a real PITA

Thanks you guys,I never thought about the diff in expansion rates between cast and extruded alum.Back to the drawing board!

Ozpulse wrote:
Ozpulse,
I will give you my opinion and findings on this particular subject as mine does vary from most of the others. In testing on the dyno and in the water we have found that the 91 through 93 octane pump gas fuel to be the best as far as power goes. The problem is that you truly do not know what you are getting from the pumps these days and some of the additives including alcohol that are used is not very good for the gaskets and pumps in our little carbs. We have also tested most of the racing fuels on the market and the only racing based fuel that we found so far that worked well was the VP brand of fuel called C12 it makes just as much power as the gas from the stations, but with less of a bad smell and none of the bad additives. I personally do not like the camp fuel and have tested it on several different occasions only to find in my testing that it just does not make as much power as the other brands mentioned. This fuel may work well for others, but in testing with my engines it just does not work as well for me. There was even another boater on Jim's Rc Boat Dock that that was shocked with the findings he made when he did the same back to back testing on water with all the fuels that have been mentioned. He basically came up with the same results as I have mentioned after spending an entire day tuning with each fuel and back to back runs logging data. You may want to do a search as he listed average RPM and MPH readings from each fuel he tested. I know all of the theories behind the high octane fuel and our low compression engines, but the results just speak for themselves in the testing that we have done.

Now to answer your original question I am very fortunate to have a small airport within walking distance from our club pond and have started using Avgas from the pump at the airport and even did some dyno testing with it before we started using it. It seems to work as well as station gas and the Vp Fuel I mentioned earlier at a much cheaper price and much easier to get. The only problem I see that you may have is that your Avgas may be different then what we are buying. The Avgas we are purchasing is called I believe 101(octane) lo-lead and is not 130 octane. We have not had to change our tune at all since switching to the Avgas and I have really grown to like it. I would suggest mixing up a small amount and doing some testing comparing it with other fuels that you have available to see what works best for you. Your test may show you completely different results, you just never know. We run 8oz of oil to a gallon of fuel.

PS.
The fuel we used to set the NAMBA G1 world record of 104.9 mph was Shell Premium with 8oz of Mercury quicksilver.

Hmmm, I may have opened a can of worms here! lol What compression are our engines running ? I know that our helicopter piston engines run between 75 and 85 psi. and of course they are air cooled with the addition of a small oil cooler, basically each cyl. is just a big version of an rc four stroke cyl. cooling fins and all.