Hey Guys.

I've got a super decathlon 89" and I'm looking for a gas engine for it.
I have no experience with the gassers and I'm looking for some help.
Any recommendations would be appreciated

How much does it weigh? and what is the wing area?

I have a Zenoah G-38 in my Dynaflite Super Decathlon (89"WS, 16.25 pound weight), and it's a very good combination. The specs suggest a 25cc engine, but I think that would not get the best out of the airframe.

Look at the ZDZ line///

You can build a 40-50cc conversion engine from a chainsaw for a LOT less than any of the commercially available engines.

Check out the conversions forum. There are lots of Craftsman and Poulan (same thing) chainsaws in this size and you can get them put together with electronic ignition for under $300.

I've done a number of these conversions. They are usually a few ounces heavier--sometimes not. They are ported and timed to run in the higher RPM range. So keep that in mind and prop them for 7500+RPM on the bench. Anything under 7000RPM and they are kinda doggy.

I have done 2 of the Poulan 54cc engines. I spent under $300 for each conversion. I bought the engine--had it machined--sent it to someone for a hub, sensor, and ignition system. Bought a BCM muffler and started flying it.

I've done a few of the Ryobi 31cc engines. Probably not enough power for your Decathlon--but fun and very easy to convert. Keep them UNDER 8000RPM unless you want to see a connecting rod detonate and fly apart.

If your worried about them being ugly--they aren't. You got a cowl on this plane--right? You'll never see the engine.

If your worried about performance--then I'll remind you that MOST gassers on the commercially available market are chainsaw/water pump conversions. DA, 3W, ZDZ are all specific RC built engines.

Brison, Zenoah, BME, Fox and numerous others are ALL nothing more than a factory converted industrial engine. They use the SAME cylinders, cranks, rods, pistons, carb...etc as a chainsaw engine. Usually the only thing different about the "professionally" converted engine is the case. Take a look at a BME 3.2 engine. The ONLY thing thats not originally from a chainsaw is the CNC machined aluminum case.

Don't be scared of a conversion engine. Save yourself a few hundred $$ and do a conversion.

What would you typically get machined on these engines. Who do you send it to. I live in Northeastern PA. What is the difference in RPMS say between a Poulan 52cc conversion and your typicall 50cc ZDZ, BME, Brison, etc?
Thanks in advance for any input.
ROb Parker

I sent my engines to Ralph Cunningham in Williams AZ. His handle here at RCU is RCIGN1. He can do the necessary machining work and provide the electronic ignition system.

My 54cc Poulan:

This saw comes with a magnesium alloy case. When your holding the saw, there is a left side and a right side. The bar oil tank is in the front and the fuel mix/gas tank is in the rear. You have to take a portable band saw and cut the tanks off the front and rear of the engine. You will end up with a crankcase--minus the bar oil tank and fuel tank. Thats all you need--is a crankcase to hold the crankshaft and bearings. You can take a Dremel and sand it down nice and pretty if you want to.

I strongly recommend an electronic ignition. You can use the stock magneto and coil if you want to save a few bucks, but it's not worth it IMO.

Ralph makes 2 different types of ignition systems. One with fixed timing and one with variable timing. Generally, you don't need variable timing on anything under 50cc.

You will need him to machine a prop hub for your engine. And you'll need some kind of a mount.

You can sometimes buy a comercially available muffler that will bolt right up to your engine. For example: I used a Bisson muffler for the moki 180 on my 54cc Poulan engine. The CL of the bolt holes had to be enlarged by 1mm on each side--and then she bolted right up.

I was swinging a 22-10 Pro Zinger at 6800RPM with a 40:1 mix and my elevation was 6000' above sea level.

I tried several different props and settled on a Pro Zinger 20-10 at 7800-7900RPM

How's that compare to a BME 50? I was turning MORE RPM than my friends BME 50 with the SAME props. He wouldn't let me get anywhere NEAR his BME with a Tach for a long time. He just couldn't admit that my cheap conversion engine from a used chainsaw was out-turning his $500 BME. He wouldn't even admit to me what size prop he was turning. He kept telling me it was a 22-10.

I snuck up to his plane one day when he wasn't looking and I had a 20-10 prop in my hand and a 22-10 prop. When I stuck my props up to his for comparison--it was obvious that he was using a 20" diameter prop. I called him on it and he was PISSED[:@]

Then he had no choice but to eat a big 'ole piece of humble pie. I started laughing and I told everyone at the field that day that he had been lying about his BME engine because he just couldn't admit that it wasn't as powerful as my chainsaw.

The peanut gallery harrassed him untill he finally agreed to put our planes side by side and run 'em up.

I was turning 300-400RPM MORE than the BME 50 with the same props. I'm not talking about 2 of the same Pro Zinger 22-10 or 20-10. I'm saying that we started his engine and tached it and then we shut it down and put the same prop on my engine and then we started it and tached it.

My engine beat him by 300-400RPM on 4 different props that day.

And I only paid $285 for mine.[8D]

Why? Because a BME 50 is nothing more than an Echo chainsaw with a CNC machined case. The crank, rod, piston, cylinder, and carb are all from a chainsaw.

Chainsaws are ported and timed to run at higher RPM than a DA or a ZDZ. If you take a look at the engine tag on most chainsaws--you will find that the MAX RPM is generally well over 10,000.

What does that mean to a modeler? You have to realize that the engine is MADE to run higher RPM than we are used to dealing with on commercially available airplane engines. So, you prop it for 7500RPM on the bench and go fly it. If you prop it like a DA--say 6500-6700RPM-------then your chainsaw conversion isn't going to perform as well as you'd like. Why? Because it's not up in the POWERBAND at 7000RPM. You gotta prop it for higher RPM and let it spin in order to get the HP out of it.

It's all about the port timing and duration. It's about how the intake ports are shaped and where they are in relation to the exhaust and transfer ports. It's about the exhaust duration and the transfer port velocity. The chainsaws are BUILT to run 10,000RPM all day for years. The velocities inside the engine ports are all different on a chainsaw. If you lug it down and prop it for 6700RPM on the bench--your not going to be happy.

Thats all there is too it. KNOW what it is capable of. Prop it for higher RPM. Don't run a 100:1 mix in the darn thing. Stick to 40:1 or even 32:1 with a chainsaw.

Sometimes a conversion engine will weigh a few more ounces than a DA or a ZDZ. Sometimes not. The problem with all the weights posted by all the engine manufcturers is that they don't give accurate--real world-- numbers.

Sure, a BME 50 weighs--lets say, for arguments sake--2LBS. HEY!! Thats a light engine!! BS--thats just the bare engine without taking into account the weight of the igntiion system, battery, muffler, prop hub, and so on.

I gaurantee you that if you put my 54cc Poulan on a scale--with the muffler, ignition system, prop hub, and battery------that it won't weigh more than an ounce or 2 more than a BME or a DA or a ZDZ. Heck, it might even be lighter. But, lets be fair and compare apples to apples. Not my apple to your stripped down bare engine block.

I wouldn't begin to compare my 54cc Poulan to a DA 50 in a prop swinging contest. No contest--the DA wins. I also wouldn't compare it to a ZDZ. Those are purpose built RC engines. No chainsaw parts inside a DA or a ZDZ.

But, I'll put my Poulan up against ANY commercially available 50cc engine that was originally an industrial engine. FOX, BME, Brison, Zenoah--bring it on!! My engine will do at least as good or maybe even beat all these engines in a prop swinging contest. I already know that it can whoop 1 of them BME 50 engines.

Now consider the cost involved. You can't touch a 50cc engine from a commercial manufacturer for under $500. Toss in the muffler, and your up to $575--$600.

I bought my chainsaw used for $45. I paid Ralph for an ignition system. I had another guy machine a prop hub for it. I bought a muffler for it. I bought a new carb for it. I bought new rings for it. I bought new gaskets for it.

Total cost was $285.

I built a Craftsman 38cc engine for alot less than that!!. I didn't have to saw the case apart. The engine simply un-bolted from the saw body. I sent it to Ralph and he supplied an igntion system and mount for it. It wasn't too expensive.

I sent it to Frank Bowman in Farmington NM and had him put a new ring in it--just for giggles. It didn't really need the new ring.

I bolted it up to a stand and ran it with a 20-8 prop. I got 7000RPM out of it on the first runs--it was still rich because of the new ring. Eventually, I got it up to 7200RPM after a couple gallons of fuel.

I had $160 in that engine.

Email for pics or other info.