Suppose I start with a ~1hp 25cc weedwacker or leaf blower motor. How much hp gain could I get from easy hopups like larger carb and exhaust + electronic ignition etc. and plz spill the beans on any other hp booster tips.

Maybe 10% more power than it already has. Hard to say. But stripping it down and running electronic ignition might yeild a little more as it removes the flywheel and magneto that do tend to take a little power away from it. A better exhaust helps of course. But going with a larger bore carb likely won't do anything. The engines are timed and tuned for torque not high RPMs.

Now one could convert them over to glow engines instead. That usually results in about a 20% to 25% improvement then.

Looks like the 1.1hp 25cc weedwackers don't stand a chance against an RCG 20cc 2.2hp and 30cc 3.9hp motors which are going to be lighter than the weed wacker. I will look at DLE similar sizes. Looks like a 30cc isn't much heavier than a 20 or 25cc and is almost 4x the hp.

Do not believe the published horsepower figures. I have never gotten that much performance out of those engines like that. The manufacturers tend to fudge the results somehow. Very few publish realistic numbers as it makes their engines look bad. But horsepower isn't the end all ultimate gauge for performance. it is really how well the engine turns a prop that you can get. Usually those high HP numbers mean they run a prop that is too small for the engine and run it way over it max torque point and its max RPM limit as well.

If so then weight vs power sez convert a 30 - 40cc chainsaw motor instead of weedwacker. I'd probably just use magneto and save the battery for more servo power. I am not going to fly competition. Just some hard climbs and regular aerobatics, no prop hanging and post stall stuff.

My Homelite conversion gets about 8 to 9 lbs thrust and 8000 rpm with a 16x8 Master Airscrew wood prop. I can fly sort of scale with that but it's a clunker at just over 3 lbs.

depends on the engine, there are professional grade engines and then there is the "homeowner" crap.

a larger carb will often get a big gain depending on what the engine started with. the big gains come with internal mods, most importantly increasing compression and increasing exhaust duration.

Really depends on how much work you want to do. An all out engine may have almost nothing left of the original. My helicopters engine was made by the helicopters manufacture. It is based on two Zenoah 26cc cylinders, and only a couple other parts Zenoah. Rated at 8 hp.

Roto Rob, That is a nice heli engine there, The original WWII prototype radio control drone planes used engines like that where the manufacturer took two air cooled outboard boat motor cylinders and made a new crankcase and crankshaft to make a flat twin cylinder engine.

ChopHop, It really depends on the plane you put the engine in and your flying style. I had used a 30cc, conversion gas engine on a Bud Nosen Mr Mulligan many years ago. It had a 108 inch wingspan and was originally designed for a .60 engine amazingly enough. Anyway the engine powered the plane just fine. At the time I thought that was a big power up for the plane. But I have seen guys put 50cc engines on it and they still wanted more power.

But one can keep the flywheel and magneto and machine down the flywheel more to reduce weight, plus you don't really need the fan blades on the flywheel if it is a open airplane engine. Some flywheels will let you reverse them on the crankshaft too. But one could dispense with the flywheel and magneto to save weight as well. Choosing a airplane that needs more weight up front works good too. Some of the old era radial engine planes had really short noses on them and needed a lot more weight up front to balance the plane, so a heavy engine would be preferable to adding a couple of pounds or more of lead weight up front too. Another modification was to make a new crankcase in order to reduce weight even more. One could go farther than just making a new crankcase and setup the engine with a reed intake instead of piston port induction too.

I just remembered this, but one relatively simple modification for performance is to increase the compression a little by shaving a little off the base of the cylinder/head. This boosts compression and also advances the port timing a little bit too. Usually the disadvantage of a utility gas conversion engine is the porting on the cylinder isn't designed for more performance as they don't need it for a trimmer or blower engine. One example is the piston port induction engines only have two transfer ports as the intake port under the piston blocks the third port. They usually don't make the two transfer ports angled or shaped well either. With reed intake designs they can put in the third port and get more power too.

You may want to go with a purpose built gas engine instead of a conversion as they include things such as better porting on the cylinder and better intake methods to get more power out of the engine.

I made a cute little inline twin out of two Kawasaki weedwacker engines. These were 24 cc and looked more like glow engines with the round cyls. Had it running on a test stand but needed one cyl for a small gas plane so took it apart for the one cyl motor. Some weedwackers used undersized carbs to limit the rpms when the cable broke, so they wouldn't over rev. So something else to look at.

Gord.

Seeing those 15 minute full throttle flights makes this tempting. I guess I'd be better off with a $150 RCG engine than a new $150 leaf blower stripped down. Maybe I'll sell the leaf blower for $80 and put it on an RCG.

chophop, I have a rcg 20cc and a 50cc and so far, I am pleased with both!

I may have to design it myself but I need a plane for a gasser that can get off and land on a 100 foot runway with a 5 foot fence. I can make a 1000 sq inch high wing Cub type, that looks like a good bet. I hoping I can make one no more than 8 lbs auw.