Hi,

Been playing with my converted Honda GX 31 four stroke. Using a spring over the pump diaphragm in the carb and an external pulse line from the manifold to the carb pump cover, I got a WT type two stroke carb to work. I made an adapter that necks down from the outlet throat of that carb to the inlet diameter of the engine, added a velocity stack, and it works very well. I converted the engine to electronic ignition, and made a straight stack exhaust outlet. With these mods and all dialed in, it turns an 18 x 8 wood prop at just shy of 7000 rpms. This isn't bad considering, but as always, I WANT MORE!

I haven't bothered to look it up, but the stock compression ratio must be horribly low. It is so low that it is almost difficult to tell which is the compression stroke when turning the prop over. It also won't even think about hand starting, though it takes off the second you touch an electric starter to it. It is so low, that I just now pulled off the valve cover and checked the valve clearances, thinking they might have tightened up and the valves weren't sealing well, but no such luck.

So that got me to thinking about the options for a low compression 4 stroke:

1. Raise the compression!! (Not likely with this engine design.)
2. Supercharge. (This engine is a great candidate for forced induction, but I'm not going to make a blower!)
3. Turbocharge. (See above!)
4. Nitro. Hmmm, now that is something I am thinking about.....

Anyone else playing with their Honda GX 31??

AV8TOR

Compression ratio is about 7.5:1 on this engine according to a Honda shop near me. Best I have gotten out of mine is about 6400 or so but that was with the stock manifold. I have made a new one to rotate the carb and matched it to the carb bore so I should see a gain. I also have a 10.5mm carb to try once I get the plane finished. Simple compression ratio formula is:

Volume at BDC divided by volume at TDC.

However this formula does not take into account dome or dish pistons, gasket thickness, bore, stroke, volume of the combustion chamber, deck clearance, etc.

Nitro may be the ticket to increase the power. I know the boat guys do different cams or grind the stock ones etc. Who knows what happens to reliability though. I won't be messing with mine much as I am in the air verses the guy with a boat and all the mods who is in the water. We have way more to loose.

Tim, when you say "nitro" do you mean glow?

Yes.

Yeah, I have a spread sheet program I worked up years ago with all kinds of math for engine work, including doing compression ratio. All you do is plug in the numbers corresponding to whatever you are working on and it spits out the result. Handy.

I might try some nitro in mine. Sure would like to raise that compression ratio though. I don't know what the deck height is in the engine, but last night I was "day dreaming" about making some offset bushings to press into the piston to raise it up in the cylinder to close up the deck height and increase the compression. Do you know of any websites that have a blow up, (diagram) of the engine?

When we speak of nitro, it is nitromethane. Cars, boats, and planes that run on glow fuel are often called "nitro" though, as most model fuels have a percentage of nitromethane in them.

Ralph at rcignitions built on of these and said it was a mistake when he raised the compression.
I don't remember if it was on his sites forum or a thread here on RCU.

You could go to his site and post questions on his forum about the GX 31.
No use in reinventing the wheel.

As I recall when doing mine there is a little room above the stroke or TDC to play with. I would think maybe .025 or so. You also could clean up the ports and runners in the valve area and may gain some. Don't know of any pages that show diagrams of this engine.

You must keep in mind Cam timing and operating RPM.

You want everything to work together.
Huge intake passages deminish air speed and kill low and mid-range.

As far as the cam goes you could remone material off the back of the cam for more lift.

Your limiting factors would be:
Valve spring coil binding and Piston to valve clearance.

There is a lot of work involved doing these things.

I would start of with adding 15% nitro to my fuel.
You can pick some up at a go cart shop.
Remember always add nitro to gas, never gas to nitro.
A little gas in nitro is lets say unstable.

I think the nitro is probably the trick. It's hard to get into seriously hopping up this four stroke, as it is pretty limited by design as to horsepower potential, and modifications on it are anything but easy.

The 6900 or so rpms I'm getting on an 18 x 8 prop will probably fly the SR Batteries Giant Eindekker I'm building for it, and it will sound really neat on that plane, but I would like a little more power...

Thanks for your thoughts,
AV8TOR

Hello av8tor, You have come to about the same performance limits that my Gx 31 did.
You might gain a little more by cleaning up and enlarging the intake and exhaust runners. There might be a little to be gained in the cam lift and duration,although most of this gain would probably be in higher rpm's than we need for aircraft operations. You are most likely right on the money to expect substantial gains by increasing the compression. The boat guys do this by pluging the wrist pin hole with bearing material and redrilling the wrist pin hole 25 thousanth's lower on the piston. Maybe someone could cnc a longer rod?
I have three of these and would love to get just a little more power to justify the weight.
Other than that I beleive this is my favorite motor.
Motorhd

That was exactly my thought for increasing the compression height of the piston, but I doubt I will ever get around to actually doing it. I also thought about a longer rod, but that would mean pressing the crank apart, etc.

All in all nitro is sounding pretty good...

AV8TOR

Avi8tor,
Let's do it and see what happens. My local kart shop did not have the nitro so I wonder where it could be found in the Portland Oregon area? This motor is built very well and it would probably stand the increase in power from the nitro. Keep us posted
Motorhd

I'm not very worried about damaging the engine. From what I've read in my research they are very well made, and the boat guys run them some outrageous rpms.

I won't have any possible access to pure nitro until the end of next month, but I may just try some of my Gas/Glow mix in it just for grins....

AV8TOR

The 31 and 35 have the same bore which is 39mm. The stroke on the 31 is 26mm and on the 35 it is 30mm. I don't remember if the wrist pin location on the 31 is the same as the 35 or not. I do know the 31 piston is taller than the 35 which would make more compression but you would need to mill about .025 off the top so as not to "kiss" a valve. All this hot rodding is fine if you need more RPM like the boat guys run. In our use for this hefty engine we are looking for useable torque and lower RPM. I think av8tor is on the right track where he is. Just my .02.

Try these guys;

Competition Motors Corp.
9399 SW Tigard St.
Portland, Oregon 97223
(503) 624-0621

You are right Tim.

If you find your self flying at full throttle most of the time and wanting more then do something different.

If you are happy at anything less then your okay where you are at.

I got the Echo finnished, made a movie will post later.

It runs betten than I hoped

While it is basically true that a larger prop is more efficient than a smaller one and turning a large prop slowly is more efficient than turning a small prop fast, the bottom line is horsepower = torque x rpms. My souped up Homelite 30cc for example, certainly provides more power turning at 9000 rpms than my brother's Ryobi 31cc at 7500, both with the same prop and/or prop load.

But a higher compression ratio, given that the engine can handle it without detonation, overheating, etc., provides more power all across the rpm band. It is one of the few things you can do to an engine that is "all good". It improves overall efficiency, and costs nothing in bottom end torque like most other modifications. Remember, these engines are supplied in a very mild state of tune in order to be "idiot proof", and run for years with very little maintenance. They are many times run with the cooling fins half clogged with trash, run on old gas, run for hours on end, etc., etc. There is almost always room for improvement for use in our applications, which are generally much more "engine friendly", even if they are modified to make more power.

AV8TOR

Hey guys, the Ryobi 4 stroke I have been working with responded greatly (nearly a 1000rpm increase) when going from 15% to 30% nitro. This pretty much verifies that the comp. ratio is low. In my case adding the nitro is about my only alternative due to engine design.@ 30% it really rocks, idles better, and I find no signs of detonation. Also I runs best on an OS A3 plug.

planepounder, what kind of ignition are you using if any and does the Ryobi 4-stroke use oil mixed with the fuel? Thanks.

Ignition consists of a glow plug and onboard battery. It must have the plug hot @ idle to be reliable, will run above idle w/o power, I just leave it on while running. I did not want to spend big bucks for onboard glow so I made a 1.2v 4500mah battery from Sanyo cells (cost about 8 bucks) gives 1.5hrs+ flying time. gas/glow I feel is the only practical way to go with this engine unless you need a 5.5lb engine.

Hey forgot to answer the oil question. I run glow fuel in the gas glow mix, so there is oil there, but it is totally unnecessary and unwanted actually, as the Ryobl is a wet sump engine. It does not seem to hurt anything and really never leaves any mess to speak of. If you custom mixed your fuel an in the gas/glow thread it might be better, but I fly some glow engines and I just use what I have on hand.

Okay, thanks. You are using a NiCad pack? Where did you get the glow plug adapter and what walbro carb is on the engine? Sorry for all the questions.

Ralph, glad you finally got it running. Looks just like mine [8D]. The video looks great. I like the way those BME mufflers sound. What are you putting it in?

Hey, questions are no problem. Yeah nicads (1500s-3) The plug adpter was made (easily) from the stock plug. The walbro is from a Stihl trimmer a WT. (I can get the # if you need it). You must convert it to external pluse (easy) and tap the valve cover for the pluse source. the original pulse hole in the carb does not align with the original hole in the carb adapter.

planepounder, If you could post some pics of the external pulse conversion that would be great.