Last updated and correction:
june 12, 2001

Ryobi hop-up and experience

The modifications pictured are for the 31cc 2-port cylinder engine. Tools needed:
Lathe, Dremel, hard-metal drill bit 1/8" (3mm), if you want to make a rotary valve.

There are very fine descriptions available on how to skin the Ryobi grass edge cutter. so I will not repeat that here. To name a few, visit
http://www.paragonaero.com/ryobi.htm,
http://www.geocities.com/CapeCanaveral/Hangar/1071/ryobi.htm
http://www.geocities.com/CapeCanaveral/Lab/6216/webww.html
http://www.simonmodell.de/shop/startshop.html (German conversions)
The pages above describe a Ryobi that will fit nicely inside a plane, and when finished, it is just fine to power a yellow cub of up to 18 pounds, maybe even 20 pounds. The German conversions claim 2.5 HP in standard configuration, which seems quite optimistic, since no modifications that influence the power output were made.
If you need some more ooomph, you need to free up the breathing restrictions built into the engine. It is these restrictions that make the engine suitable for grass edge trimmer work, and that we can do without.

First requirement: A larger carb.
The Ryobi as standard, is fitted with a 12mm carby which has a 8 to 9mm choke, which is very restrictive, and should be upgraded to a 16mm carby with a 11 to 13mm choke. This is a very popular Walbro size for chain saws and the like.

Second requirement: Larger manifold passages
In order to take advantage of the increased carby diameter, the standard reed valve should be scrapped and a manifold with enlarged mixture flow must be made. A lathe is handy here, but stock parts can be changed, and parts can also be made of resin or plastic stock. For compression ratio and porting changes, see the CAD section. (download)

Option cheap, Fit a larger reed valve, and modify the induction manifold to a trumpet shape. The standard intake manifold can be used. This mod allows for a larger 16mm Walbro carby. Select one with about 10-12mm throat diameter. Power output is satisfactory, but not yet great.

Option one, large butterfly, or double butterfly reed valve.
For that purpose, a new back cover with a Y-branch, or better still, a double Y-branch manifold must be made. The double Y-branch drawing (see CAD preview) shows two types of valve: a cloverleaf shaped reed valve, and

Option two, A flat plate check valve;
the check valve has an extreme small flow resistance, smaller than a reed valve, because it needs hardly any differential pressure to stay wide open. It uses the same manifold as the reed version, but the valve stop is slightly different. The valve plate should be light and flat. I made mine out of a piece of 0.5mm carbon sheet I had in my scrap box. It is closed by the gas back flow, and has just a small spring (out of a ball point pen) to bias the valve towards closed position and keep it there when the engine is at rest. Power is as good as with the rotary inlet, and low end torque is slightly better!

Option three, a rotary inlet valve
While the above options both suffer their share of inertia problems (the flat check plate is worst), the rotary inlet is inertia-free, and can operate at high speeds. In the form shown, it cannot however, offer the low flow resistance of the check valve, so low end torque is slightly less.

Third requirement: A redesigned crankshaft bearing fit. (To download the drawing, click here)
The Ryobi relies on a press fit of the bearings in the housing to locate the shaft. Because of this, the fit on the shaft is very tight. Thus the bearing is not free to compensate any material expansion as the engine warms up. I experienced bearing failure after two gallons of fuel in one engine.
In order to be able to relieve the fit of the bearings on the shaft a bit, it is necessary to install a distance ring between the bearings, and a snap ring in the shaft, in front of the front bearing race. This is needed to prevent the shaft from being pushed in, when the electric starter is applied.
The tight fit of the outer races in the housing was retained.

Field experience:
The Ryobi engine does not like muffler back pressure, and responds with rpm drop on any increase. Serious noise reduction has to be achieved with a roomy muffler of twin chamber design. (see MAN article "hey, keep the noise down"). My testbed is my old trusty Freight Train, which is a bit over weight at 20 pounds. The standard engine would only just fly the plane. After the modifications, it is a very lively combo.

Prop performance, using Menz wooden props and low back pressure exhaust
the standard was as it says, standard, without modification of any kind.

The rotary inlet shows promise, because it still has a very restrictive timing.
After some modifications, it managed 6800 rpm on the Menz-S 18x10, and 7000 on a like prop, but with true pitch(*) of 18x10. Menz-S uses tangent line as pitch reference.

Setup #6, as used to comply with our noise regulations, clearly shows an RPM cut-off at about 6000 rpm, and a gain below that value.
During our fly in 2001, I had a chance to piggy back and tow glider planes, using the setup #6 with the 18x10 Menz prop. So far, this prop proved the best both static and in flight. For the largest glider present, it needed more skill than I could provide to get a good tow (tow plane 20lbs, glider 16lbs, total weight 36lbs), so the second haul was ended in the second turn, and before a lot of height had been achieved. No glider came to harm though.

Final static tests, using set-up #6.
This table shows that my prop choice was dead wrong, and I could have gained some extra 25% thrust in the low-speed hauls and climb-out using the 20x8 Menz prop. Even after exhaust shortening this prop showed most thrust, and would have performed well at low speeds.

Further shortening of the tuned pipe length hurt performance over the range 6000-6800.