diceco said: BTW I've just completed an analysis, using the methods outlined in Gordon Jennings two stroke tuning book, and discovered that for the bifurcated transfer port cylinder, the transfer ports are plenty big enough to give peak power at 7500 rpm but the exhaust port is not. I further determined that the duration number of 150 deg for the exhaust port, that's been mentioned by several, is just right if obtained by raising the top of the port 0.11 inches. This not only gives greater duration but larger area also. In so doing Jennings suggests an ellipsoidal shaped port and proper radiusing and chamferring of the edges of the port. I'm looking forward to see what the performance gain really is. Even if one doesn't use the calculations in the book, his general comments on what makes two stroke motors work is very good reading.
I put the Ryobi back together after re-working the exhaust port and ran it up. It gained 255 rpm prop speed and 1.0 lb thrust; a significant amount to be sure! In the current condition the APC 18x8W turns 7630rpm and it pulls 15.0lbs thrust. These numbers are corrected to 59 deg F ambient temperature and 29.9 in Hg barometric pressure. The raw numbers, for those of you who don't believe in normalized data, were 7600rpm and 15.2lbs thrust. These were obtained by averaging three data points taken within about 3 minutes of each other.
The attachment shows a drawing of the exhaust port measurements; stock, objective (150 deg duration), and actual (160 deg). Unfortunately the port ended up being larger than I intended. A hand file is NOT a precision machining tool, at least in my hands!
A note of caution about porting the Ryobi. Gordon Jennings recommends cutting a shallow ramp leading away from the edge of the port, the purpose of which is to guide the piston ring(s) back into the cylinder wall from the bulging condition that a wide port might cause. I am certain he is speaking of doing this in an iron cylinder liner This should not be attempted on the Ryobi because of it's AC (aluminum chrome) construction. The chrome lining is very thin and no attempt should be made to cut a "ramp" in it. One should be sure there are no sharp edges in the chrome lining at the port lip, and do create an ellipsoidal port shape as suggested by Jennings. The bottom of the stock port is pretty much straight across. I very slightly cut the bottom of the port to make a slight curve in it's edge but this was only for the benefit of getting the ring(s) back into the cylinder for the last bit of travel before BDC. This will do nothing to the effective port area because the piston doesn't go below the bottom of the stock port at BDC. The slight cut to the bottom of the exhaust port is not shown in the drawing.
I also measured the cylinder head volume and found it to be 2.5cc. I then added the deck height 0.12cm and used the bore 3.5cm and stroke 3.25cm to calculate a compression ratio of about 9.5:1. Better than I thought it might be after hearing so often of those "low compression" weedie motors! Yeah it would be nice to raise the CR and get more torque but there is no practical way I can think of to do this. First off, the deck would have to be machined, then the cylinder base, but now you've messed up the port timing, so they would have to be reworked! Not worth the effort IMHO.
And one last test: After running up the motor on the APC 18x8W I switched to the Master Airscrew 18x8 Classic and got 260rpm more prop speed but 1.2lbs LESS thrust. NOT ALL PROPS MARKED WITH THE SAME DIAMETER AND PITCH PERFORM THE SAME!!
When I get a chance I intend to go back in my records and track the performance history and modifications of this motor from new-cut-out-of-weed-wacker configuration to the current configuration, which is about as far as I think I can go with this motor. The only other thing I might try is a free-er flowing exhaust system.
diceco