apologies for digging up a somewhat 'old' thread but i just couldn't resist a reply.

Kudos to all the research (and blood) that you guys have put in to all this. I have just read all 20 pages and i'm so glad i did. I can not believe that i didn't come accross this page AGES ago! Although not a direct comparison, i have a modded/hybrid Ryobi in my BOAT!

I came accross this thread looking for some ways to get more power from my rig but as it turns out, i have done most of them already! I have gone through almost all of these mods on mine and just by chance ended up with a similar combination- An Old style crank/case with old head and the newer MTD style twin ring piston with a Walbro WT-257 (1/2") as it gave me the best performance. I am also running mine with a large tuned pipe and velocity stack and it SINGS! I don't have figures (sorry) but listen to it in the video below, it is pulling some huge RPM (est . 12k) and i have never had any problems with mechanical failure, i.e rod etc. Mine uses the I beam style rod. The head is modified for water cooling/boat use, the flywheel is machined down and i use a pulley and electric motor to start it.

I think there is still some more potential for the engine in my setup but what i find baffling is there has not been too much talk of what fuels/lubricants have been run in them when the tests were performed - this can easily affect stats between the engines. I run mine on 98 octane premuim fuel (not just high octane, it has more power units per cc) mixed with Castrol Racing TT fully syn oil at 20:1. I have not seen any evidence that running it at lower concentrations of oil makes it run faster and running more oil at these RPM's has to be better. I have NO carbon build up with hours and hours of running. The internals of the engine are like new. Not even any tanning and only what i would call normal wear. The crank bearings however seem to suffer. I suspect that is more because of the occasional water ingestion rather than the speed/forces they endure.

I have also noticed for those that have the entire new engine style (with compression relief, 2 rings etc) that the area around the crank where the ports start, on all of mine there are some fairly reasonable casting marks causing an edge on them. Older ones do not seem to have this. I would suspect that removing them with a file/dremel would make a measurable difference with the flow past them.

Check these:
[youtube]http://www.youtube.com/watch?v=IBM6XZyVhMg[/youtube]



I have many more pics if anyone is interested.

Long live the ryobi!