Chapter 4 - The Engine
So, my searching for fast 50cc motors, at least those readily available in the US, led me to kids' dirt bikes. KTM 50 SX Pro Sr, circa 2001-2006 or so, seemed to be the best compromise. Parts are plentiful enough, and their clutches were frustrating enough to keep prices for motors somewhat low. The newer generation(s) of the KTM 50 have better cylinders and clutches, but they cost more, at least at the time. And the version I ended up with certainly ain't bad! Hard numbers are hard to come by, but seems to be something like 14,000rpm, 10hp, 9000rpm clutch out of the box. Not bad for a single-speed!
Of course, I am trying to nudge the limits of 50cc single-speed, so I dove into the beginnings of a plan to extract more power. More power would necessitate a better clutch, so I ended up with a Tomar Hole Shot. Apparently, much less frustrating than the stock 3-shoe, and a quality item. Of course, the real tough part of single-speed, well, speed, is balancing clutch meltdown/explosion with acceleration with top speed with durability. Pick two, maybe three of those. I'm sure many, many of you know too well.
But to start, I just wanted to see what kind of power could be extracted from the stock cylinder. It has pretty good ports, better than pretty much all moped kits, with a 3-port exhaust and 7 transfer windows (4 main, with the boost port broken into three interconnected windows). The main downside is the rather sharp bend in the main transfers, and less-than-ideal reentry of the auxiliary exhaust ports back into the main:
I used these port molds to start modeling the engine for simulation in EngMod2T, a fantastic software. Some of the world's most accomplished 2-stroke tuners use it and it helps them push the envelope in their classes - think 50hp from a 125cc kart engine - they have found good correlation between the simulation and the dyno. Like any mathematical modeling, there are significant assumptions baked in (especially because EngMod is a 1-D simulation), but if you're in the ballpark, one of the most powerful aspects of simulation isn't the raw numbers (50hp from a computer doesn't mean anything), but the ability to compare design/tuning options back-to-back. That said, as with most things, garbage in = garbage out.
So at the time (May 2016) I explored some options - given a need for high clutch stall, should I go for a crazy high-RPM setup, or closer to the stock motor? If I'm going for raw performance, durability be damned, high rpm setup (exhaust duration increased about 20deg, transfers about 10, for a total timing of something like 205/130, with pipe to match) makes much more power under the curve:
As for progress, that was about as far as I got back then. For the next 3+ years, everything sat on shelves.