Two-Stroke 250 MX

What it is

Bore is 66.4 mm, stroke is 72 mm, and the single cylinder sweeps 249 cc. Those three figures match the Yamaha YZ250. Nothing else here was copied from one. The torque comes out of the same Wiebe heat release solved against the same crank-slider geometry as every four-stroke on the roster. The cycle is the difference: one power stroke every revolution, and gas exchange through holes in the cylinder wall that the piston uncovers on its way past.

The spec table reads 10.6:1. The ratio this engine compresses through is 6.97:1, and the simulator's spec panel prints both. Compression cannot begin while the exhaust port is still uncovered, and a piston-controlled port is symmetric about bottom dead centre: this one opens 96 degrees after top dead centre and closes 96 degrees before it. So the model starts compression from the cylinder volume at port closure instead of the volume at the bottom of the stroke. Yamaha quotes the bike at 8.9 to 10.6:1 for the same reason. The power valve moves the port, and the corrected number moves with it.

Volumetric efficiency peaks at 0.95, and that peak sits at 8,000 rpm against an 8,800 redline. Under the rest of the range there is a floor of 0.5. That is an expansion chamber written as a curve. A tuned pipe sends a pressure wave back down the header and stuffs charge that had already left the cylinder back through the exhaust port, and it does that near one frequency. Below six thousand the engine breathes at half of its own best. Yamaha never published a crank horsepower rating for its motocross bikes, so there is no factory number to hold this one against.

Things to try

• Pull Exh port opens down from 96 toward the 75 the slider allows. That is a taller race port. Blowdown starts earlier, Trapped CR on the panel falls to 5.19:1, and the bottom of the rev range goes away to pay for the top.
• Take the same slider the other way, up to about 122. Trapped compression climbs to 8.83:1, close to the 8.9 Yamaha quotes with the power valve shut, and the engine pulls off idle instead of waiting for the pipe to arrive.
• Trapping eff starts at 0.72, so more than a quarter of the delivered charge short-circuits out of the exhaust and takes its premixed fuel with it. Drag it toward 0.95 and watch the L/h readout fall. A real engine buys that back with a power valve and a pipe.

Seen in

Yamaha YZ motocrosser

Same layout

Racing on the sand at Margate, England. The fat chrome expansion chamber under the engine is the two-stroke's tuning device, and the port it feeds is the one this page is about.

Photo: Andrei I · Pexels