Yamaha YZ250
The YZ250 is the 250 cc motocross two-stroke that stayed in production long after the class went four-stroke, because the engine was right. Unusually for a high-performance two-stroke it is undersquare — 66.4 mm bore against a 72 mm stroke — which is a deliberate choice for tractable, usable torque rather than peak power. The preset's 8.9:1 geometric compression is the highest of the two-stroke set. Modelling it against the RG500 preset is the clearest demonstration available of how stroke choice shapes a two-stroke's character, using the same underlying model for both.
Preset parameters
Values below are the preset the calculator loads — you can change every one of them in the form. Angles are absolute crank angle (CA) on a 0–720° cycle.
Reference figures
Manufacturer figures carried by the preset for comparison. They are reference values, not calculator output.
Notes for modelling
- Long stroke plus high rpm means high piston speed — the dynamic results show what the small end is carrying.
- Its powerband is wide by two-stroke standards; that is port timing, and it is editable.
- Re-tune the expansion chamber after any port change.