ASK THE MXPERTS: THE X-FACTOR VERSUS THE NON-X FACTOR
Did MXA ever run the YZ250X on the dyno? I am curious to know how much horsepower it made overall and at different points on the rpm scale. I have scoured the internet and forums for results from this particular bike but to no avail. I know it’s not the motocross version, but I am dying to know how the YZ250X and YZ250 compare and contrast on the dyno.
Yes, we did dyno our test YZ250X, but only in your imagination can you expect the X-model to compete with the YZ250 motocross model. Here are the reasons why.
(1) Cylinder head and cylinder. The head volume was increased by 2.1cc from 21.5cc to 23.6cc. This added volume reduced the compression ratio from 8.9:1 to 7.9:1. The exhaust port height was raised 0.5mm, and the power valve shape was updated to match the exhaust port height.
(2) Pipe. The YZ250X expansion chamber is totally different from the YZ250 pipe. It is designed to tuck in tighter. The YZ250X pipe is narrower by 35mm.
(3) CDI. The CDI timing has been retarded in the mid-to-high rpm range for a broader and more gentle pull.
(4) YPVS timing. The Yamaha Power Valve System’s (YPVS) governor spring and preload settings open sooner for a softer hit.
(5) Transmission. The YZ250 and YZ250X share the same 14/50 final gearing. While the YZ250X’s first and second gears are identical to the YZ250 ratios, third gear is lower (comparable to adding one tooth to the rear sprocket,) fourth gear is taller (like going two teeth smaller on the rear sprocket), and fifth gear is so tall that it would equal five teeth fewer on the rear sprocket.
(6) Clutch. The clutch spring rate has been reduced by 10 percent for easier lever pressure. Pressure is 20 percent less at the lever thanks to the cam on the actuating arm having more leverage.
On the track, the YZ250X engine has a milder hit and is tractable from idle to 6500 rpm, then there is a significant dip in power right before the YZ250X picks up at 7400 rpm. Amazingly enough, at peak, which is at 8800 rpm, the YZ250X and the YZ250 are almost identical. Because of the lower compression ratio, the YZ250X has more over-rev than the YZ250. MXA test riders liked the YZ250X, and it was capable of being raced on a motocross track, as long as the rider was a Vet or a Novice and the track didn’t have any big jumps (since the YZ250X suspension has lighter compression damping).
On the dyno, the Yamaha YZ250X does not make more horsepower at any point on the curve until they merge together at 9000 rpm (which is slightly above the 8800 rpm peak). It should be no surprise that the YZ250X can’t run with the YZ250 motocross bike; it was designed for woods and off-road conditions, at which it is very adept. Below are the comparative dyno numbers.