It's a busy day in the Moto3 world. KTM has unveiled its new 250 four-stroke and Dorna have also released nearly-final rules for the class. Looking at the motor with those rules in mind, KTM seems to have a winner. Here's a step-by-step breakdown of KTM's engine, the new rules and how this will effect Moto3 (and hopefully small-displacement streetbikes).
Many people, us included, were expecting to see motors based off production 250cc MX bikes. A well-built KTM 250SXF motor could probably deliver 54bhp with ram air factored in. To get that power, you'd need to use a wildly different and 5mm larger 81mm piston, an 8.5mm longer rod and a de-stroked crank to achieve an 81x48.5mm bore and stroke. Monster cams, careful porting, a custom header, ram air and a 14,000rpm rev limit. The KTM factory ignition box has a 13,800rpm limit and the motors work fine at that speed, so another 200rpm shouldn't kill them. The shorter stroke, lighter piston and lighter overall rotating assembly would also smooth things out a bit compared to stock.
It would've been cool, but Dorna has updated the rules and made production motors illegal. The motor we've already seen from Honda has the same bore and stroke as the 04-09 CRF250R, 78x52.2mm. We'll only know for sure once Moto3 racing starts next year, but the smaller piston and longer stroke should mean the Honda will have a less extreme and more versatile power curve. Easy to use, but ultimately not the fastest way to do things. Because of that, my money is on KTM.
The physical size and shape of a 250MX motor is also less than ideal for road racing. MX bikes have lots of ground clearance, traditional cradle frames with motor mounts down low, very little need for lean angle and much longer swingarms than road racing bikes.
By the looks of KTM's motor, they're likely using a larger piston and shorter stroke. Using specially developed forum troll photoshop science, I've calculated that the exposed part of the cylinder on the KTM Moto3 engine is about 36mm tall. On their 250SX-F, it's 40. We also know that the stroke of that bike is 54.8mm. That's 1.37mm of thick aluminum and liquid cooling for every mm of stroke. Apply that same number to the Moto3 engine and we end up with a guesstimated stroke of 49.3. That would dictate a bore size of 80.3mm. This is obviously fuzzy math, and without taking the motor apart, there's no way to tell for sure, but building a motor with a larger piston will almost certainly make more top end power.
The outside appearance contains more clues that point toward big power. Dual headers with sharp bends right after they exit the ports will help hot exhaust gases build momentum and create a vacuum in the cylinder. The arrow straight intake will dump a ram-aim pressurized column of air and fuel right on top of intake valves with no restriction. These design features work best at extremely high RPM, which goes along with the prediction of a large piston.
Those are 8mm bolts holding the cases together. Using that as a point of reference, you can measure the exposed cylinder height.
A motor that sacrifices everything else for top end power would make sense for a Moto3 racer. A 250cc single racing on a GP circuit will spend most of its time at wide-open throttle, so outright power is much more important than in MotoGP.
Teams will be able to modify the prototype engines by machining parts, porting the heads and changing out things like cams, pistons and cranks in the search for more power. This contrasts sharply with the current electronics war going on in MotoGP pits. With a spec ECU and throttle body, all that's left to tune with are the actual hard parts. This means a return to old-school hot-rodding, lots of dyno time, and the most powerful four-stroke 250cc singles the world has ever seen.
There also won't be any speed to gain from transmission developments. The rules expressly forbid systems like the one currently in use in Honda's RC212V MotoGP bike. There's also the rule under engine supply that says " In order to ensure this price limit, any Team competing in the race will be granted the right to buy the engine used in the race from another Team, at the end of the race for a fixed price of 12,000 Euros (details TBA)." It's doubtful anyone will have to use that, but the threat of having your cheater motor purchased by another team for less than you paid for it should be enough to keep people honest. Maybe.
What does it all mean? First, interesting racing and a variety of different machines. The bikes should get faster and the combined rider+bike minimum weight will make things fair between jockey sized younger riders and full-sized older riders. Teams will hire engine tuners and their job will be extremely important because teams that find an extra horsepower here and there will have the fastest bikes.
Here's the new Moto3 rules in full: