KTM’s 250cc four-stroke and Moto3′s final rules

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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 81×48.5mm 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, 78×52.2mm. 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:

Moto3 rules via MotoMatters

  • walter

    Yes!
    Small bikes.

  • http://pinkyracer.com pinkyracer

    that’s so cool! I’m so glad it’s not limited to one manufacturer. still not cool enough to replace the beautiful music of 2-strokes, but I know we must let them die out so that we can survive. :-( I wish we were getting rid of 2-stroke garden equipment instead.

    • Sean Smith

      Meh, let the 2-strokes die. The new bikes are superior in every way. Plus, this should help build interest in small sportsbikes. It’s ridiculous that you have to buy a 600 if you want a decent chassis and suspension.

      • Charles

        Aside from not blowing up as often, aside from not oiling down the track when they blow up, aside from costing a quarter as much to buy and a quarter as much to run, aside from weighing in the same with fewer exotic parts, aside from having actual powerbands and being real racing engines, I’d almost be inclined to agree with you except that the only reason four-stroke racing motorcycles exist is that Honda prefers them and every racing organization wants Honda in their series.

        • Penguin

          Aside from not blowing up as often? Please! My right knee still aches when it is cold because one fine day an NSR250 piston decided to give the bore a not so tender embrace and become one piece of metal.

          I got my current job by swapping stories of RD250LC’s and NSR250s seizing up with my boss. I say let the four bangers come and when KTM release a 250 Sportsbike then I’ll have my name down for one. (Probably have to go on a diet first though)

        • Sean Smith

          So a 14,000rpm 250 making 50hp, with 2-stage injection, traction control and tunable engine braking isn’t a ‘real’ racing engine? Even if it’s designed and used solely for prototype racing? Reminds me of Harley guys that dismiss my GSX-R as ‘only’ a 600 and not a ‘real’ bike.

          These are GP bikes. They’re supposed to be wild and exotic. That’s one of the things that makes them interesting.

          • http://twitter.com/metabomber Jesse

            I welcome our 200hp / liter, naturally aspirated overlords.

          • Charles

            It’s a real racing engine – but racing a 250F against a 250T would be like bringing a knife to a gunfight.

            A two-stroke racing engine seizes a piston – the mechanic pulls the head, cleans the aluminum off the cylinder walls with some muriatic acid, a new piston, ring, pin, bearing and clips go in, and they’ll make it for the next session for sure. A four-stroke racing engine seizes a piston – the mechanic pulls the engine and installs the spare, later to rebuild the blown-up engine back in the sanitary room at home base.

            If I were designing an engine to be run in a hostile environment, which a racetrack certainly is, it wouldn’t be something that relies on a continuous, high-pressure stream of oil and can’t be rebuilt in the pits. Honda’s ’60s four-strokes were almost certainly the greatest racing engines of all time but they were about as far removed from the engine a racing team would want as you can get.

            • Sean Smith

              All good points, but these bikes are replacing 125s, not 250s. Those were replaced with CBR600RR powered Moto2 bikes.

              This is also GP racing. Ease of use and cost are just about irrelevant. Besides, you can rebuild a single cylinder four-stroke in the pits. My old boss used to do them in the back of his trailer for District 37 desert racers 4 at a time. Even replacing cranks, holed cases, etc isn’t a big deal on a motor like that.

  • http://www.muthalovin.com the_doctor

    Should be pretty rad. Hopefully KTM will be successful enough to be interested in the big event.

  • http://www.pedalgents.com holdingfast

    that engine would look so good in my livingroom!

    • John

      That engine would look so good in a monkey bike!

  • Your_Mom

    I am surprised that anyone would believe that the manufacturers would convert MX engines to road-racing use. There are too many shortcomings that, by the time they were removed, the converted MX engine would no longer resemble its original configuration. There are three failings that come to mind with only a minimal analysis.

    1 – MX engines almost exclusively use roller-bearing crankshafts. They do so since roller-bearing elements can deal with brief periods of less than ideal lubrication supply as a result of the dynamics of a moving mass of oil as the chassis gyrates over jumps, etc. However, roller bearings have much higher friction which results in power losses. For this reason, all modern road racing engines use plain bearing crankshafts and rods.

    2 – Straight intake tracts increase cylinder filling which increases BMEP which increases torque. While arguably most significant at peak RPM cylinder filling increases torque at all engine speeds. MX bikes use their crippling curved intake ports due to packaging constraints – not for any superior gas flow characteristics.

    3 – MX engines have incredibly low cylinder deck heights to minimize their height. They are inherently taller than their 2 stroke-cycle predecessors due to the cam and valve-train which a 2 stroke-cycle engine does not need. In their efforts to keep the engine package as small as possible, the MX engine designers have opted for an extremely short cylinder which mandates a correspondingly short connecting rod. Unfortunately, such short connecting rods increase piston thrust loads to unacceptable levels for a road-racing engine. These thrust loads result in significant increases in friction which again creates a loss of power.

    I suspect that it is for these reasons as much as a rule mandate that the manufacturers have opted for a clean sheet design based on first principles. While converting an MX can be done it is not the way to win a world championship – which is what both KTM and Honda are aiming for.

    Best Regards.

  • Your_Mom

    Sean writes: “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.”

    I know of no principle of gas flow that will “…build momentum…” due to “…sharp bends right after they exit the ports….” If anything, a sharp bend will decrease momentum as the gases negotiate this curve. As the exhaust gases have the assistance of the piston movement to aid their exit, this is not as much a crippling factor as a bend in the intake tract.

    Best Regards.

  • Kevin

    Thank you Mom I was confused/dismayed by this as well. I wpould like a response on this point from Sean.

    • Sean Smith

      Honestly, I don’t have a simple explanation for that, but I will say this:

      –The smaller diameter of twin pipes (versus a larger single) should help keep speed up.

      -Those bends are continuing the turn that the gas is forced to make as it exits the port. Continuing one turn and making it into a long, smooth curve eliminates the need for a second turn a few inches later.

      -Making the turn at peak velocity and heat will hurt flow less than making the turn later.

      -KTM built this motor for maximum power and they did the pipes this way.

  • http://www.postpixel.com.au mugget

    That is one miniature engine!!

    Also that’s some analysis.. how do you know that stuff? HFL has an engineer on their hands? :O