Technology: major (r)evolution on KTM two-stroke crossers
Yesterday, market leader KTM showed their new motocross models to the general public. These received a major update, which gives the Austrian brand a significant lead over their competitors. Both the four-stroke and two-stroke engines were redesigned, but the step taken in two-stroke technology is certainly a very big one.
Today we fillet the three two-strokes from KTM. The SX125, SX250 and SX300 are completely new. The two-strokes get fuel injection, an electric starter and other nice things. KTM was one of the few brands that continued to develop their two-stroke motocross bikes. That turned out to be a golden move because die-hard two-stroke enthusiasts worldwide were served by the Austrian engineers. In Japan, only Yamaha continued to build two-strokes and put new stickers on their old 2006 models every year.
Yamaha finally saw what the fuss was about last year and gave their YZ bangers a solid engine update. This did not really come close to the power delivered by a similar KTM, but at least the blue machine was now more or less competitive again. However, the update that KTM sent out to the wide world yesterday is of a different order of magnitude. Let's just say it's a small milestone in two-stroke technology. With the introduction of these new dirt bikes, KTM is perhaps taking the biggest step in two-stroke technology ever.
Premix in da house
Spy photos previously gave us insight into the big turnaround at KTM. They showed that a carburetor was no longer necessary and that there was only one option to fill the cylinders: EFI or electronic injection. KTM had introduced the TPI injection on their enduros a few years ago, but it was not retained. Instead of screwing two injectors into the flush ports, they are now located in the throttle body.
Together with Keihin, a 39mm throttle body was developed that can withstand the rigors of competitive motocross. This is done by a specially tuned ECU that works with the Keihin throttle body to always deliver the correct air-fuel mixture by continuously analyzing water temperature, air temperature, ambient pressure, crankcase pressure, engine speed and throttle position. The ECU was developed by Vitesco, a German company from Regensburg that specializes in drive systems for the automotive sector. Newly designed injectors also ensure optimal combustion speed, with an improved and therefore finer fuel mist being produced for cleaner ignition and less fuel waste.
In contrast to their TPI system on KTM enduros, the motocross bikes will need premix as usual. So there is no separate oil tank and therefore no separate injection of two-stroke oil. Adding oil to the petrol remains a must if you want to race on a modern two-stroke. It is not yet known whether the mixing ratios have changed compared to the models with a carburetor.
KTM says the 2023 KTM SX range is a realistic alternative to four-stroke engines, offering optimal power delivery in all conditions without the need to manipulate carburettors in terms of jetting. From now on, KTM owners no longer have to worry about adjusting a carburetor, changing the height of the throttle needle and replacing the jets.
The reed valve
The reed valve has received a major update, with newly added plastic flaps on the outside of the reed valve housing for better sealing of the entire intake tract. This new design ensures that the cylinder does not overfill with fuel in extreme uphill or downhill passages without seizing or running rich. Boyesen continues to supply the carbon cover plates or reed flaps.
The electric power valve
With the introduction of EFI, the power valve in the exhaust port is controlled based on the throttle position and engine speed. Because the valve is automatically calibrated, there is no more hassle with incorrect adjustments of the power valve, because the ECU automatically calculates the ideal position. This results in linear and predictable power delivery at all throttle positions.
The electric starter
From now on, the three KTM two-strokes are equipped with an electric starter. Kicking is not necessary, it is not even mentioned anymore. The machine runs at the push of a button, a luxury that is now also available to two-stroke enthusiasts. The start button can be easily switched via a newly designed combined Start/Stop switch on the right side of the handlebar.
The starter motor is now even more integrated into the engine compartment and is supplied without an intermediate shaft, which saves weight and enables a compact engine design with perfect integration and protection against rotation. The weight saving goes even further, with an ultra-light 12,8 V 2 Ah Lithium-Ion battery placed under the saddle close to the center of gravity.
The mappings
With the addition of EFI it was possible to introduce different engine mappings on the 2-stroke SX machines. You can easily switch this via the switch on the left side of the handlebar. Map 1 provides a standard, more moderate curve for linear, predictable power. Map 2 is more aggressive with a more direct throttle response and a more explosive power delivery.
The exhaust
The 2023 KTM SX series benefits from a more compact exhaust design, which has improved ground clearance and reduces the risk of damage from deep ruts or outside objects. The silencer also benefits from the compact treatment, made of lightweight aluminum and stylishly finished with a premium black coating.
Conclusion
KTM has set a major milestone in two-stroke technology with these new two-stroke models. It was an open secret that Austria was working hard to banish the carburetor permanently to the museum. The convenience of the starter motors that have been on their four-strokes for years is now also common on two-strokes. A logical consequence is a power valve that works electrically and is controlled by the ECU.
With this introduction, the brand's two-strokes will suddenly have a lot of electronics on board. In terms of reliability, this is no problem at all, because in Mattighofen they have had years of experience with this on their SX-F models. However, the rider and the person who services the machine will have to take a completely different approach to tuning and maintenance. From now on, the characteristics of the power delivered will be much more determined by ECU tuning. Before that, tuning was applied almost entirely mechanically. In any case, KTM has nothing to fear from the competition in the coming years. Two-stroke technology remains popular and with injection on their machines they serve a large group of motocross riders who swear by pre-mix gasoline.
We are eagerly awaiting a test with these new “ring ding ding's” from Austria.
Tekst: Danny Hermans
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