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Photo: KTM 300 EXC TPI - Six Days ISDE Edition 

Two-strokes are still a hot button when it comes to talking about dirt bikes these days. The old "two-stroke vs. four-stroke" debate has been beat to death and many of us are sick of it, but it rages on regardless. 

OK!! So four-strokes won, the Japanese factories, AMA and EPA got what they wanted and it's over...two-strokes, once the powerhouses of motocross, have now been relegated to the shed, gone out of fashion and not used by any top racing team in MX or SX. 

But the two-stroke is still gasping for air thanks to KTM and maybe even Honda... who officially stated they'd converted to a four-stroke company years ago, so that's a surprise! 

Why is it still here?  

Because it's awesome that's why...two-stroke engines pack more horsepower per pound than four-stroke engines, and even if that statistic was equal, the number of complicated, fragile and expensive parts in a modern four-stroke will always cost more to replace. 

Granted the replacement interval for four-stroke motorcycle engines has gotten longer and longer but you'll always have the complication and expense factors to think about...and that's good for the manufacturers...a nice balance between reliability and the need to replace worn parts makes for a good bottom line, but that's another discussion. Obviously less moving parts and making more HP/lb are excellent attributes that appeal to motorcyclists and maybe not so much to the manufacturers at large. 

Three years ago, I wrote an article that talked about advances in two-stroke technologies and the possibility that these technologies (EFI, DFI, TPI) could help the two-stroke gain more market share. One of the conclusions was that EFI using DFI was too expensive, bulky and heavy to be a reality on off-road motorcycles and that has turned out to be the case when looking at how the technology is presenting itself in production form. 

KTM have been the leaders in two-stroke motorcycle engine design and accompanying technologies so it was only natural that KTM would be the first major motorcycle manufacturer to provide a viable cleaner-burning technology to the two-stroke arena. The first bikes to display this technology are the KTM 250 EXC TPI and the KTM 300 EXC TPI. Honda also has filed a similar patent but has not put any examples into production and looking at the patent drawings, it appears to be an industrial design featuring a pushrod, not suited for high-performance applications.  

What does this advancement mean, and is this the saving grace technology that two-stroke fans have been waiting for? 

No. But OK, it's a great advancement in terms of the accomplishment - but how does it impact the market as a whole? It's great if you ride enduro bikes in the EU...but will TPI bring two-strokes back to off-road bikes?  

Maybe, but motocross only bikes won't be included. Why not?  

Because the Japanese factories have a lot of time and effort invested in four-stroke technology and it's not going away. They influenced the sanctioning bodies and promoters to implement unfair displacement rules that favor four-strokes. 

So why did KTM do it? 

Because a lot of folks ride two-stroke enduro bikes and KTM sells a lot of them both in Europe and here in the USA! Although KTM doesn't make two-stroke streetbikes per se, they do have two-stroke enduros with plates and lights and these enduro models are homologated for use on EU public roads, which means they have to adhere to tough new Euro4 emissions limits, as well as be prepared for the upcoming Euro5 restrictions. 

I've spent a lot of time in the EU and small bikes matter...in fact small motorcycles are the norm not the exception. You see lots of small two-stroke bikes and scooters...but the EU impose restrictions on emissions so these bikes need to have some kind of emissions/clean air technology if they are to survive and prosper.  


Photo: New for 2018 KTM TPI Unit

KTM has come to the conclusion that Transfer Port Injection (TPI) is the EFI delivery system that has won the war against its Direct Fuel Injection (DFI) rival which was the technology explored earlier by KTM. Why? 

DFI seemed good and had been proven on the street in a few smaller two-stroke applications and a bunch of four-strokes but when all the support hardware and electronics were installed on an off-road machine, things didn't play as well. Weight, expense and complexity all played into KTM abandoning the DFI technology. But they didn't stop looking for a cleaner burning two-stroke solution. TPI wins that war until something better comes along. 

TPI wins for now because of its unique new design which according to KTM features "two lateral domes, holding the fuel injectors supplying fuel into the rear transfer ports. Thus the loss of unburnt fuel is reduced for less emissions, a more efficient combustion and reduced fuel consumption. A little tube in the back of the cylinder is connected with an intake pressure sensor, which supplies pressure data to the control unit."  

KTM continued: "The TPI engine is fitted with a newly developed throttle body made by Dell`Orto. It features a diameter of 39mm. The airflow is regulated by a butterfly connected with a twin-cable throttle cam, which is operated by a new handlebar throttle assembly.  A throttle position sensor provides airflow data to the control unit, while a bypass screw allows the regulation of the idle speed. The cold start device also opens a bypass supplying more air. Via an oil intake tube oil supplied from the oil pump is mixed with the incoming air to lubricate crankshaft bearings and cylinder/piston etc." 

Another great advantage is no more pre-mixing the fuel with oil, there is now an oil tank and metering system controlled by the ECU and this allows a very precise, variable and minimal mixture of oil to be burnt resulting in lower emissions.  


Photo: KTM 2-Stroke TPI Engine for 2018

Smaller niche companies that make two-stroke off-road machines in the same niche could be expected to follow this trend, possibly by licensing the technology from KTM or creating their own variants. This could include brands like Sherco, Ossa, Beta, TM, Gas Gas and the KTM-owned Husqvarna brand. 

So more cool bikes in Europe but what does it mean for riders in the USA? 

It could mean a little or a lot. Could this mean that KTM will be able to certify two-strokes with an emissions label indicating for "on-highway use"? This would be the hurdle to cross...once they've passed emission in places like Californis, the whole pie is up for grabs – here is what the California regulation says now: 

"Off-highway motorcycles must have an emission label affixed to the vehicle indicating certification by the manufacturer for on-highway use when converting to on-highway or dual registration. Registration guidelines for off-highway motorcycles converting to on-highway or dual registration require verification of the emission label." 

For KTM two-stroke riders, this technology would make sense to scale to the whole two-stroke lineup from EXC to SX. Imagine a version of the 250 EXC with lights that's legal for the streets here...and you ride it to work like the ultimate hooligan, or just be able to get a street plate to ride to your favorite riding destination. 

In conclusion, most people thought two-stroke was dead but maybe, just maybe, technology like TPI will make it feasable for KTM to expand their offerings using this technology. The first step it seems would be the ability to pass emissions both in the EU and USA, then maybe apply it to US market machines.  

If that happens we'll be first in line to try them. 


ThumperTalk wants Your Comments On This Article:

Is this a stopgap or a technological step forward?

Would you ride a two-stroke street bike?

Aren't those Six Day graphics awesome?

Tell us what you think below!!

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User Feedback

Both 2t and 4t will live on in one way or another, and the debate of one vs. another is dumb as they are tools for different jobs. While the emissions standards may make things difficult, there are many places in the world that don't have those standards. Where they do exist, there are still many people that love the bikes and will find ways to ride them.

If anything is going to revolutionize things in the near future, it's the electric bikes. All it takes is a breakthrough in battery tech, and we'll be living in a different world.

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Tested the Husky 300TI and was impressed but not a patch on the old open class 2 strokes considering the leap in technology (300TI vs CR500), it was a bit flat mid range. what a bike that would be if Honda were to  bring back the CR500 as a TPI.




Edited by Derekj1563
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5 hours ago, ged93 said:

From 2020 EFI would be the future for 2 stroke , now manufacturers can do some tricks to pass emissions tests , but with EURO 5 and 6 that won't be enough , even the ktm solution for tpi it's not enough as only the direct injection can pass ( if you see some TPI unboxings , you can see that it still comes with closed exhaust , so they had not solved completely the problem ) .

Beta , Sherco and Tm are ready with EFI , lets see how it goes .

Emissions? Two strokes and most dirt bikes have been sold as closed course race machines for decades now and not subject to emission testing in the states. 

Many states do not even require emissions testing for vechicles, my state included. 

Fuel injection will only raise purchase price and cost to modify, and likely produce less power out of the box. 

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Let's remember that Honda won Baja on a specially built two stroke engine that was - as memory serves <G> injected - but shelved the technology to concentrate on four strokes.  I don't like honda but I will give them credit when it's due.. We all owe them for what they have accomplished in getting Japanese motorcycles accepted in America.  Of course their R&D is more than the other big three combined sp cutting edge technology is Honda's thing....

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