The 2-stroke motorcycle was a mainstream fixture to the off-road enthusiast up until 1997, when the environmentally cautious California Air Resources Board (CARB) approved drastic changes to the off-road motorcycle emission standards. The new emission standards resulted in the inability to register nearly all 2-stroke bikes, as they were now deemed as an air pollutant, ultimately leading to their demise.
While later CARB approvals allowed for the registration of “non-compliant” bikes, riding became seasonal to restricted areas. The red sticker provided for non-compliant bikes did not deliver the same riding freedom as the green sticker. The void of a compliant off-road bike forced manufacturers to focus on developing a motorcycle that delivered the freedom to ride outside of restricted areas, while receiving the approval of CARB. Manufacturers looked to create a 4-stroke motor with performance and riding ease comparable to a 2-stroke motor.
The first few generations of 4-stroke motorcycles were heavy and difficult to start, and off-road enthusiasts were not impressed. With continued improvements, 2004 turned out to be the year the 4-stroke motor accomplished the consumer’s riding approval and surpassed anything available in a 2-stroke.
However, consumers quickly became aware of the new costs associated with the enhanced performance of 4-stroke bikes. With an increased 30% to 50% of moving parts used by the 4-stroke motor, many do it yourself mechanics grew dependent on dealership mechanics with specialized equipment to repair their motors. The service departments may have enjoyed the spike in business, but many frustrated riders left the sport due to the increased repair costs. The increased cost of maintenance on 4-stroke motors continues to be a hot topic in the industry, as they have become the primary engine type.
So, why do some manufactures still produce 2-stroke bikes, and why, as consumers, do we continue to rebuild, overhaul, and restore 2-stroke bikes nearly 20 years after their decrease in popularity? Those of us who rode prior to the 4-stroke revolution long for the lightness, ease, and low maintenance provided by the 2-stroke. We have patiently waited for an updated and successful 2-stroke bike capable of competing in the same arena as the 4-stroke machines.
The release of KTM’s TPI 2-stroke motors is the answer to our longing and patience. The KTM TPI 2-stroke is the first major motor development to an off-road 2-stroke motorcycle in over twenty years! I had the opportunity to speak with Dave Sulecki from Wiseco about the motor, and more specifically the piston components, and if they were similar to current 2-stroke pistons. His answer, an outstanding yes, plus more! KTM’s new TPI (Transfer Port Injection) system is similar to what snowmobiles have been effectively operating with for years. Snowmobiles use direct injection and fuel injection (BRP had the first TPI motors), and Wiseco is precisely familiar with this motor configuration.
As the TPI system is not new to the Powersports industry, KTM has updated the technology utilized in the TPI 2-stroke. Fuel is injected into the Transfer Port by using two fuel injectors, which creates fuel air turbulence at higher revolutions per minute (RPM), allowing for a consistent fuel delivery. With a single injector on a 2-stroke motor at high RPM, the fuel is not dispersed in a consistent manner, leading to an inconsistent fuel burn. 2-stroke motors, unlike 4-stroke motors, do not have a specific stroke for mixing.
TPI motors can manage fuel delivery in a very precise way, as opposed to a carbureted motor. TPI motors are far more efficient, and can adjust the exact amount of fuel delivered. Carbureted motors send a specified amount of fuel on a pulse signal from throttle opening and crankcase pressure. FI and TPI Motors run far leaner in specific conditions, while constantly adjusting to throttle response. This results in a leaner mixture, which produces less unburned hydrocarbon emissions.
With a leaner mixture, the motor also creates more heat. Wiseco being a long time 2-stroke piston manufacturer, you can imagine the excitement to development a piston for this innovative platform. Wiseco pistons for the TPI engines are similar to those of modern carbureted 2-stroke engines, with the exception of the piston crown. The crowns on the TPI pistons are designed to ensure they can handle the increase in operating temperatures.
The pistons feature hard coat anodizing on the crowns and ArmorGlide coating on the skirts. Hard coat anodized crowns has proven to better resist the wear cause by combustion chamber conditions, and ArmorGlide skirt coating is a Wiseco proprietary skirt coating that remains for the life of piston, reducing wear and providing smoother operation. The features of Wiseco’s TPI pistons allow the piston to dissipate heat and ensure engine reliability and performance, along with less unburned hydrocarbon emissions.
The KTM TPI 2-stroke’s oil is injected directly into the crankcase, differing from previous 2-stroke motors that relied on premixed oil and gas. The days of 2-stroke motors fouling plugs are over with TPI. Carbureted 2-stroke motors usually allow unburned gas to settle at the bottom of the crank, causing a bellowing of smoke when started, nicknamed “loading up.” Anyone who has ever rode a carbureted 2-stroke has experienced this after a stall or tip over. With TPI, oil is still burned for lubrication, but can be metered as needed, and will not create excess burn. It has been proven under idle that the mixture will be extremely lean, while adjusting the mixture as the bike increases in RPM. This means more consistency in lubrication and less unnecessary oil burning.
So, what does this all mean to the consumer? For starters, KTM is pioneering technology and initiating industry standards by building efficient and cleaner burning 2-stroke motors. KTM has worked tirelessly developing this bike in Europe, where they encounter far stricter emissions regulations than North America. The KTM TPI 2-stroke easily passes all emission standards in Europe. Previous CARB standards that basically retired the 2-stroke and started the 4-stroke revolution should not get in the way. With potential emissions issues being resolved, does this mean we will see the KTM TPI greatly accelerate the 2-stroke revival? The thought of using a far simpler motor with less moving parts will greatly reduce repair and maintenance costs, as rebuilding a TPI motor is no more difficult than traditional 2-stroke motors. Wiseco has continued to put money and time into 2-stroke performance innovation, leading to parts that complement new 2-stroke engine technology such as KTM’s TPI.
All bike and engine/engine component photos courtesy of KTM.