Along with cylinder port design, another important aspect of performance is fuel mixture. There is a perfect ratio of fuel and air (called the stoichemetric ratio) at which each engine will produce maximum power. At high altitude, the amount of fuel must be decreased to match the diminishing amount of air. That's why most of us are very familiar with the fact that jetting down is important to performance at altitude.

There have been a couple of attempts in snowmobiling to eliminate the manual act of changing jets. In the early to mid 90s, Polaris and Arctic Cat sold snowmobiles equipped with fuel injection. Unfortunately, early fuel injection turned out to be costly and hard to service with no real demonstrable advantages over standard carbs. Ski-Doo's High Altitude Compensator (HAC) and Polaris' Altitude Compensating Carburetion System (ACCS) of the mid '90s took a similar approach to solving altitude fuel mixture compensation. Both employed a metering device that physically changed size with atmospheric pressure changes and altered float bowl pressure, thereby changing fuel flow. Of the two, Ski-Doo's generally worked better, although both were plagued by moisture ingestion problems and inconsistency between units. In 1999, for example, most of our customers opted to have the ACCS removed because their sleds ran better without it.

Enter DPM. Ski-Doo's latest system has been very impressive at actually performing altitude correction. We tested extensively last winter and were very impressed with its ability to alter fuel flow. So how does DPM work? The principle behind DPM is the same as HAC or ACCS. Carburetors work by applying atmospheric pressure to the float bowls. When the engine intakes and creates a vacuum, that pressure pushes fuel out of the bowls, through the jets, and into the throat of the carb, where it is atomized and mixed with air, and finally ingested into the engine. When you change jets, you alter the size of the orifice that the fuel can flow through, thus altering the amount of fuel that gets to the engine. DPM works on the other side of the equation: it alters the amount of pressure that is put in the float bowl. With the same size jets, less pressure in the float bowl means less fuel flow. The advantage to DPM is that it is not dependent on a physical process to detect changes in altitude. Instead, an electronic pressure sensor is located in the airbox. The sensor's information is then used to control a solenoid located in the DPM “rail.” The solenoid acts as a gate keeper between atmospheric pressure and the float bowls. While the solenoid always cycles at 30 Hz, it simply varies the duration that the valve is open and closed, thereby altering the percentage of amount of atmospheric pressure that the float bowl sees. Because the system is electronic, it is very precise and all units can be calibrated exactly the same. Since the carbs are still used, the system also features the advantage of being able to alter the baseline by changing jets if the engine is modified. Overall, DPM is by far the most effective system ever introduced to keep fuel mixture optimum at changing altitudes.