SZAM
01-29-2003, 11:11 PM
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, most snowmobilers opted to have the ACCS removed because their sleds ran better without it.
The neat thing about Ski-Doo is that they take a more technological approach to solving the high altitude compensation issue. They use a system called DPM for Digital Performance Management which monitors several factors including barometric pressure, engine temperature, and air temperature. It then alters the jetting by changing float bowl pressure and also manipulates the timing curve to keep the engine performing well. The downside is the complexity of the system. With plain carburetors, you’re at least reasonably sure that if the jetting is the same, it will perform the same.
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.
Hope this helps someone
The neat thing about Ski-Doo is that they take a more technological approach to solving the high altitude compensation issue. They use a system called DPM for Digital Performance Management which monitors several factors including barometric pressure, engine temperature, and air temperature. It then alters the jetting by changing float bowl pressure and also manipulates the timing curve to keep the engine performing well. The downside is the complexity of the system. With plain carburetors, you’re at least reasonably sure that if the jetting is the same, it will perform the same.
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.
Hope this helps someone