SnowTech Tips: Airbox Temperatures
Winter 2000 - 2001

Many sleds blend warmer under hood air with colder outside air. Several Arctic Cat models draw airbox intake from just above the footwells, but as this photo shows, warmer under hood air is also drawn in from the front (intake foam is removed for clarity).

One of the more curious aspects of a snowmobile is the variation of intake temperatures. Here we have a carbureted snowmobile, with jetting that is calibrated to supply sufficient fuel for, say –20 degrees. At cold start-up, the jetting may even be a slight bit too lean on the bottom, requiring a blip of the choke.

As the ambient air temperature warms, the jetting and fuel delivery remains fairly constant (there is some slight compensation). Your engine that was properly jetted for –20 can actually be running quite rich at 20 above.

Now consider the effect of heating the incoming air. Why, or how? Many sleds, maybe without your knowledge, warm the incoming air before it enters the engine. The thermometer may say –20, but your sled is sucking warm air from under the hood that can be much warmer. The key here is the inconsistency and variation that can occur. Your jetting may be even further off than you ever realized; depending on the intake design and operating conditions. Deep snow, heavy load running at slow ground speeds can be a prime example of how the airbox intake temperature can vary wildly from the actual air temperature.

By placing a temperature-based float bowl pressure regulator like the Holtzman Tempa-Flow in your airbox, carb jetting is adjusted as actual airbox temperature changes.

The factory recommended jetting specs take all of this into account, but the fact remains that the actual air temperature inside the airbox can be fluctuating more than you realize.

Sucking warmed air into the engine isn’t all that bad in certain conditions. The venturi effect speeds up the air as it enters the carb, and the result in high humidity conditions can be icing of the carbs. Yamaha for years has liked to blend warm, under hood air with colder, outside air to help keep the carbs free and clear. On most Arctic Cat models, the airbox intakes, located above the foot wells, are ingesting an amount of warm under hood air exiting the rearward outlet vents.

Testing has indicated the airbox temperatures can be as high as 30-40 degrees warmer than the actual air temperature. Could this maybe explain why some sleds get such poor fuel economy?

EFI and DPM equipped sleds have airbox sensors, and the result is improved fuel economy and performance as the fuel delivery is far better matched to changing air temperature; ambient air temp and incoming airbox “blended” air temp. While there may not be much you can do to better regulate this wide ranging variable, there is something you can do to compensate for its affect; install a float bowl pressure regulator based on temperature like the Tempa-Flow from Holtzman Engineering (715-479-8727).

The $148 Tempa-Flow attaches to the carb float bowl vent lines and regulates the float bowl pressure by sensing the air temperature. It applies no pressure correction at –20; any temp warmer and it reduces float bowl pressure. Jet your sled for –20, and the Tempa Flow does the rest, providing improved fuel economy and performance. Since it is a mechanical device, it doesn’t react as quickly as an electronic system like DPM or EFI, but it will keep the jetting far closer to ideal than before.

Sleds that already route the vent lines to the airbox will require little, if any, jetting changes and are prime candidates for placing a Tempa-Flow in the airbox. However, if your vent lines are not currently routed to the airbox, a significant increase in main jet size may be required if you relocate the vents to the airbox due to the loss of under hood pressurization of the float bowls.

This is a logical product that provides real world benefits on sleds that have no temperature compensation.