howto in the hood

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Your hood, that hunk of high-tech plastic covering your sled's motor that is the moving billboard for your OEM, serves more than as a statement of who you are; it has a purpose. Heat and poor ventilation are two things that detract from a sled motor's performance, but can be rectified with hood design.

Helpful hoods
When an OEM designs a snowmobile, much attention is given to its hood. They come in varying sizes: big ones for Polaris' Dragon, and most Arctic Cats, or shovel blade-sized like on the Ski-Doo's XP or Yamaha's Nytro. Regardless, the hood, and its corresponding side panels work together to exhaust under-hood heat, allow fresh air to flow through, prevent powder from dumping onto the two clutches, prevent the motor from ingesting its own exhaust and provide sound deadening.

Hoods are designed with airflow in mind. Air is easily pushed in when the sled is speeding along, and hot air is pushed out mostly near the console and footrest area. The process of exhausting heat comes from outlet vents placed above the pipe and behind the hood on each side of the fuel tank and near the footrests.

When designing a new chassis, engineers place heat sensors under the hood and on vital engine components, such as wiring and coolant lines to verify if a component can withstand the heat produced by the engine. If a component cannot, it's either rerouted, insulated or redesigned with more heat-resistant materials.

Heat deflecting foil is strategically placed to protect the hood and engine parts. For added protection, beyond the foil, aluminum plates are placed where needed. For the most part, OEMs try to move wiring and other parts away from heat sources. By rerouting, the OEM can minimize the amount of foil and heat deflection plates used.

Often, OEMs test their hood designs when conditions are the hottest.

The hood and the air box also work in tandem to reduce noise. Enviro groups often point to the 2-stroke pipe as an unlawful noisemaker. However, much of the noise comes from air induction; air moves in through the air box and into the carbs or throttle bodies and this often creates more noise than the pipe.

The hood must pull in cool air for cooling the motor AND for the air box to induct fresh air for combustion. To control noise emissions here, the sled must be cool and have fresh air for the air box. Hot air is exhausted down on each side of the fuel tank, the muffler is insulated, sound deadening material is placed by the pipe and on the hood's underside and chambers are built into the air box.

The aftermarket
It is not unusual for speed guys to cut up their hood to allow more airflow, correcting what they believe to be a design flaw, or to increase airflow and to vent heat away from their snowmobile's modified motor. Remember though, like gutting an air box, increasing airflow to an engine by cutting away some or all of the hood's foam, or opening up holes or vents, increases noise and increases airflow to the air box and motor. If airflow is increased, re-jetting or re-mapping the Electronic Control Unit (for electronic fuel injected motors) for the increased flow, may need to be done. Consult your favorite aftermarket company for advice.

If you're experiencing rich conditions, consider this phenomenon, sometimes air intake channels behind the windshield are exposed to negative pressure. When the sled is moving, air may not travel down through the vents, but will swirl atop the vents similar to a water vortex behind a rock or waterfall. The only way to correct this is to open the vent behind the windshield to free its path, experiment with different windshield heights or add additional vents. Aftermarket companies often know about this negative air pressure gremlin and have components to correct it.

Many mountain mod guys use aftermarket hoods to lighten their sleds and to increase airflow. Some hoods, such as the titanium-framed and mesh-wrapped Diamond S hoods, are superb for both. Additionally, some mod guys will add hood screens and vents to their sleds. AmSnow's experience with these products has been favorable, but when adding side-panel and/or hood vents, snow can directly channel to the two drive clutches causing belt slip, and when under load, belt slip is not fun.

Many riders are adding superchargers or turbochargers to their snowmobiles. Managing underhood heat and pulling in fresh air is extremely important for these applications, especially turbocharged sleds. Aftermarket companies who sell turbos or blowers have instructions on how to modify your snowmobile's hood to fit the turbo or blower, but to also effectively manage airflow and heat.

Another concept you may not have noodled over is how your snowmobile's hood works in unison with the windshield. Directing air into the hood, to move heat and to feed the intake system, is paramount. Equally important to the OEM and the sled's driver is moving air and snow up and over or around the operator. Of course, "up and over" is dependent on driver height and if the driver is standing or sitting.

Though a windshield is not relevant to under hood heat, it is important for comfort and warmth. Finally, you and I may not think much about our hoods, but the hood is vital to your sled's performance, noise reduction and to component longevity. So, when polishing up your sled, take a moment to study your hood and remember its function.