Your snowmobile's engine, exhaust, clutches and brake system are all sources of underhood heat. Left unchecked, that heat can cause problems in the sled's ability to function, from improper fuel injection calibration to premature wear on moving parts, like the drive belt, clutch cams and bushings.
The factory venting designs on most modern snowmobiles are adequate for stock engines under moderate conditions. But you have to keep in mind that the hood and bellypan vents are designed primarily for trail sleds and trail riding conditions. Mountain sleds, which typically share the same plastic as trail sled counterparts, operate in much different conditions. Trail sleds operate at high ground speeds with little drag on the drive train. The constant stream of air flowing in through the stock hood vents is enough to keep ambient underhood temps in check and provide fresh air to the intake system. And the low operation demands of a snowmobile cruising down a groomed trail means that the moving parts aren't generating excessive amounts of heat.
A mountain sled with the same hood and bellypan (and identical venting capabilities) has a lot more going against it. The load on the drive train is much higher, given the resistance from the longer, deep lug track, the deeper snow and the constant fight against gravity. Mountain sleds are typically moving at slower ground speeds with higher engine rpm than trail sleds, meaning that the flow of air coming in through the hood vents is slower and the internal source of heat is higher (from the higher rpm). What's more is that mountain sleds are often covered with snow, sometimes blocking off the stock hood vents for long periods of time when the load on the engine and drive train is at its peak.
Right off the showroom floor, a mountain sled has an inadequate ventilation design.
Updating The System
There are two factors to a sled's venting system that need to be addressed when modifying it: fresh air (coming in) and hot air (getting out). Fresh air vents are any vent placed low in the belly pan or on the front side of the hood, where forced air will enter through them. Hot air exit vents are any vents higher up on the hood or cowling or toward the rear of the engine compartment, like on the cowling near your knees or up near the gauge pod. Depending on how fast the sled is moving and how deep the snow is (if it's blocking side vents or nose vents), any vent may serve as a fresh air vent or a hot air exit vent.
Full Of Hot Air
Typically, hot air will rise and cold, dense air will settle to the lower areas of the engine compartment. The quick inflow of cold air meeting the hot air will create immediate condensation, creating a very humid climate under the hood of your sled. Snow coming through the vents and hitting the exhaust and other hot parts will create steam-another evil in the system.
Hot air and steam rise, so the most logical area to place exit vents would be up high, near the gauge and headlight area of the hood for when the sled is stopped or moving at a slow ground speed, like working up a tight canyon or through trees. When there is a stream of air coming in through the front vents, then more rear vents will also help the hot air exit. The best combination for a mountain sled would be to add venting to the higher and more rearward areas of the engine compartment. Side panel vents, like those from R&M Lighting Products, though low on the sled, help heat from clutches and exhaust mufflers escape by thermal radiation. Heat radiates off of these parts and is quickly transferred to the cold sources of air right next to these parts. At high ground speeds, these side panel vents will act as exit vents as cool air enters through the hood vents, flows through the engine compartment and is forced out through the side panel vents (and other vents).
For fresh air, there are a few different things to look at. First, a mountain sled can have powder blocking off everything on the front end of the hood and belly pan. Some sled builders will add vents to the back inside wall of the front suspension wells. Some will add vents to the hoods, actually cutting out portions of the hood and adding some type of vent screen, like Starting Line Products' Pre Filter material. Side panel vents can also function as fresh air sources, drawing in cold air as hot air exits through higher vents. The key here is to have vents in the higher areas of the engine compartment.
Many mod sled builders also take into account the engine's intake system when adding or modifying hood and belly pan vents. If the sled's intake is up high, like on a Polaris RMK, some vents in the cowling near the speedometer will help prevent the engine from drawing in steam through the intake.
On mod sleds where the intake is located inside the open engine compartment, like on most turbo sleds, having a source of clean, cool air is important to maintaining consistent operation. Same goes for turbo sleds with an intercooler. In these cases, the hood vents can be modified to open up more air flow near the intake source.
Another venting-related problem mountain sleds come across is clogged vents. Again, going back to the trail sled example, when these hoods are designed on a computer or in a clay mold, they're designed for prime conditions and high ground speeds. Some negative side effects of these cool-looking hood vents are that they become catch basins for powder snow. If a vent is blocked off with snow, it will have a negative effect on underhood temps. The Arctic Cat M sleds are notorious for plugging up the front hood vents. A simple screen like the Pro Lite hood vent screen kit from Black Diamond prevents snow from getting packed in the front vents, ensuring a constant open portal for air to enter the engine compartment.
There are a few main things to remember when looking at hood vent kits: Nearly every mountain sled could benefit from added vents; heat and steam rise, so adding vents low really only helps when you also add vents high; mountain sleds operate at low ground speeds, so vents function differently than on trail sleds.
Upgrading your sled's vent system can save you money by prolonging belt lift, reducing wear on clutch components and prolonging brake component life. It will also help maintain consistent power output by not over-heating the engine and providing a cooler source of intake air.