The Unofficial Guide to Hopping Up, Modding Out and all Things Related to Going Faster on the Snow

2008 Underhood Performance Directory

October 2007 Feature

Mod-Stock is about one thing and one thing only-snowmobile mods. You want your sled to go faster and we're here to show you how.

Within the following pages you'll find all the parts and company information for everything that has to do with underhood performance. Mod-Stock 2, which follows this issue later this fall, will cover everything else. We build our own project sleds-including Project Puff-with the directories in these two special issues.


Horsepower comes from the engine and there's no quicker way to more of it than going bigger with the internals of the engine. Big bore kits, which change the displacement of an engine by replacing the cylinders and pistons with larger ones, are the old guard of high performance modifications. There are also other options within an engine. Porting changes the performance of an engine without enlarging its displacement. Case porting cleans up the edges of the cases and increases the volume of air and fuel it can hold before transferring to the combustion chamber. Piston coating and treatment processes prolong the life of the part, while cylinder repair shops repair important cylinder linings. Heads are an important piece of the engine performance combination and there are often several different head domes available, which alter the engine's compression ratio. Four-strokes usually don't have the need for larger bores and aftermarket engine internals. Turbos and superchargers have dominated the four-stroke performance segment and the engines have proved reliable under most applications.


Once the biggest sales item in the aftermarket segment, exhaust pipe sales have been hampered by sound regulations over the last half decade. However, technology has caught up and exhaust makers are able to offer high performance tuned exhaust pipes that stay within reasonable decibel output levels. Pipes have made a comeback and are a very economical approach to performance improvements. Twin pipes do make the most peak horsepower, but don't overlook the low end grunt and broad powerband of a single pipe.


Silencers are the key to a sound-friendly, high performance exhaust system. Silencers help control backpressure, which is an important aspect to two-stroke exhaust systems. Another big factor of aftermarket silencers is weight reduction. Depending on the make and model of your sled, you can see an eight to 25-pound weight reduction on two-strokes and four-strokes. Silencers are inexpensive and can add up to 15 hp, give or take a few.

Fuel Delivery and Intake

Here's where you'll find a huge assortment of fun mods, from nitrous systems to turbo chargers. Years ago, this section used to be mostly about carburetion, needles and jets. Technology has given way to throttle bodies and EFI, which opens the doors for high-tech mods. Nitrous systems produce more power by further atomizing fuel with more oxygen molecules. Turbos-for both two-strokes and four-strokes-make tremendous amounts of power by increasing the amount of combustible air and fuel in the combustion process. Superchargers (currently four-stroke applications only), do the same thing as turbos, but are driven off of the engine's crank for a direct response rather than the exhaust pressure, which can be delayed slightly. Either way, you get 50 to 300 additional ponies out of the setup.

This section also covers parts for the tried and true method of carburetion. Reed valves and cages, air boxes and filters, intake screens and anything to do with getting more air, more fuel or more of both into the engine.


It's often said that all the power you can make is worthless if you can't get it to the ground. The engine's horsepower goes through the primary clutch and is transferred to the secondary through the drive belt. If any of those components aren't doing their job, the sled's performance will suffer. The primary clutch consists of moving parts (spring, moving sheave, cover and weights or cam arms) and non-moving parts (fixed sheave and shaft). The moving parts are what you want to focus on. Spring rates control the rpm at which the clutch engages the drive belt. The weights determine how rapidly the movable sheave closes and at what rpm it all happens. It sounds simple, but too many factors-like elevation, horsepower, temperature, carburetion, snow conditions, etc-make it anything but simple. The secondary is the same way. Ramp angles, spring rates, rollers . it all depends on the same factors. Fortunately, there are several companies there to help you figure out what your sled needs to perform and what can be done to make it perform even better.

Drive Train

Once the power has made it from the primary clutch to the secondary, it has to travel through the drive train. That includes the jackshaft, chaincase upper and lower gears and chain, drive shaft and drivers. Many new Arctic Cat models feature a gear drive drive train that eliminates the jackshaft and chaincase. It's new technology, but there are aftermarket options available for it, even retrofit kits to put the Diamond Drive on other makes.


Did you know that you can drop a couple pounds of rotating weight by upgrading your brake system? Lightweight rotors, ceramic rotors, high performance brake pads and brake lines and new levers can make a huge difference in how solid your sled's braking system feels. High performance rotors and pads tend to resist brake fade better than stock parts, too.

Gauges and Electronics

Mod sleds act much differently than stock sleds. You really need to know what's going on beneath the hood of your mod or you might find yourself on the dead end of a tow rope. Or, you might also be leaving potential power on the table. Gauges are tuning aids. Power is the byproduct of the engine producing heat by combustion. Having a sensor in that heat path (exhaust gas temp gauge) lets you know whether you're making too much heat (too much air), too little heat (too much fuel) or just the right amount (perfect air/fuel ratio). With gauges, you can not only monitor EGTs, but rpm, shaft rpm, water temp, airbox temp, oxygen sensor, speed and other parameters.

Aside from gauges, you'll also find other electronic devices such as PCM override fuel-management boxes. These boxes are the keys to unlocking computer-controlled fuel 
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