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
Fuel Delivery and
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
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.
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