Porting Snowmobile Engines

Crankcase and Cylinder porting is probably the most controversial subject among performance sledders. Part of the problem is understanding the "pulse" or "charge" nature of the airflow in a 2-stroke engine. The other is how elevation change affects horsepower. Port timing for Flatland HP setups is different from Mountain Sled setups. Today computer software is available to design each part of the 2-stroke cycle. The laws of physics have not changed and the mathematical formulas are readily available. To obtain maximum torque and horsepower requires applying the laws and the mathematical formulas to the pulse/charge effect of the airflow. We will work with your engine to achieve your goals. Decide on your priorities: reliability, RPM & pipe (unless you plan modify or build a pipe), elevation, and we will design specific engine mods for you.

A squish test is required before the engine is disassembled. A compression test is recommended as a reference for comparison. With the engine apart, measurements can be made to determine the work required. Then the computer and experience will be used to design & map what is to be implemented on the engine.

Decide what rpm the engine will run at, with available pipe(s), or custom built/modified pipe(s). Then check the piston speed for the stroke in your engine. Some engines can be run much faster, others, are running near maximum piston speed. Suggested maximum piston speeds: Trail sleds = 3700 feet per minute, Hill Climb/Race = 4000 feet per minute. You can run faster speeds, but you risk reliability and longevity. Note: Watercraft = 3300 fpm & 3500 fpm.

Example #1: 700 Polaris twin engine has a stroke of 68 mm and runs at 8300 rpm, what is the mean piston speed?

Find RPM @ 4000 fpm: 4000' x 12" = 48000" / 5.354" = 8965 rpm (700 Polaris)

Example #2: 700 SX Yamaha triple engine has a stroke of 59.6 mm and runs at 8500 rpm, what is the mean piston speed?

Find RPM @ 3700 fpm: 3700' x 12" = 44,400" / 4.6929 = 9,461 rpm

Another way to find mean (average) piston speed:

Use this formula: MPS = S x 0.1666 x rpm / 25.4

MPS = mean piston speed (feet per minute) and S = piston stroke in millimeters.

700 Polaris example:

Do you want horsepower or torque? Both, right!! Remember This: Speed ='s HP & Acceleration ='s Torque. High Horsepower numbers can be very misleading due to high RPM. The true test of a properly modified engine is Torque. The engine must produce high torque over a wide rpm range, or hillclimb and race track times will be slower due to poor acceleration. And the test of the engines efficiency is the Brake Mean(average) Effective Pressure. Off and on the throttle tests the engines ability to perform at all rpms. For this reason, BMEP gives the true indication of how effectively the engine is operating, regardless of its displacement or its operating rpm. To achieve high BMEP numbers all parts and surfaces must be smooth and work together to increase airflow from the carb to the exhaust flange.

See BMEP page w/ examples & 4-stroke BMEP formulae

I use the TSR computer software to calculate/predict the horsepower.

If you are interested here is the BHP (brake horsepower) formulae:

BHP = PLAN / 33000
P = Brake mean effective pressure in psi
L = Piston stroke in feet
A = Area of one piston in square inches
N = Number of power stokes per minute

Example: Polaris 700 twin cylinder which will run at 8,000 rpm, deliver an average pressure of 135 psi, has a bore of 81 mm and a stroke of 68 mm. What is the predicted BHP?
P = 135
L = 0.223 (68/25.4 = 2.677"/12"=0.223 feet)
A = 7.988 square inches
N = 16,000 (8,000 rpm x 2 cylinders)

You can play with the variables, P= psi & N=rpm, in your engine to determine how to get the HP you want! Are the numbers you used realistic??