For 20 years, Phil Mickelson provided Snow Goer readers unique insight into the world of snowmobiles from an engineering perspective. His Tech Professor column admittedly sometimes was difficult for many readers to read or understand – particularly the first time through – but Mickelson had a knowledge base that was unmatched in snowmojournalism at the time. For the ultra-technical reader, his understanding of the science and physics behind snowmobiles and snowmobile engines provided an education they weren’t getting elsewhere.
It all started 25 years ago this fall, when Mickelson joined Snow Goer as a contributor working from his Duluth, Minnesota, home and penned his first Tech Professor column. It is printed in its entirety below. Reading it brings us back to the days when more performance shops were modifying two-stroke engines to create big power.
Mickelson penned his Tech Professor column until March of 2011 after he retired from writing; in September of 2013, he passed away. Here’s a link to a column about our beloved Phil. Below is the first Tech Professor column, directly from the December 1991 issue of Snow Goer.
Torque is a unit of work; a force applied over a distance. If you apply a force of one pound to an object and move it one foot in the direction of the force, you’ve done one foot-pound of work.
Work done by a rotating shaft is called torque. Lifting a one-pound weight with a shaft having a radius of one foot will require one pound-foot of torque. In the world of physics, the order of the terms, foot-pound vs. pound-foot, is used to distinguish the linear unit of work from a unit of torque. In the world of mechanics, however, the unit of torque is almost always referred to as foot-pounds or inch-pounds.
Power defines the rate at which work is done. The math equation is this: Average power = work done /time interval. And, one horsepower is defined as 550 ftlb/sec.
In an engine the force is created by burning gasoline which heats the trapped gases above the piston. The pressure generated forces the piston to travel the length of the stroke. The piston, which is having linear work done to it, transfers that work to the crankshaft through the connecting rod. Some work is lost in that transfer but, hey, what can we do? The linear work done to the piston translates to torque at the crankshaft. When we rate the torque delivered per unit of time, we define the power of the engine.
The pressure above the piston is constantly changing, but to predict the output of an engine, engineers determine an average pressure on the piston during the power stroke. This average considers the compression ratio of the engine, the fuel used, the breathing or volumetric efficiency of the engine and a host of other factors defining an engine’s output. The figure they end up with is called the Brake Mean Effective Pressure (BMEP).
The torque an engine produces is a factor of the BMEP multiplied by the engine’s displacement. To increase the torque output of an engine, you either make the engine bigger or you improve the BMEP. It’s that simple!
There are three ways to make the engine bigger: increase the bore size, increase the length of the stroke, add cylinders to the engine. If those three techniques were applied to three separate engines, the displacement remained equal in-all three and the BMEP was equal, the torque produced would be equal. That’s right, a long stroke engine doesn’t produce more torque than a short stroke engine of the same displacement. Torque output will increase by whatever means you use to increase displacement provided you don’t mess up the BMEP.
Adding cylinders to increase displacement is obviously no garage job but it offers the advantages of smoother power delivery to the crankshaft, increased port area, additional cooling surface, etc. While this sort of modification was unheard of a few years ago, it’s amazing how many speed shops are now offering multi-cylinder conversions. If you can afford it, it’s a great way to make torque!
Stroking crankshafts has become a more popular way of increasing displacement in the last few years. To increase the stroke of an engine, you must relocate the connecting rod journal at a greater radius from the crankshaft’s axis of rotation. Changing connecting rod lengths has nothing to do with the stroke length of an engine. A change in rod length may also be required when stroking an engine to gain needed clearance or reduce piston side loading, but the rod length has no bearing on the displacement of the engine.
When the stroke length of a two-stroke cycle engine has been changed, care must be taken to restore the port timing and compression ratio to a workable set of specifications. Generally, shim plates or multiple gaskets are used between the crankcase and the cylinder and some port grinding is almost always required. Stroking the engine without paying attention to the ports almost always results in a decrease in the BMEP and you can end up with less torque from your long stroke engine.
The most commonly employed technique to increase displacement is boring the cylinders to accept larger diameter pistons. Limits to how big the bore can be are imposed by the pistons available, space available in the cylinders, available transfer passage cross sectional area and stud placement, to name a few.
Boring generally has no effect on the port timing of a cylinder, but it will result in a compression ratio increase that must be kept within workable limits.
A 100cc increase in displacement from boring an engine will produce the same increase in torque as a 100cc increase from stroking the engine provided the BMEPs are kept equal.
Once you have the engine as big as you dare make it, your other option to increase torque is to raise the BMEP. The easiest way to raise cylinder pressure is to talk with the boys fiddling with nitrous oxide injection. Changing to fuels that pack along a little oxygen and nitrogen are, in fact, the easiest way to increase the BMEP. You can increase it to the point your heads blow off! But, fuels are tricky to deal with, expensive to buy and they aren’t legal in most forms of racing.
Almost all other forms of modification you’ve heard of are aimed at increasing the BMEP, i.e.: compression ratio increases, tuned exhausts, bigger carbs, porting, ignition advances, etc. The aim of all these modifications is to move more fresh mixture into the cylinder and trap it there as the exhaust port closes and expand it as far as possible before that port opens again. Every particular port and pipe configuration will work best in a given RPM range, but discussion of torque per unit of time brings up the subject of power and that’s another story. Today we’re just talkin’ torque. SG