Let's Talk Engines

Lets start with the basics for a clear understanding as we talk about this. Any engine is just an air pump, 2cy, 4cy, single cylinder thru a V8. The more air you can pump through it, the more power it can make. It’s that simple.

With the current 120 sleds we have 3 brands and actually 4 engines. OEM Polaris uses a Robin, Ski-Doo a Honda, and Arctic Cat started with a Spirit and now has a Yamaha engine for this year. All of these engines are about 120cc. Overhead twin valve designed to be in the 3 ½ HP class. All were designed for use in the industrial world to power small pumps and generators.

Again, for a clear understanding, let's look at one of these engine from one end to the other.

Air enters the engine threw a carburetor; its job is to mix the correct amount of fuel with the incoming air for a clean and efficient burn. Gas is stored in the small bowl on the bottom of the carb. It is controlled by a float. As you use fuel, the float drops letting in more fuel, at a certain point the float presses on a needle valve and shuts of the flow of fuel. It simply keeps a usable level of fuel in the bowl. In the center of this bowl is the main jet. This is nothing more then a brass nut with a specific size hole drilled in the center. From here a passage is drilled up into the throat of the carb. As air is drawn through, it pulls fuel up the passage and they mix together.

From this point the mixture travels through the intake manifold, which is nothing more then a tube to connect the carb to the head. Here it enters the port, again just a tube or passageway to get it to the valve. As the valve opens (we’ll get to that next) the mixture enters the cylinder and the combustion process starts. Once the plug fires, driving the piston down and moving your sled forward. The crank makes the piston change direction and start moving upwards. Now, the exhaust valve starts to open, the spent gasses flow past the valve, down the port, and out the pipe. Game over.

At this point let's talk a little about camshafts and valve train. These parts are one of the two main differences between a 2-cycle and a 4-cycle engine, the other being the extra cycle between firings on a 4-cycle.

On these engines the cam has a shaft which has two lobes on it, picture these as egg shaped. The end of the cam has a larger gear cast in. This mates to a gear on the crank and rotates the cam. The cam turns rotating force into a lifting force. Riding on the outer edge of each egg shaped lobe is a lifter, a flat disc with a stem on it. When the lifter is 180 degrees from the top of the egg, it is at its lowest point and here the valve is completely closed. As the cam rotates, the lifter rides up the outer profile of the egg shaped lobe. It continues to rise up, opening the valve until it reaches the top of the egg. This point is considered max lift, in our case, stock OEM cam, around 1/4 of an inch. Now it continues rolling and works its way back to the bottom of the egg where once again, the valve is completely closed. If you look at a cam from its end, you will see the lobes do not line up, they are off set. This is referred to as lobe centerline. The timing of the lobes allows the intake to open first, fill the cylinder with fresh charge, give it time for the plug to fire it, then start opening the exhaust valve to get rid of everything.

The last parts missing from this puzzle are simply the pushrod which mate to the lifters in the block and move that lifting force up to the top of the head where they mate with the rocker arms. Look at rocker arms as a simple teeter-totter. It takes the force moving up (or down) on one side, and converts it to pushing down on the other side as it rotates on its center point, just like a teeter-totter. There are springs on the valves that keep pressure on everything as this happens.

There you have it - the basics of how your little 120 engine works. Maybe worth noting: this is also how the engine in the car you drive each day works also.

So, with the basics down, lets get onto the good stuff. How to make your sled faster.

Building Horsepower

Before we can start grinding ports, or buying cams and carbs we need to get some basic understanding out of the way. The difference in HP between a good engine, and a great engine is making sure each and every part works well with all the other parts. While from the outside, these engines may look similar, internally they are very different. Things like, bore and stroke ratio, rod length port design and lay out, valve size, cam design and carb size are just some of the variables, not only in peak hp, but where in the rpm range, they make it. It’s important to remember that each of these engines was designed simply to power some piece of power equipment, usually at steady speed of 3600 RPM’s. Beyond making more HP, we have to deal with using parts that will live at the new power and rpm levels we are building to. An example would be the stock rod may be fine for 3 ½ HP at 3600 rpm’s, simply doubling either of those numbers may be far beyond the limit of that part. Something many home builders never think about is safety issues when building an engine. That stock cast iron OEM flywheel maybe perfectly save with the governor working at 3600. How good is it a 6000-9000? All I will say is nothing but stock engines will be run on our dyno, I don’t care what your willing to pay. I won’t stand next to it, my kid won’t stand next to it, and I certainty hope, you won’t put your child next to it. They do explode, like a grenade.

The last thing to think about is something that really needs a lot of serious thought.

The first questions from most customers is, How much HP can I get, and how much will it cost? It should be, what do I really need and how will it last? As we go down the road of making power, the further we travel, the harder it gets to find gains and as we push those limits, the more trade off we have on reliability. With some of our serious class racing customers I have to have the talk. Are you better off spending the big money on things like Titanium valve train parts, the “to the limit carb”, radical cam, the things that separate the very front runners, OR, are you better off working on chassis, handling and working with your driver? As you work towards the upper limits here, you have to accept the reality that, the frequency of rebuilds and possibility of catastrophic engine failure go up with the power increases.

Think deep, and define what will really work best for you. Spend YOUR money wisely.