substitutes for cubic inches

"There are no substitutes for cubic inches." So goes a famous hot rod slogan that seems to be a fitting description of the recent trends in snowmobile engines. Big twins are all the rage right now, but are they really the best design for a snowmobile?

Are there other alternatives to just making the engines bigger and harder to start? If we take a quick look at some basic engineering formulas, a number of interesting alternatives emerge. Expanding the old basic formula of HP=Torque x rpm gives us a much clearer look at the variables involved in producing power. Engineers often use the following formula when evaluating an engine:

HP= (BMEP) x (displacement) x rpm
6,400,000

The two first expressions, BMEP and displacement, is torque broken down in cylinder pressure and cylinder size. BMEP is, in this formula, expressed in PSI (pounds per square inch) and the displacement in CC (cubic centimeters). The dividing constant below the line is adjusted to compensate for the apparent contradiction of having one part of the formula in inches and the other in centimeters, but these are the terms most popularly used. BMEP means Brake Mean Effective Pressure, and is an expression of cylinder efficiency.

It is calculated off the torque figure and adjusted for piston area and represents the average pressure pushing on a piston surface through the full stroke. Evaluating the engine by using BMEP figures, tells us how efficiently the cylinder is filled with gasses and how effectively it is burnt. Just having a larger cylinder does not necessarily give you more power, unless you can also fill it with fuel and air, and also burn it effectively. Several factors work against larger cylinders.

In a larger cylinder, there are longer distances for the scavenging air to travel, and at higher engine speeds it gets harder to completely fill the cylinder. With longer strokes there is also a limit to how fast you can turn the engine. While the cylinder displacement goes up with the square of the bore, the wall area only increases directly proportional to the bore.

This means that as the volume of burnt gasses that want to escape the cylinder increases faster than the port area available to let it in or out, the larger cylinders require higher ports or more time to do the job. As we take a closer look at the formula it becomes clear that the maximum horsepower can be obtained by a lot of small cylinders turning at a very high rpm. This does not just pertain to two stroke engines, but four stroke engines as well.

When you have a racing class with fixed displacement, as most snowmobile, motorcycle and car racing does, ultimate power can be obtained by better cylinder filling or higher rpms. As cylinder filling is better in small cylinders at higher rpm, the natural evolution goes towards more and smaller cylinders turning at higher rpms.

In the late 1960s and early seventies motorcycle Grand Prix road racing saw a tremendous development in this direction. Spurred on to beat Honda's multi-cylinder four stroke engines, Suzuki built a three cylinder 50cc engine which produced 19 hp at 18,000 rpm, while its V4 125cc produced 42 hp at 16,500 rpm. The difficulty of filling even those small cylinders at the high speeds is evident by the relatively low BMEP number of 135 PSI.

In its rush to find power, Suzuki went extreme in small cylinder size and high RPM. There is a limit to how complicated you can get and how high you can rev all engines before frictional losses eat up your gain and the design gets too heavy and bulky. Developing these extreme engines meant large sums in development, and with 14 speed gear boxes not even the best race drivers knew what gear they were in. This extreme engineering came to a quick end when the racing organization changed the rules to permit only one cylinder 125s, two cylinder 250s and four cylinder 500s, with no more than six speeds in the gearbox.

This forced the development to focus on maximizing cylinder efficiency. Great energies went into adding transfer and exhaust ports, and to maximize intake and exhaust systems. A single cylinder 125cc GP engine must today produce over 50 hp to be competitive on the GP circuit.

How does this relate to snowmobiles? Formula One Oval Racing was the first beneficiary. The Rotax factory not only builds snowmobile engines but also motorcycle racing engines. The rotary valved Rotax twin engines appeared with five transfers and three exhausts in the late 1970s, and is still dominating oval racing 20 years later with the same basic design.

The 340 Rotax F1 engine produced 105 hp at 10,000 rpm, giving BMEP figures of close to 200 PSI. While regular snowmobile engines 20 years ago operated with 120 PSI in BMEP figures, it is not unusual to see BMEP figures as high as 160 PSI in production engines today. Engines with case reeds, five transfer passages, triple exhaust and power valves are now commonplace.

When comparing twin or triple engines of the same displacement, the triple will always be able to outperform a twin in sheer horsepower numbers, better cylinder filling and higher rpms, guaranteeing more power. There are other performance parameters than just power guiding the snowmobiler in his choice of machine. Light weight is a very important factor, and this explains the trend back to twins again. As the triples gained in power, they also gained in bulk. A heavy machine with more power than you can handle soon gets to be a tiring exercise out on the trail.

Twins also have shorter cranks than triples, and this helps turning the machine because they have less gyroscopic forces from the crank trying to keep you going straight. If you can make a lighter triple with a shorter crank- you will be a success. This has been proven by the Polaris 600 XCR monoblock engine, and the presently very popular Yamaha SX 700R.

Yamaha does not give away much bulk to the larger twins and by lightening up the flywheel, it has also reduced gyroscopic forces.

So where are we going with this? It seems the present trend is toward better cylinder filling rather than even larger cylinders. Surprised? Take the example of the new Polaris 600 EDGE power valve engine. Customers were impressed when this 600 outperformed the 700s in a straight drag race. The improved cylinder efficiency made up for the lack in displacement to the older designs.

This is a typical example where cubic inches has been substituted by better efficiency. The next step will probably be to improve the efficiency of the 700 and 800 twins, and as usual the aftermarket is a couple of years ahead. There are now aftermarket cylinders with seven transfer passages, triple exhausts and power valves available from several companies, such as PSI and Union Bay to name a few.

There are going to be a lot of cylinder improvements coming in 600 to 800cc twins to try and reach the same BMEP figures as the triples. Once the cylinder efficiency goes up, we will probably also see more power with a slight increase in rpm. This increase will probably be in the neighborhood of 500 rpm, bringing peak power up from 7500 to 8000 on most of the bigger twins.

There are very practical limits to just adding cubic inches, and before we see any big future jump in power from big displacement twins, better cylinder filling at higher rpm will have to be addressed.