developing 2stroke exhaust systems

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Today's 2-strokes are light and powerful, but their largest power surge came with the expansion chamber exhaust. The tuned wave technology doubled power output.

When early snowmobiles were built back in the 1960s, this idea hadn't yet reached our shores. Tuned exhaust systems were under steady development in the motorcycle-racing world, and didn't appear on racing snowmobiles until the early '70s. Before the expansion chamber appeared, megaphone exhausts were the hot setup, and they produced the most incredible ear-splitting noise you could ever experience.

Megaphones helped get the exhaust out of the cylinders, but a lot of unburned air and fuel came out with it and simply got lost. Racers noticed that certain mufflers and some combination of backpressure seemed to work better than a megaphone, and a lot of combinations were tried. This took place back in the '50s, and some 250 DKW racing motorcycles were seen in 1952 with what looked like an expansion chamber. The results were inconclusive, and the factory teams went back to using semi-enclosed short megaphones.

Rocket science helped
It took a rocket scientist to figure out the correct combination. Walter Kaaden had been a development technician under the noted Werner Von Braun, but rather than following Von Braun into the U.S. space program, he returned to the East German MZ factory to develop racing motorcycles (see sidebar).

Thanks to Kaaden's pioneering work with expansion chamber exhausts, today we can enjoy light, powerful 2-strokes. His expansion chamber worked by harnessing the energy of exhaust pulses and using them as a reverse turbocharger. The fresh gasses are sucked into the header pipe by the front megaphone section, and then pushed back into the cylinder again by the original pressure pulse returning after it has been reflected by a rear cone. The rear cone has a very restricted outlet commonly referred to as a "stinger" that controls the backpressure.

For the pressure pulse to return at just the right time to push the fresh gasses back into the cylinder, the length and volume of the expansion chamber has to be tuned to the maximum engine speed. This means the reverse turbo action only works in the mid- to upper-speed ranges, much like a turbo in a 4-stroke. Under a certain rpm, usually around half of the maximum rpm, the pipe doesn't add power.

Yet you can feel when the engine "comes on the pipe;" there's a steep increase in power. To improve low rpm performance and prevent fresh gasses from escaping, modern 2-strokes now use power valves in the exhaust ports too. This shortens the exhaust port timing and lets the pipe tune better at a lower rpm, extending the effective powerband. Power valves haven't only broadened the powerband, but also reduced the sound level and improved emission values at the critical low engine speeds where the pressure pulse is out of tune.

Tuning remains an art
As much as Kaaden developed this system, designing and tuning expansion chamber exhaust systems still remains an art, where experience counts.

Some claim that there are no more gains to be had in expansion chamber design because they can be calculated on sophisticated computer programs. Yet computer programs are always simulations of already existing pipes - no significant improvements have been made by a program. Instead, all major improvements have been made on the dyno or in the field, and the result later added to the computer database.

This is not to say that computer programs for pipes have not improved through the years, but the programs are always behind the latest dyno developments. Also, there are still large differences when comparing snowmobile pipes to motorcycle road racing or motocross pipes, so before you pick a program you hope will work for a snowmobile, make sure the author has some experience with snowmobiles and didn't base the program on a 125cc, 12,500 rpm road racing bike.

Computer programs can suggest a basic design, giving you a starting place, but experienced tuners can always make improvements on the dyno and in the field. There are several different designs of tuned exhaust systems including one expansion chamber per cylinder, two cylinders feeding one chamber or three-into-one systems.

Two-into-one systems are good up to 8000 rpm, but after that, pulses from each cylinder start to interfere with each other. The two-into-one system has excellent low rpm power and strong acceleration, and also is favored on production machines because of its lower cost.

Three-into-one systems also are flexible, torquey and less expensive to make. If you want maximum power, one expansion chamber per cylinder is most efficient and superior in power when you start revving the engine above 8000 rpm. That's where the two- and three-into-one systems start signing off.

With tuned exhaust systems playing such a large part in overall performance, it's an area where experienced racers can improve performance.

Certainly books can be written on tuning expansion chambers and mufflers, and this is just a quick overview. We plan to cover this science more extensively in future issues of American Snowmobiler. With more emphasis on quieter exhaust systems, most aftermarket companies are working to meet new DNR test parameters for quieter systems for the trail.

Companies that want to compete in this changing marketplace are spending their time and resources on new systems, and we should see a number of new products to meet these challenges soon.