Donaldson Snowmobile Muffler
A time back I was building an Arctic Puma for vintage racing. Naturally I was looking for a stock muffler which could add power and meet the rule book for noise control. I was told it was the Donaldson. I bought one and wanted to see how it was tuned so I tore it apart.
The Donaldson Company, Inc., Minneapolis, Minnesota was/is a big and smart company who knew how to engineer sound management systems. When the snowmobile industry/sport was growing wildly in the 70s Donaldson jumped in with ‘Power-Tuned’ mufflers because the manufacturers were just using simple cans which added little power. Donaldson added a new feature--tuning.
The principles of exhaust tuning for two-stroke engines had already been established in outboard and motorcycle racing. Racing “chambers” had separate but multiple stages which when calibrated to a specific engine dramatically increased performance. These were the length and diameter of; header pipe, divergent cone, center section, convergent cone and stinger. Divergent cones pulled spent combustion out of the cylinder along with fresh unburned fuel. The convergent cone acted as a block to exhaust flow and returned the un-burnt fuel into the combustion chamber where it met the carbs' fuel/air mix for an increased explosion (more power.) Specifications of all the parts “timed” this flow activity.
The Donaldson included these features in their snowmobile mufflers but the emphasis was on sound management with a nod to tuning. Their mufflers were made for a universal fit. The only distinction was a single or twin cylinder engine. For manufacturing convenience their mufflers were built for both applications. The muffler shown here was built for a single. There is only one inlet and one outlet. The inlet port was centered in the body and the exhaust off to one side. Donaldson opened the second inlet for twin cylinder engines. It is blocked with a plug in these photos.
In this muffler, for a one Lunger, the divergent cone (megaphone) is the center expanding tube. The one on the right for a twin is inert. This megaphone extracted the exhaust. The exhaust flowed around inside the outside jacket (central cavity) and banged into the plate on the exhaust end. This acted as a convergent cone which returned the fresh fuel charge. Spent exhaust exited the side tube.
So there you have it--The Donaldson muffler exposed after 45 years. I doubt this will come up in conversation but now you have useless knowledge in your head.