Hot Stuff
Have you ever wondered why snowmobiles produce such awesome power? A triple-cylinder, 900cc engine can crank out more than 200 horsepower using a triple-pipe exhaust system. And a 700cc twin, using twin pipes, will run smoothly all day long at 8700 rpm, cranking out 150+ horsepower. But a 780cc watercraft motor may well come to an abrupt and unexpected halt if it's asked to deliver a mere 120 hp at 8000 rpm.
The difference is that snowmobiles, like grand prix motorcycles, and motocross engines, operate at high BMEP (brake mean effective pressure), usually between 160 and 190 pounds. Those engines produce their power at high rpm, anywhere from 8500 to 14,000 rpm.
On the other hand, personal watercraft engines usually run between 6700 and 7500 rpm, producing anywhere from 50 to 120 horsepower, on average, at a relatively low BMEP of 125 pounds.
I remember my 1970 era snowmobile reaching a top speed of around 45 mph, whereas today's trail sleds top out somewhere around 100 mph, and racing snowmobiles are topping 150 mph. Where did all this power come from? One of the largest contributors to the new-found power is a properly designed and tuned exhaust system.
The problem in designing an efficient exhaust system is controlling the temperature throughout the entire length of the pipe. The complexity of gas expansion, and cold and hot spots in the pipe contribute to many problems, especially in twin or triple-pipe systems. Field experience and dyno work confirm that the exhaust pipe temperature has a great effect on engine performance. Multiple-pipe systems have to be shielded, and may need to have differing pipe lengths in order to produce consistent levels of performance in all cylinders. Also, consider that some engine compartments don't have enough room to install a bilge pump, much less a second exhaust pipe. The problem becomes even more evident if the engine is equipped with electronic fuel injection.
The problem that develops with electronic fuel injection is that the injection "map" stores information which controls fuel delivery, but that information is accurate only when the pipe is at its peak operating temperature. During actual use, the pipe may be colder, or the engine may not be fully warmed up, and in that instance the computer that controls the fuel injection will think that the engine is turning much faster than the pipe will permit. In this situation, a 500 rpm error on the part of the computer can cause mid-throttle burn down.
If you compare dyno tests of a hot engine/cold pipe and a hot engine/ hot pipe, you'll see graphically how much difference there is in performance between these two setups.
The sonic wave of a pipe is also related to the temperature. The fact is that the speed of sound in an exhaust pipe depends not on the pressure in the pipe, but on the pipe's temperature. And the performance of a pipe (and therefore the engine it's attached to) is directly related to its sonic properties. In a personal watercraft engine, a variation of 100 rpm can be the difference between winning and losing a race. This is because the jet pump is very sensitive to the rpm ratio; a loss of as little as 50 rpm can translate into a decrease in top speed of as much as 1.5 mph.
So you can see that the temperature of the exhaust pipe is crucial to getting the most performance possible out of your engine. This means you have to get the pipe good and warm before you'll get everything you want out of it. The problem here is that at any given instant during the warm-up process, each bit of pipe has its own average gas temperature, which comes from the heat entering the pipe from the cylinder and the heat being given off by the pipe itself. As a result, every part of the pipe will have a variance in sonic velocity. In the hotter sections of the pipe, the speed will be higher, while in the cooler pipe sections, the speed will be slower. In other words, the hotter parts will act as though they are shorter, while the cooler parts will act as if they're longer. The solution, therefore, may seem obvious: lengthen the hotter sections and shorten the cooler ones, or else just wait until the whole thing is as hot as it's ever going to get. That seems obvious, but it's incorrect. The problem is that even when the pipe is warmed up, all its sections' temperatures are constantly changing.
For example, a race engine runs very hard on the straight-aways, then cools down going through corners. At any given moment, the pipe may suffer from being too long or too short for the power demands of that particular section of the course. This results in the bogging down often experienced in various sections of the power band during a race, or even a hard riding session. If you never changed the rpm or the load on the engine, getting the pipe to work perfectly wouldn't be all that difficult, but once you factor in all the aspects of real-world use, you can see how intricate and elusive the art of designing a high-performance pipe that works well in all areas truly is.
If you bear all this in mind, you'll probably find yourself a little more appreciative of what the pipe makers are up against. Just think what they go through trying to make your engine perform the way you think it should. It's because of all the variables that I think it's undeniable that the selection and tuning of exhaust systems will be the greatest challenge facing engine builders and tuners looking for more horsepower, especially with the growing environmental demand for cleaner-running power plants. Where this will lead is anybody's guess, but don't be surprised if you see the employment of more and more space-age materials (for their light weight and durability), cleaner-burning fuels and lubricants, and maybe even catalytic converters and/or direct-to-cylinder fuel injection, (which has the potential to burn cleaner than modern automobile engines).
If you choose to tinker with your exhaust pipe, take a long, hard look at the almost limitless potential for mistakes. If you still think some tuning of your pipe is required, remember that if you shorten the length of the head pipe, you will move the rpm and torque upward in the power band; a shorter midsection will increase only the rpm, and changes in the stinger diameter can result in higher or lower operating temperatures, increasing the chances of piston meltdown.
Knowing the difficulties in making a good exhaust pipe (especially when you consider that a multiple-pipe setup increases these difficulties exponentially) should give you new respect for the people who manufacture high-quality tuned pipes. Nowadays, most systems on the market are very good in delivering power. If your system doesn't seem to be working the way it should, don't automatically assume you have a lemon - check to be sure the pipe you're using is designed to deliver power in same rpm range as your engine. A pipe that's designed for 7500 rpm will never work very well with an engine that's only running 6800 rpm.
George Grabowski HPT Sport USA