If the negative pulse is timed to reach the open exhaust valve during the overlap period, it will draw in a fresh mixture of air & fuel from the intake before the piston descends on the intake stroke. This process is called “exhaust scavenging”, as it uses the energy from the inertia force created from the out going exhaust pulse, to cause a low pressure area in the exhaust port & combustion chamber during the valve overlap period (when both exhaust & intake valves are open at the same time). It is possible to use the energy from these forces to help extract exhaust gases from the engine. This means the pulse must go from 0 to 200 mph & back to 0, about 3,000 times per minute. It is important to know that the exhaust exits as long columns or pulses of spent gases, and at peak RPM, the pluses can reach velocities of about 300-350 feet/second, and a frequency of about 3,000 per minute. Again, these problems only exist with “open” systems such as “straight (drag) pipes, open megaphones & extremely short systems.īefore you can understand how the Thunderheader ™ works, you should have some insight on the dynamics of exhaust pulsing. Some exhaust systems can affect your air/fuel mixture to such a degree the engine will “load up” at certain RPM’s making it undesirable to keep the engine at that speed. Most “open” exhaust systems will produce good top-end HP, however in most cases the torque will suffer because of the lack of exhaust scavenging, due to reversionary problems associated with “straight through” systems.Īnother aspect worth discussing is throttle response. You don’t want to increase HP, at the cost of loosing substantial amounts of torque. When comparing the results, be sure to look at the overall performance, not just the peak horsepower. Be sure to record the torque as well as the horsepower with each test. Be sure to re-jet for each exhaust system, so you will get the best results possible with that system. Record your test results, then change the exhaust system only, and start your testing again. Horsepower is measured at power peak (the highest RPM which horsepower continues to increase), power peak for most performance equipped EVOLUTION models, is usually between 5800-6750 RPM’s, depending on the breathing capacity of the engine. Tune the engine to produce maximum HP & torque. The test engine must be equipped with high performance components, such as a cam, ignition, air cleaner & Thunderheader ™.
The best way to test or compare high-performance exhaust systems is with a Dynamometer.
This means it has to breathe, if you install a high performance cam, carburetor, air cleaner, ignition & exhaust system, you can expect to increase your horsepower about 30-40% over stock, on most HARLEY-DAVIDSON EVOLUTION models. Your engine must have the proper components necessary to produce maximum horsepower output. The addition of a high performance exhaust system alone is not enough to achieve maximum horsepower & torque. Not only will the exhaust system affect peak horsepower, torque & RPM, it also can change throttle response, gas mileage, vibration & engine temperature. The exhaust system has a very dramatic effect on how a engine operates throughout the entire RPM range. A stock cam & air cleaner will limit the performance of your engine, even with a high performance exhaust system installed. If your engine is equipped with at least a high performance cam & air cleaner, your performance gains will be significant. For motorcycles used only for racing, other modications can be considered. For the motorcycle to remain legal for street use, other modications must be limited to those that have been approved by the California Air Resources Board (CARB).
How a Thunderheader ™ exhaust system will affect your engine’s performance depends on other modifications that are made.
Thunderheader ™ high-performance exhaust systems, are specifically designed for the HARLEY-DAVIDSON V-Twin engine. Click Here For Dealer Only Access to Tuning Maps!