Maximum Horsepower Output

Tuning the exhaust system can be an important component in achieving maximum power when optimizing the performance of an engine for racing or dyno shootouts. While not recommended for street bikes, the use of drag or straight pipes can maximize the horsepower produced by any specific engine combination. The RPM range that the straight pipes produce their maximum power is very narrow. The best ways improve the performance of straight pipes is to "tune" the exhaust length to the required power band.

Determining Exhaust Pipe Length

Opinions differ as to the best exhaust pipe lengths for any engine. There appears to be two major "schools" of thought on the best exhaust system length. The "short" pipe and "long" pipe calculations performed below present both views. Short pipes seem to win from an esthetics point of view, but the longer pipes seem to produce a better power curve. The decision of which to use is up to you.

Any formula that calculates header pipe lengths must take into account conditions such as exhaust temperature, gas speed, exhaust valve duration and the RPM the engine is running at. Each formulae makes different assumptions about these items resulting in different results from the same basic input parameters. The formulas used here result in short and long pipe length being calculated.

For serious performance efforts, the pipe lengths are calculated for a 3-step drag pipe. This 3-step design has generally proven to give the highest horsepower results over any other design. 3-step pipes are generally a custom build pipe.

There are two pieces of information that must be supplied in order to determine the exhaust pipe lengths for an engine.

1. The RPM's for the middle of the desired power band is needed. For Harley-Davidson applications the following RPM values would be typical: a street engine will be 4000-5000, a street/strip engine 4500-5500, race engines 5000-6000 and dyno shootout engine 5500-6500.

2. The opening point of the exhaust valve is required. This number must be reported as degrees BBDC ( Before Bottom Dead Center ). The default value used in the calculations is for an Andrews EV-3. The EV-3 is a general purpose cam that works well in stock engines and reacts positively to more extensive engine modifications such as increased compression and air flow. Determine the value for the camshaft in your engine. Values for stock HD, Andrews and Crane cams are listed below.