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.
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.
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.
Some specifications for common camshafts are