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stolen from MAP tech papers .
TUNED INTAKE LENGTH - It is possible to improve volumetric
efficiency by harnessing the natural kinetic energy and the
resonant pulses that occur during the intake cycle. When the
intake valve closes, a pressure pulse bounces back out the intake
tract, and then in again toward the valve. By making the intake
tract the proper length, the returning pulse can be timed to arrive
at top dead center of the next intake cycle, forcing extra air in and
keeping exhaust gases out of the intake port. In order to utilize
this pulse, the intake tract, from the carburetor bell mouth to the
inlet valve, must be the correct length. See fig.#2 (Appendix A).
There are actually several pulses that can be used corresponding
to the 2nd, 3rd and 4th time the pulse is reflected. These pulses
are called harmonics. The 2nd harmonic being the strongest.
There is a simple formula for calculating the length using the
second harmonic.
Tuned Length (in inches) is 132,000 divided by RPM
For instance, if our usable peak RPM was 7000: 132,000 divided
by 7000 - 18.8 inches. This is a very long tract and difficult to
accommodate on a motorcycle. The use of the 3rd harmonic will
result in a slightly shorter tract:
97,000 divided by 7000 = 13.8 inches.
Still a rather long distance. What real benefits can we derive from
intake tuning? The real joker in this whole thing is that although
we may derive some peak power from increasing the ram effect,
it will only work during a relatively narrow RPM band (a little
over 1000 RPM usually). It also has the effect of reducing engine
power outside of the narrow RPM band. Is it worth it? For an allout
road racer it may be. For a modified street bike we will
probably lose more than we gain. A close look at the latest factory
road racers would indicate that this theory has been all but
abandoned and carburetors are now being placed as close to the
head as possible with relatively short inlet bells. Perhaps the mid
range losses were found to be greater than the top end gain.
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If you love it , let it go . If it comes back to you , you've highsided ;-)
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