Skip Gurnee. wrote:
> Greg and everybody
> No clue whether this is right from an engineering point of view, but I
> can't resist:
> Perhaps the loss of power could be in the tuning of the pipes. The
> negative pressure of one exhaust wave pulls the next wave out, and so
> on, thereby creating less backpressure at the cylinder, thereby creating
> more horsepower. If the wave isn't there to pull, i.e. it's in another
> pipe, the power is lost. Oboy, lots of variables here; diameter of
> pipe, length of pipe vs. length of exhaust wave, etc. etc. ad infinitum.
> Any engineers out there wanna comment?
> -S.
>
Exhaust systems are a very complex relationship between sound waves, pressure
waves,
and gas flow. Any pipe that has compressible fluids moving through it (i.e.
air,
exhaust, etc.) has a natural harmonic based on the diameter and length of the
pipe, as
well as the mass of the fluid. The basic theory is to utilize the harmonic to
create
a depression at the valve at the instant that the valve opens, thereby sucking
the
exhaust from the cylinder.
The smaller the diameter of the pipe, the higher the frequency. Even if two
smaller
pipes had a larger cross-section, and, ultimately, could flow more, their
frequency
would still be the same based on the diameter/length/mass equation. It is quite
possible that smaller pipes move the harmonic higher than the useful rpm range
of the
engine, and therefore, the benefit of the depression, or "pulse", is lost.
If the engine only ran at one speed, it would be very easy to design the optimum
exhaust, but since the speed changes dramatically, the task is more difficult.
The
length of the pipe would need to change by several feet to maintain a
depression at
the valve at the rigth time. Add to that physical limitations under the car
and the
noise requirements, and you can see how many compromises must be made.
Interestingly, I've talked to several engineers at big automotive companies who
claim
that much of the real-world function of a tuned exhaust pipe goes rapidly down
hill
when mufflers are inserted, and judging from the number of different
configurations
available today at the dealerships, one would think that they are right and that
almost anything goes. Certainly if there were one configuration that was much
better
than all the rest, every automaker with a mid-size V6 car would adopt such a
system!
Good luck!!!
Brian Schlorff '61 TR-4 '64 TR-4 '72 TR-6 '79 Spit
---------------------------------------------------------------------
Power British Check out Power British
371 E. Main St. on the 'net!
Norristown, PA 19401
(610) 270-0505 http://www.powerbritish.com/~britcars
---------------------------------------------------------------------
|