Bill Babcock wrote:
>well to start with, unless the first quote is out of context, it's simply
>wrong. Shorter primaries emphasize high RPM power, because the negative sonic
>pressure wave arrives at the exhaust valve sooner (too early for mid- and low-
>range power), if the pipe is short enough the exhaust valve won't be open yet.
>
>But it's very true that there are too many variables for an effective thumb
>rule.
>
Yet one more reason for wanting a dyno at home. :) But, the variables
involved are pretty fascinating, and the science of it is one where
failing to account for all the variables can lead to quite unpredictable
results. And, there are instances that seem to defy the science.
Think, for instance, about dragster engines. These typically run at
revs well below the top end of F1 engines, but the exhaust systems used
are invariably "zoomies" that are little more than short stubs. Doesn't
exactly make sense, according to conventional wisdom--until one realizes
that there's so much boost at the front end of the engine that an
exhaust of almost any appreciable length is a source of fluid drag in
the pipe, and that the vehicle is so fast that the large volume and
speed of air passing the end of the exhaust tube has an eductive effect,
so extraction designs are not necessary. On such engines, fancy tube
tuning often results in less power, not more.
What's important to understand is how the basics influence each other.
Because, for example, small changes in diameter create larger changes in
cross-sectional area, small changes in diameter have large effects on
gas speed, and big increases in gas speed can create substantial
decreases in pipe energy due to drag. Therefore, a pipe length that's
supposed to come in hard at a particular rpm may not, because the pipe
diameter is too small or too large.
The other tendency a lot of people have is to think of exhaust systems
in isolation. For normally-aspirated engines, this simply isn't
true--one has to look at the whole system, intake and all. If you're
stuck with a specific length of intake from free air to valve head, then
the optimum exhaust system is dependent upon that intake length. You
can move the torque peak up and down the rpm range a bit, but often at
the loss of engine flexibility. Philip Smith has some dyno tests done
by Jaguar on their early test XKE engines equipped with Lucas mechanical
injection and slide throttles reproduced in one of his engine books,
charting volumetric efficiency against intake length (adjusted by air
horns). With some pretty long horns, the engine could achieve, as I
recall, 108% VE, but the engine was exceptionally peaky--torque dropped
off noticeably on either side of the maximum--not exactly an ideal
circumstance with a 4-speed gearbox.
But, there's good discussion of intake/exhaust variables in Philip
Smith's _Design and Tuning of Competition Engines_ (find the 5th
edition, if possible), and in another book by Smith and a fellow named
Morrison, which I think is titled _The Scientific Design of Intake and
Exhaust Systems_, and has a lot of theory in it, although the
experimental examples date back to the `30s and `40s and seem quite
crude compared to current equipment.
Cheers.
--
Michael D. Porter
Roswell, NM
Never let anyone drive you crazy when you know it's within walking distance....
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