[quote= machten ...
" Bob, I'll keep my powder dry on calcs for the moment. I will say this this however...
I've seen and played with some simple internet exhaust length calculators "
____ It would've been of interest if you had provided links to any of those which you found to be possibly worthy of further examination by the rest of us.
__ Did you compare any of their answers using the sample*example that Harvey chose (and which I've maintained as the accepted working-example -[* DESMO-cam @ 6500RPM]) ? _ As it would be of interest to learn if any of their result-answers matched closely or not !
" I've looked at your own maths "
____ I assume that since you haven't pointed-out any math-errors, that you must not have found any ?
Anyhow, it doesn't really matter if you-yourself bothered to check that all my math was actually correct,, as
I know it's all without errors, (just as
anyone-else with a calculator could realize) ! _ It's just that those who don't have a clue,, could've been more assured, with the likes of yourself confirming that the math is all good.
" I don't think the physics are anywhere near so perfect a model. "
____ There's actually no legitimate arguing with the perfection of the actual math-logic ! _ However I do believe you're quite correct to imply that all the involved physics seem too complex to pin-down to the degree of perfection that yields result-answers within fractions of an inch (or perhaps even within a half-foot, with much certainty) !
__ What actually makes it next to impossible for the math-result to reach the actual factual bottom-line
answer,, is that the speed of sound within the ex.pipe, just can't seem to be pinned-down to a
good/solid known to be absolutely true degree.
And whatever that actual sound-speed boils-down to,, it's so very fast, that in any case,
no matter how tight it's tolerance-range is
fairly kept limited down-to,, it's varied result range is just-simply too great (to trust the bottom-line result [in inch-increments*] as being near perfect enough) !
(Meaning that while we might be able to trust result-figures in 'yard' or 'meter' increments,
inch-increments however imply an accuracy that's really not trustworthy, [for bike-exhausts with their limited pipe-lengths].)
Harvey wrote:I can clear that up Bob, I have used the lower road bike sound speed, of 1500 ft/sec.
____ That's a perfect answer, Harvey !
Instead of you seemingly having not used your-own presented formula (for attaining your posted "41.5" result), you actually rather had simply plugged-in a lower sound-speed into that formula.
(I had just assumed that you'd stuck-with the same '1600' ft/sec sound-speed that you had seemed to've settled-on.)
____ I've been fairly suspicious of that seemingly oversimplified formula (that Harvey has presented), but I've recently since found that it's end-result has provided the same result/answer as that achieved with the long/drawn-out math-figuring...
Just as plugging-in the '190' duration (of the DESMO-cam's 80+110 degrees) into that formula yielded 46.76+ inches,, the long-math method I perform,
also resulted with '46.76+', just as-well (when I did the math-figuring with merely just the 190-degree duration) !
__ However I still find there to be an issue with the directions for exactly what valve-timing numbers should be entered into that formula (to figure the desired length of ex.pipe for a particular RPM.range)...
__ By only entering merely-just the number of degrees existing between the opening of the ex.valve & the opening of the in.valve, (such as the 190-degrees of the DESMO-cam), the neg.sound-wave will arrive back at the intake-valve just as it's begun opening, (which is the
good intention of the formula).
However the chosen RPM entered into the formula will then tend to be misleading because...
For instance, with '6500' desired as the target-RPM to prefer max.torque to center-peak at,, the 'result' of the-
formula will rather be
off-target (to the
lower-edge of the
actual enhanced power-band's
beginning-point [that starts immediately
above 6500]) !
That's because at
any lower RPM than 6500, the neg.sound-wave will then return too-soon before the in.valve opens (and then not able to fully perform it's useful effect [anywhere below the 6500-point]). _ And-also (with that raw formula), the v.overlap-range will only allow the power-band to be enhanced just within the RPM.range directly
above the target-RPM
only !
So in the example-case of a DESMO-cam & 6500-RPM, the enhanced RPM.range will
BEGIN at 6500 and extend to 11,100-RPM* (due-to the D.cam's 135-degree overlap-period), rather than
center-around the 6500-RPM !
(* As by 11100RPM, the ex.valve will then be closed by the time the neg.sound-wave returns [from the tuned-end of the ex.pipe].)
__ So if anyone expects to use
that formula to rather find the 'bull's-eye' of the target-RPM power-band, (instead of simply the RPM.point at which it merely
begins),, then ya have-to include
half of the v.overlap-degrees in
addition to the duration between the valve-openings ! _ In
that way, the entered RPM will then be rather
centered within the targeted enhanced power-band range. _ And-so with the chosen (DESMO-cam) example-case of picking 6500-RPM as the preferred center-point for the
enhanced power-band,
the 6500-point will then be centered within the 4600-RPM power-bandwidth (with a 2300 rev.range left on
both sides, [instead of all 4600 left only
above the 6500-point]).
Enlightening-Cheers,
DCT-Bob
,,,,,,,you can't fool us,,,,,,,,,,the truth is that you are snowed in over there 
,,,,,,,,,,,can't get out,,,,,,,,,,,,cabin fever 
........ Got to talk to some body about anything. 
My copy is 1962, right as we were trying to get 15hp out of a 125 BSA Bantam !
) and my better half had other plans for me and motorcycle activities were banned! (well, perhaps not "banned", but the consequences of spending too much time on it didn't bear consideration). I have managed a few illicit peeks, but that's all. Give me a few days to digest what's here.
