Daniel,
I don't quite follow.
You calculate mph/rpm in 3rd and 4th gears, but how do you get to the rpm
increase in the last line?
Also, let me try to make my point logically.
I am in 3rd gear and accelerate to 6000rpm, then I shift to 4th. The rpm
drops. By how much does it drop is the point of discussion here.
The rotational speed of output shaft of the transmission is the same
before and after the shift, because you are going the same speed right
before and right after the shift. Only the input shaft of the
transmission changes speed, namely the engine rpm.
BUT, when I rev 3rd to 6000 and then shift, the speed of the output shaft
is the same no matter which rearend I have installed, it just translates
to a different speed. More specifically, the output shaft is at
6000rpm/1.357 = 4422rpm when I am at 6000rpm in 3rd gear. This is not
influenced by anything past the transmission. Then I shift to 4th and
the engine rpm drops to 4422rpm.
SO, the rpm drop from 3rd to 4th only depends on the transmission ratios,
not anything else.
ERGO, I think I was full of it after all :-)
As an aside:
What DOES change is the speed (mph) at which this shift occurs.
It occurs at a higher speed for a longer diff, which means a higher load
(through wind resistance mainly). That means that at every point in the
rpm and gear range, the load on the engine will be higher than with the
shorter diff. So it will take you longer to go from 4000rpm to 5000rpm in
4th, but you are also going faster. I am not sure what I am trying to
say with this. Just an observation.
> 3.9 diff x wheel rpm = 3.9 x 911 = 3555.45
> 3.7 diff x wheel rpm = 3.7 x 911 = 3370.70
>
> 3.9 diff 3555.45 divided into 60,000 = 16.875mph per 1000rpm direct gear (4th)
> 3.7 diff 3370.70 divided into 60,000 = 17.800mph per 1000rpm direct gear (4th)
>
> 3.9 diff direct = 16.875 divided by 3rd gear (1.357) = 12.3455 mph
> 3.7 diff direct = 17.800 divide by 3rd gear (1.357) = 13.1171mph
>
> rpm increase 4th to 3rd on 3.9 diff at 3000rpm = 1,101rpm
> rpm increase 4th to 3rd on 3.7 diff at 3000rpm = 1,071rpm
>
> RPM increase is greater on 3.7 (taller diff) and this is why all you guys
> with 4.22 diffs don't worry about the wide ratios on the Jap boxes. Fit a
> 3.9 diff and you might think twice about the Ford box (er hum which I happen
> to have in my Sprite, er hum with close ratios anyway).
>
> Daniel1312
>
Ulix __/__,__ ___/__|__
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'67 Sprite '74 X1/9
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