Last comment and I am driving off to the airstrip at Maxton North
Carolina..
RPM X tire height in inches divided by 336 X gear ratio = speed in mph
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> From: Daniel1312@aol.com
> To: ulix@u.washington.edu
> Cc: spridgets@autox.team.net
> Subject: On the other hand was Re: Or maybe not, was Re: I'm full of
$#1^...:-)
> Date: Thursday, April 29, 1999 3:01 PM
>
> Hi Ulix, list,
>
> It is late but I will try again as below somewhere.
>
> << 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?>> me neither and it is bed time in the UK!!!
>
> <<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.>>
>
> With a 3.9 diff at 6,000rpm in third gear you are travelling at
12.3455mph x
> 6 = 74.073mph. When you shift to 4th in theory (the speed will bleed off
a
> bit in practice) you are still at 74.073mph but the revs are now
4,389rpm.
> The rev drop is 1,611rpm.
>
> With a 3.7 diff at 6,000rpm in third gear you are travelling at
13.1171mph x
> 6 = (faster than was the case with the 3.9 diff but it is the rpm that
> matters here) at 78.70mph. When you shift to 4th in theory (blah, blah,
etc)
> you are still at 78.70 but the revs are now 4,421. The rev drop is 1,579
> which is of course the wrong answer because I want it to be less but at
least
> it does matter what the diff ratio is: (I think we need Ron for the Math)
>
> <<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
> >
>
>
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