In a message dated 3/1/99 10:42:33 PM Eastern Standard Time,
wayne@richmond.net writes:
<< Grand Turismo Jaguar sells a 6-speed tranny that will reduce
your 0-60 time by over 1.5 seconds. Its because of lower gearing in the
lowest
gears. >>
Regarding the comment above:
I am sure the 6-speed is quicker to 60 than the 3-speed auto, but not because
of the reason stated above. There are 2 primary reasons for its better 0-60
performance. Number 1, a manual transmission transmits more power to the
wheels than does an automatic tranny (it has lower losses). And 2, since it
is close-ratio, it can pack more gears into the 0-60 sprint (3 vs 2) so that
the total area under the engine power vs road speed curve is greater.
As an example of what I mean by this…quickest acceleration would be achieved
by a car equipped with an infinitely variable transmission keeping the engine
at its power peak at all times. This would always assure that the power at
the rear wheels was maximized, producing the greatest excess torque to
accelerate the car. In my example, the area under the engine power vs road
speed curve (a straight horizontal line at 262 hp, extending from 0 mph to 60
mph) would be a nice large rectangle - the largest possible area under the
‘curve’. Since we don’t have an infinitely variable tranny, the next best
thing is more gears as in a close ratio 6-speed. If you plot engine power vs
road speed for each of the gears on the same graph paper, it is very obvious
that the ‘area under the curve’ for 3 speeds is much greater than the area
under the curve for 2 speeds (for the 0 to 60 mph range). This is why the
6-speed is so much quicker than the 3-speed from 0-60mph, not because of its
lower ratios.
A perfect example of this is the comparison of the XJ-S with 3-speed auto vs
4-speed auto. The 1st three gears are the same in both trannies, the only
difference being the rear end ratio, the 3-speed having a 2.88 and the 4-speed
a 3.54. This means that the 4-speed car has significantly higher overall
ratios in the 1st three gears than the 3-speed car. So presumably it will
accelerate a lot faster because of the ‘lower’ gearing, right? Wrong. The
3.54 car accelerates to 60 in 7.3 seconds, 0.2 seconds faster than the 2.88
car (both cars use only 1st gear and portions of 2nd gear in the process).
Essentially, the area under the power vs speed curves is the same, regardless
of rear end ratio. If the rear end ratio was so high (6.0) that the 4-speed
car used 3 gears in getting to 60 mph, then it would be faster because of the
increased area under the power vs speed curves (0-60 mph in 7.0 seconds). Of
course the top speed of the car would only be 95mph - totally impractical. So
more gears (and close ratio) is the way to go.
This explains why 0-60 times are essentially independent of rear end ratios
within a reasonable (and practical) range.
Regarding the acceleration times I posted previously:
OK, I obviously have started a controversy regarding the common wisdom of
going to a higher rear end ratio, and the seat-of-the-pants improvement in
acceleration as a result. I will try to explain the data I have posted and
why it doesn’t contradict this seat-of-the-pants feel that a higher ratio is
quicker (which it is).
First, the data for in-range acceleration times is based on allowing the
transmission to kickdown, if it can. So, for a 3.54 car, at a given road
speed, it may not be possible to change down to the next lower gear when
giving the car the boot, while the car with a 2.88 may change down a gear,
thereby raising the engine’s rpm to a higher speed and a potentially higher
power. In this case the 2.88 car will accelerate faster than the 3.54 car
stuck in a higher gear. This example illustrates why the data I posted is
valid, and does not contradict the feeling of snappier response when the
throttle is depressed on a car with a higher rear end ratio.
The following data is single-gear acceleration times for all three gears and
selected speed ranges (meaning the transmission is NOT allowed to shift into
the next lower gear). Note that this data confirms what everyone knows, a
higher rear end ratio is snappier, as long as you can stay in one gear over
the speed range of interest. So for most drivers, the lower rear end gearing
will provide a lot of punch, and can be very worthwhile. However, this does
not translate to better 0-60 times, which was the original statement to which
I responded.
So, the moral of the story here is that if you are looking to improve your
0-60mph time, rear end gearing really won’t get you very much improvement.
But if you really are looking just for snappier part-throttle response, then a
rear end change is just the ticket.
I hope this clarifies things regarding the previously posted data, and its
seeming inconsistency with the ‘real world’ seat-of-the-pants feel about rear
end gearing.
Steve - 89 conv’t/93 XJRS
1st Gear
RER 5-20mph 20-40mph
2.50 2.40 2.60
2.88 2.15 2.35
3.07 2.00 2.00
3.31 2.00 2.00
3.54 1.75 1.95
2nd Gear
RER 20-40mph 40-60mph
2.50 4.60 4.78
2.88 4.06 4.07
3.07 3.75 3.75
3.31 3.44 3.59
3.54 3.12 3.44
3rd Gear
RER 40-60mph 60-80mph
2.50 7.06 7.88
2.88 6.12 6.47
3.07 5.65 5.99
3.31 5.18 5.06
3.54 4.70 5.30
is that lower gearing