When a standard engine is tuned to produce more power, the power band (that
range of engine speeds where power output is pretty much at a maximum) often
becomes narrower due to a steeper power curve, with peak torque occurring at
higher engine speeds also. A transmission's ratios are designed to maximize
acceleration by getting the most torque to the wheels at the lower shift
point for that particular ratio and maximum power to the wheels (more or
less) at the upper shift point for that ratio. Thus, as the power band gets
narrower and the torque peak and power peak get closer together, they no
longer match the gear ratios of the standard transmission. As you shift
from 3rd to 4th, for example, with a highly tuned engine and wide ratio
gears, the engine speed might fall way below the speed at which peak torque
is occurring, so that the car "bogs down" until its able to slowly reach the
peak point of its torque curve. So the answer is to use a close ratio
gearbox. The problem is, that with a close ratio four speed box, for
regular driving use, either 1st will be too high or 4th will be too low.
The answer, of course, is to add a speed, so that the full range of road
speeds expected to be experienced by the car are covered by an appropriate
ratio. If the car were only to be driven at high speed, (such as stock
cars, which lap at high speeds and don't accelerate away from a standing
start), you wouldn't need this extra ratio, but for everyday driving, using
an engine with a narrow power band and peak torque occurring at high engine
speed, a close ratio five-speed is a necessity. This is the reason Japanese
motorcycles are equipped with 6-speed transmissions (and maybe even
7-speeds?).
Maybe some numbers would help:
For the TR3, maximum power was 89 BHP at 4700 rpm. Peak torque was around
3250 rpm, with the torque curve crossing the power curve at about 3550 rpm.
The ratio of the peak power engine speed to the speed at which the torque
curve crosses the power curve would be 4700/3550, which is a ratio of
1.324:1. The TR3's 3rd speed ratio was actually 1.325, according to my shop
manual. Now, lets say that the peak power is increased to 140 BHP at 5500
rpm, peak torque increasing accordingly, and let's say that the torque curve
crosses the power curve at 4400 rpm, so that the optimum shift point is
moved to this engine speed. Then the ideal 3rd/4th ratio would be 1.25:1,
closer than standard.
-----Original Message-----
From: C. Centore <anacon@ziplink.net>
To: autox <triumphs@autox.team.net>
Date: Tuesday, September 07, 1999 3:13 PM
Subject: Close ratio gear box
>
>Helo all,
>
>Could someone explain the difference between a close ratio gear box a a
>so called standard set of gears. What advantage is gained by this type
>of set up? Say if one were to change the gears in a TR3 tranny what
>difference would it make?
>
>
>
>Wondering Chuck
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