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In the U.S., most enthusiasts don’t think about upgrading their vehicle’s transmission until it becomes absolutely necessary. By absolutely necessary, we mean the point at which the engine produces more torque than the transmission can handle. The end result is often broken shafts, bent forks and worn synchros. In Japan and elsewhere, a number of enthusiasts upgrade the vehicle’s transmission to not only handle additional power, but also to optimize the transmission’s gear ratios for increased performance. Acceleration is improved as the engine spends more time in the sweet-spot of the power band. While this doesn’t affect the peak horsepower getting to the ground, the average horsepower put to the ground is increased.
Now take a look at first on this imaginary transmission. The input gear has 12 teeth and the output gear has 36 teeth. In this case, the input shaft would need to spin three times (36/12=3.0) for every revolution of the output shaft. This would illustrate a first gear with a 3.0:1 or 3-to-1 ratio. This three-to-one ratio would multiply the torque from the engine by a factor of three to help the vehicle accelerate quickly from a stop. Of course, speed would be limited by this ratio. When the engine is spinning at 6,000 RPM (the input shaft is also spinning at this speed), the output shaft of the transmission would only be at 6,000/3.0 or 2,000 RPM. At 2,000 RPM shaft speed our imaginary car may only be able to reach 45 MPH at 6,000 RPM of engine speed in first. Now, let’s say that fifth gear in this same transmission is “1-to-1”. This is another way of saying that fifth gear has a 1.0:1 transmission ratio. In this case, the number of teeth on both the input and output shaft gear when fifth is selected would be equal.
So what’s the advantage of having close ratios in the transmission? Close-ratio transmissions produce less RPM drop between gear shifts. In the above examples, an 8,000 RPM shift from first to second would result in a drop in engine speed to 6,770 RPM with the close-ratio transmission or 6,670 RPM with the wide-ratio transmission. Maybe not a big deal on the 1-2 shift, but look at the 3-4 shift. The engine speed at an 8,000RPM shift would drop to 6,220 RPM in the close-ratio example or to 6,000 RPM on the wide-ratio transmission. Depending on the shape of the engine’s power curve, the RPM difference may mean that the engine is down a measurable amount of power on the gear shift.
