Why a sports car gearbox has closer steps between gears, how engineers measure that closeness, and how the Porsche 911's three 1967 to 1971 gearboxes show the difference.
A close-ratio transmission has smaller gaps between its gears than the alternatives for the same model, which keeps a sports engine in its power band. Porsche's 1967 to 1971 911 gearboxes show how the step size changes engine speed.
A close-ratio transmission is a gearbox whose steps between gears are smaller than average. Carmakers use the term mostly for sports cars, where the driver is expected to change gear often and keep the engine in its most useful speed range. The label is relative, and it only makes sense when compared with the other gearboxes offered for the same model.
What makes a transmission close-ratio?
There is no fixed threshold or industry standard that says when the steps between gears count as close. A transmission is close-ratio because it is more tightly spaced than the alternative fitted to the same make and model. As a result, what one manufacturer calls a close-ratio box may not be any closer than another maker's ordinary manual.
Carmakers often use the term when offering one or more options in place of the standard gearbox. A sportier box with closer ratios can be sold alongside the usual unit, and that is how Porsche used the label for the 911 between 1967 and 1971.
A close-ratio gearbox is only ever close relative to its alternatives, and the test is whether the engine stays in its power band.
How is gear spacing measured?
The closeness of a gearbox can be described by the cumulative average spacing between its gears, which is the geometric average of the steps. It is found by taking the (n-1)th root of the product of the ratio changes, which simplifies to the overall range from the highest to the lowest ratio, raised to the power of one over the number of steps.
Most transmissions cover roughly the same total range between their highest and lowest gears. That means the more gears a box has, the closer together they must be. A continuously variable transmission has an almost unlimited number of ratios, yet it is not normally called close-ratio, because it has no fixed gears unless they are programmed in.
Why does the power band matter?
Petrol engines can run over a wide range of speeds, from idle to redline at roughly 700 to 6500 rpm or more. The power band is the narrower range where the engine gives the best balance of fuel use, torque and power, and the transmission's job is to keep the engine inside it while the car covers a wide range of legal speeds.
When the car speeds up and the engine passes the speed of peak power, the driver or the transmission shifts to a higher, numerically lower gear. That drops engine speed back into the band, allowing acceleration to continue. If the next gear is much lower than the one before, an upshift can pull the engine below the band, which is why close ratios help during hard acceleration.
A wide-ratio box, by contrast, needs fewer shifts and covers a greater range of vehicle speeds, but the engine has to work across a wider range of revolutions. High-performance engines are often tuned for peak output in a narrow range, so a close-ratio box lets them stay there. Sports cars are the natural home for such gearboxes, because their drivers are expected to enjoy shifting often.
How did the Porsche 911 use close ratios?
Three transmissions were offered for the Porsche 911 from 1967 to 1971. The standard unit was the 901/75. The 901/76 was marked for hill climbs, and the 901/79 was marked for Nurburgring ratios. Each gear is compared with the one before it, so the step between third and fourth gear on the 901/76 is 85 percent, meaning the engine turns 85 percent as fast in fourth as in third at the same road speed.
On the standard box, each gear averages 75 percent of the preceding one, which comes from the ratio of the first to the last gear over the four steps, 0.821 divided by 2.643, raised to the power of one quarter, giving 0.747. The Hill Climb box averages 81 percent, and the Nurburgring box averages 77 percent. The Hill Climb gears are therefore closer to their predecessors than either of the others, which makes it the close-ratio option.
The Nurburgring pattern is different in character. Its step from first to second is the largest single change of the three, but the steps from second through fifth are relatively small. The gearbox was designed for sustained high-speed running rather than acceleration from a standing start.
| Transmission | Label | Average gear step |
|---|---|---|
| 901/75 | Standard | 75 percent (0.747) |
| 901/76 | For hill climbs | 81 percent; third to fourth step 85 percent |
| 901/79 | Nurburgring ratios | 77 percent; largest step from first to second |
What difference did the Hill Climb ratios make?
The 1967 Porsche 911 S had a 2.0 L flat-six that produced 160 hp (120 kW) at 6600/min, and peak torque of 179 N m (132 lbf ft) at 5200/min. With the standard gearbox, shifting from second to third at 6600/min drops the engine to 4890/min, which is 6600 multiplied by 1.32 and divided by 1.78. That leaves the engine a little below its peak torque speed.
On the Hill Climb box the same shift leaves the engine at 5120/min, calculated as 6600 multiplied by 1.55 and divided by 2.00. That figure almost coincides with the speed of maximum torque, so the engine is working closer to its best point on the next gear.
The Nurburgring set is also slightly closer than the standard gearset, making it more useful for sporting applications. Its fifth gear is taller, which allows higher top speeds on the faster circuit. The standard box, with its numerically lower fifth gear, keeps engine speeds lower at highway speed, which reduces noise and fuel use but costs acceleration at very high speeds.
How have close-ratio gearboxes changed over time?
In the 1960s most manual cars had four forward speeds and a 1:1 top gear. The designation of wide or close ratio mostly affected first gear. General Motors' four-speed Muncie gearboxes showed the difference: the M20 wide-ratio unit had a first gear of 2.52 or 2.56:1, while the M21 and M22 close-ratio units had a first gear of 2.20:1.
Fuel economy was not a priority then. Close-ratio boxes were usually paired with low final drive ratios of 3.5:1 or higher to offset the high first gear, and that combination used a lot of fuel. After the oil crises of the 1970s, final drives went to 3:1 or lower. To cope, manufacturers added overdrive fifth gears with lower first gears, so that 1970s close-ratio boxes had first gears of 2.64:1, while wide-ratio boxes were 3:1 or higher. The 1970s close-ratio units therefore often covered a wider range than the 1960s wide-ratio units.
Can adding gears make a box close-ratio?
One way to create a close-ratio transmission is to add gears while keeping the lowest and highest ratios the same. Some six-speed transmissions are labelled close-ratio on that basis. The test is the overall spacing from first to sixth gear. A six-speed can be legitimately called close-ratio only if its top gear matches the comparable five-speed, so that the change from low to high happens in smaller steps.
Some six-speeds keep the ratios of a five-speed and add a taller sixth gear, which lowers engine speed further at highway speed. Such a box is better described as a double overdrive, depending on the fifth and sixth ratios. Automatics can also be close-ratio, and with six-, seven- and eight-speed automatics the ratios sit closer together, which fits the mathematical definition.
Where do continuously variable transmissions fit?
Before the 1970s manufacturers' manual gearboxes typically had three or four gears, and three-speed automatics were the norm until fairly recently. To improve fuel economy, manufacturers began offering five-speed manuals, and six-speed manuals followed in the 1990s. Now six-, seven- and eight-speed automatics are being offered.
More gears allow tighter spacing, which keeps the engine in a narrow band. A five-speed box must cover a wider power range and so compromises engine efficiency, while an eight-speed box can keep the speed range narrow and let the engineer optimise efficiency at one engine speed. Automatics also upshift whenever possible, because engine efficiency improves when the load is high.
The continuously variable transmission pushes this strategy to its limit, giving an almost unlimited number of ratios. Because it has no fixed gears, though, it would not normally be considered close-ratio.
