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A close-ratio transmission describes a motor vehicle transmission with a smaller than average difference between the gear ratios. They are most often used on sports cars in order to keep the engine in the power band.
A close-ratio transmission is one which is described relative to another transmission for the same vehicle model. The relativity applies only for the transmissions offered for a single make and model; that is, there is no specific threshold value or accepted industry standard that determines whether the steps between gears constitute a normal or close-ratio transmission. What one manufacturer describes as a close-ratio transmission is not necessarily closer in ratios than another manufacturer's normal manual transmission.
Often, manufacturers use the term "close-ratio" when offering one or more alternatives to the transmission fitted as standard equipment: for example, an optional, sportier transmission which offers closer ratios than the standard, such as Porsche offered with the three transmissions listed below for the 911 from 1967 to 1971.
Mathematically, the "closeness" of a transmission can be characterized from the cumulative average spacing between, or geometric average of, gears. This is defined as the (n−1)th root of the products of each gear ratio, which simplifies to the overall range of gear ratios and the number of (forward) speeds:
where
In general, most transmissions have approximately the same total range between the highest and lowest gears, so the more gears a transmission has, the closer they are together. This is apparent from the expression above: as increases, the average spacing will increase. A continuously variable transmission (CVT) has a nearly infinite "number" of gear ratios between its highest and lowest ratios, which means the CVT has infinitesimally small steps between gear ratios. However, because CVTs do not have specific (fixed) gear ratios unless programmed as such, it would not be considered a close-ratioed transmission.
Internal combustion engines found in passenger automobiles are capable of operating over a relatively wide range of speeds: idle to redline for petrol engines is approximately 700 to 6500 RPM or more; however, the power band, which is the optimum range of engine speeds considering fuel consumption, torque, and power output, is usually smaller. [1] The automotive transmission is used to maintain the engine speed within the power band while operating the vehicle over a wide range of legal speeds.
During acceleration, when the vehicle's speed increases to the point that the engine speed exceeds the speed at which maximum power is developed, the driver or transmission shifts to a higher gear (numerically lower ratio), which reduces engine speed, keeping it in its optimum power band, and allows continued acceleration. It is possible for the next higher gear ratio to be so much lower than the preceding ratio that upshifting lowers the engine speed excessively, resulting in the engine speed falling outside its "power band"; for maximum acceleration, the engine speed of an automobile should be kept in this power band. A wide-ratio transmission requires the engine to operate over a greater range of engine speeds, but requires less shifting and allows a wider range of output (vehicle) speeds.
High-performance engines often are tuned for maximum power in an even more narrow range of operating speeds. A close-ratio type of transmission is designed to allow an engine to remain in this relatively narrow range of operating speeds and generally are offered in sports cars, in which the driver can be expected to enjoy shifting often to keep the engine in its power band.
| Gear | Ratio | |||||
|---|---|---|---|---|---|---|
| Standard (901/75) | Hill Climb (901/76) | Nürburgring (901/79) | ||||
| Value | Step | Value | Step | Value | Step | |
| 1st | 2.643 | — | 2.833 | — | 2.643 | — |
| 2nd | 1.778 | 67% | 2.000 | 71% | 1.600 | 61% |
| 3rd | 1.318 | 74% | 1.550 | 78% | 1.318 | 82% |
| 4th | 1.000 | 76% | 1.318 | 85% | 1.080 | 82% |
| 5th | 0.821 | 82% | 1.217 | 92% | 0.926 | 86% |
| Avg Step | 74.7% | 81% | 76.9% | |||
| Overall change | 0.31 | 0.43 | 0.35 | |||
This table compares the ratios of three transmissions offered for Porsche 911 vehicles from 1967 to 1971, the first being the standard 901/75 transmission, the second being the 901/76 transmission denoted "For hill climbs", and the third being the 901/79 transmission denoted "Nürburgring ratios". It includes the step between successive ratios; for example, the step from 3rd to 4th gear for the 901/76 transmission is 85%, meaning the engine speed in 4th gear is 85% of that in 3rd gear at equivalent vehicle speeds.
For the standard Porsche transmission (901/75) described here, each successive gear's ratio is, on average, 75% of that of the preceding gear:
By similar calculations, the Hill Climb transmission has successive gear ratios which are 81% of the preceding gear, on average, and the Nürburgring transmission has successive gear ratios which are 77% of the preceding gear. Thus, the Hill Climb transmission's gears are "closer" in numerical ratio to the preceding gear than that of the standard or Nürburgring transmission, making it a close-ratio transmission.
Note the step from 1st to 2nd for the Nürburgring transmission is the largest single change for any of the three transmissions, but the successive step changes from 2nd through 5th are relatively small; this transmission is intended for sustained high-speed operation instead of acceleration from a stop.
The 1967 Porsche 911 S was equipped with a 2.0 L flat-six engine which produced 160 hp (120 kW) at 6600/min and 179 N⋅m (132 lbf⋅ft) of torque at 5200/min. [3] Using the Standard transmission gear ratios above, when the driver shifts from 2nd to 3rd gear at 6,600/min, the engine speed would fall to 4890/min (which is 6600 × 1.32 / 1.78). In this case, shifting up to 3rd gear causes the engine speed to be slightly below the speed at which maximum torque is produced. By using a close-ratio gearbox, such as the Hill Climb example above, shifting to 3rd gear would drop engine speed to 5120/min (6600 × 1.55 / 2.00), which almost coincides with the maximum torque output of the engine.
Likewise, the Nürburgring gearset above is also slightly numerically closer than the Standard gearset, making it more useful for sporting applications. However, the Nürburgring specification has a "taller" (numerically lower) 5th gear ratio than the Hill Climb gearbox, allowing for higher top speeds necessary for this faster racing circuit.
The Standard gearset with its numerically lower 5th gear, will allow even lower engine speeds at highway speeds, thereby reducing engine noise and fuel consumption, but compromises acceleration performance at very high speeds.
| Gear | Muncie [4] | Borg-Warner Super T-10 [5] [6] | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| M20 (1963–65) | M20 (1966–74) | M21 & M22 | (1974–77) | (1977–79) | (1979–81) | (1980–82) | ||||||||
| Ratio | Step | Ratio | Step | Ratio | Step | Ratio | Step | Ratio | Step | Ratio | Step | Ratio | Step | |
| 1st | 2.56 | — | 2.52 | — | 2.20 | — | 2.43 | — | 2.64 | — | 2.88 | — | 3.42 | — |
| 2nd | 1.91 | 75% | 1.88 | 75% | 1.64 | 75% | 1.61 | 66% | 1.75 | 66% | 1.91 | 66% | 2.28 | 67% |
| 3rd | 1.48 | 77% | 1.46 | 78% | 1.28 | 78% | 1.23 | 76% | 1.34 | 77% | 1.33 | 70% | 1.46 | 64% |
| 4th | 1.00 | 68% | 1.00 | 68% | 1.00 | 78% | 1.00 | 81% | 1.00 | 75% | 1.00 | 75% | 1.00 | 68% |
| Avg Step | 73.1% | 73.5% | 76.9% | 74.4% | 72.4% | 70.3% | 66.4% | |||||||
| Overall | 0.39 | 0.4 | 0.45 | 0.41 | 0.38 | 0.35 | 0.29 | |||||||
In the 1960s, cars equipped with manual transmissions typically had four forward speeds and a top gear offering a 1:1 ratio. The designation of wide versus close ratio affected the lowest gear ratio; [7] for example, the four-speed Muncie transmissions offered in General Motors performance vehicles included the M20 "wide ratio" transmission, which had a first gear ratio of 2.52 or 2.56:1, while the M21 and M22 "close ratio" transmissions had a first gear ratio of 2.20:1. [4]
At that time, fuel efficiency was not a primary concern and the "close ratio" transmissions generally were paired with a low final drive ratio of 3.5:1 or higher to compensate for the relatively high lowest gear ratio, resulting in a large rate of fuel consumption. Following the oil crises of the 1970s, final drive ratios went to 3:1 or lower to improve fuel economy and to accommodate this, vehicle manufacturers began adding more forward speeds into the gearbox, typically pairing an overdrive fifth with an even lower first gear, resulting in what would have been considered a very wide ratio transmission. [7] "Close ratio" transmissions now had low gear ratios of 2.64:1 while "wide ratio" transmissions were 3:1 or higher, meaning that "close ratio" transmissions produced in the 1970s often had a larger range of ratios than "wide ratio" transmissions from the 1960s! [7]
| Gear | Civic (1978) [8] | NSX (1991) [9] | Civic Type R (2019) [10] | |||
|---|---|---|---|---|---|---|
| Ratio | Step | Ratio | Step | Ratio | Step | |
| 1st | 3.181 | — | 3.071 | — | 3.625 | — |
| 2nd | 1.823 | 57% | 1.727 | 56% | 2.115 | 58% |
| 3rd | 1.181 | 65% | 1.230 | 71% | 1.529 | 72% |
| 4th | 0.846 | 72% | 0.967 | 79% | 1.125 | 74% |
| 5th | — | 0.771 | 80% | 0.911 | 81% | |
| 6th | — | 0.735 | 81% | |||
| Avg Step | 64.3% | 70.8% | 72.7% | |||
| Overall | 0.27 | 0.25 | 0.2 | |||
One way to create a close-ratio transmission is to install more gears into the transmission without altering the lowest and highest gear ratios. In this manner, some six-speed transmissions available in consumer vehicles are labelled as "close-ratio". Again, the defining issue is the overall spacing of gears between 1st and in this case 6th gear. As an example, consider three manual transmissions fitted to Honda cars, each with an overall change in ratios () of 0.2 to 0.3, but with a different number of gears; the six-speed transmission has closer ratios than the four-speed transmission.
Whether a six-speed transmission can be legitimately called "close-ratio" depends on whether it keeps the top gear unchanged relative to that of a comparable 5-speed model, thus causing the change in ratios from low to high gear to occur in smaller steps (i.e. closer ratios) between gears. Alternatively, some six-speed transmissions have ratios essentially the same as a 5-speed transmission, and add an even higher (numerically lower) 6th gear that allows even lower engine speeds at highway speeds. In this case, the transmission would be considered a "double" overdrive transmission, depending upon the 5th and 6th gear ratios. [11]
By extension, an automatic transmission could also be called close-ratioed. With the advent of 6-, 7-, and 8-speed automatic transmissions, the ratios become closer and closer together, which meets the mathematical conception of what constitutes a close-ratio transmission.
Prior to the 1970s, manufacturers' manual transmissions generally had three or four gears. To meet requirements to maximize fuel economy, manufacturers began offering 5- and, in the 1990s, 6- speed manual transmissions. Likewise, 3-speed automatic transmissions were the norm until fairly recently, but now 6-, 7-, and 8-speed automatic transmissions are being offered.
By reducing the spacing between ratios allowed by having more gears, a vehicle's engine speed can be kept in a narrow band. With a 5-speed transmission, the power range must be relatively wide, which requires compromising the engine's efficiency. With an 8-speed transmission, the power range can be kept relatively narrow, which allows the engineer to optimize engine efficiency at a particular engine speed, and the transmission attempts to keep the engine operating at that speed.
(Engine efficiency improves greatly when the load on the engine is maximized; hence, automatic transmissions also upshift whenever possible in an attempt to lower the engine's speed as much as possible, which increases load and efficiency.)
The recent introduction of continuously variable transmissions (CVTs) attempts to push this strategy to its logical conclusion. This allows a near infinite "number" of gear ratios, which this implies an infinitely close-ratioed transmission. However, given that there are no gears or specific gear ratios, one would not really consider such a transmission close-ratioed.
An automatic transmission is a multi-speed transmission used in motor vehicles that does not require any input from the driver to change forward gears under normal driving conditions. Vehicles with internal combustion engines, unlike electric vehicles, require the engine to operate in a narrow range of rates of rotation, requiring a gearbox, operated manually or automatically, to drive the wheels over a wide range of speeds.
An epicyclic gear train is a gear reduction assembly consisting of two gears mounted so that the center of one gear revolves around the center of the other. A carrier connects the centers of the two gears and rotates, to carry the planet gear(s) around the sun gear. The planet and sun gears mesh so that their pitch circles roll without slip. If the sun gear is held fixed, then a point on the pitch circle of the planet gear traces an epicycloid curve.
A torque converter is a device, usually implemented as a type of fluid coupling, that transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the torque converter connects the prime mover to the automatic gear train, which then drives the load. It is thus usually located between the engine's flexplate and the transmission. The equivalent device in a manual transmission is the mechanical clutch.
Overdrive is the operation of an automobile cruising at sustained speed with reduced engine speed (rpm), leading to better fuel consumption, lower noise, and lower wear. The term is ambiguous. The most fundamental meaning is that of an overall gear ratio between engine and wheels, such that the car is over-geared, and cannot reach its potential top speed, i.e. the car could travel faster if it were in a lower gear, with the engine turning at higher RPM.
A transmission is a mechanical device which uses a gear set—two or more gears working together—to change the speed, direction of rotation, or torque multiplication/reduction in a machine.
A continuously variable transmission (CVT) is an automated transmission that can change through a continuous range of gear ratios. This contrasts with other transmissions that provide a limited number of gear ratios in fixed steps. The flexibility of a CVT with suitable control may allow the engine to operate at a constant angular velocity while the vehicle moves at varying speeds.
A manual transmission (MT), also known as manual gearbox, standard transmission, or stick shift, is a multi-speed motor vehicle transmission system where gear changes require the driver to manually select the gears by operating a gear stick and clutch.
A semi-automatic transmission is a multiple-speed transmission where part of its operation is automated, but the driver's input is still required to launch the vehicle from a standstill and to manually change gears. Semi-automatic transmissions were almost exclusively used in motorcycles and are based on conventional manual transmissions or sequential manual transmissions, but use an automatic clutch system. But some semi-automatic transmissions have also been based on standard hydraulic automatic transmissions with torque converters and planetary gearsets.
A direct-shift gearbox is an electronically controlled, dual-clutch, multiple-shaft, automatic gearbox, in either a transaxle or traditional transmission layout, with automated clutch operation, and with fully-automatic or semi-manual gear selection. The first dual-clutch transmissions were derived from Porsche in-house development for the Porsche 962 in the 1980s.
The Honda Stream is a car manufactured by the Japanese automaker Honda from 2000 to 2014. The second generation model was officially presented on 13 July 2006. It has been described as a multi-purpose vehicle (MPV) or as an estate car.
A gear train or gear set is a machine element of a mechanical system formed by mounting two or more gears on a frame such that the teeth of the gears engage.
Hewland is a British engineering company, founded in 1957 by Mike Hewland, which specialises in racing-car gearboxes. Hewland currently employ 130 people at their Maidenhead facility and have diversified into a variety of markets being particularly successful in electric vehicle transmission supply.
The power band of an internal combustion engine or electric motor is the range of operating speeds under which the engine or motor is able to output the most power, that is, the maximum energy per unit of time. This usually means that maximum acceleration can be achieved inside this band. While engines and motors have a large range of operating speeds, the power band is usually a much smaller range of engine speed, only half or less of the total engine speed range.
A gear stick, gear lever, gearshift or shifter, more formally known as a transmission lever, is a metal lever attached to the transmission of an automobile. The term gear stick mostly refers to the shift lever of a manual transmission, while in an automatic transmission, a similar lever is known as a gear selector. A gear stick will normally be used to change gear whilst depressing the clutch pedal with the left foot to disengage the engine from the drivetrain and wheels. Automatic transmission vehicles, including hydraulic automatic transmissions, automated manual and older semi-automatic transmissions, like VW Autostick, and those with continuously variable transmissions, do not require a physical clutch pedal.
The Turboglide is a Chevrolet constant torque, continuously variable automatic transmission first offered as an option on Chevrolet V8 passenger cars for 1957. It consisted of a turbine-driven planetary gearbox with a 'switch pitch' dual-pitch torque converter stator. It had a die-cast aluminum transmission case, like Packard's Ultramatic of 1956. Turboglide cost about $50 more than the Powerglide 2-speed automatic. It was available in all V8-powered 1957-1961 Chevrolet models except the Corvette. General Motors produced 646,000 of these transmissions during its production.
Nuvinci Cycling technology currently under development for other applications, including wind turbines, light electric vehicles, outdoor power equipment, and automotive front-end accessory drives.
A dual-clutch transmission (DCT) is a type of multi-speed vehicle transmission system, that uses two separate clutches for odd and even gear sets. The design is often similar to two separate manual transmissions with their respective clutches contained within one housing, and working as one unit. In car and truck applications, the DCT functions as an automatic transmission, requiring no driver input to change gears.
A motorcycle transmission is a transmission created specifically for motorcycle applications. They may also be found in use on other light vehicles such as motor tricycles and quadbikes, go-karts, offroad buggies, auto rickshaws, mowers, and other utility vehicles, microcars, and even some superlight racing cars.
