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Overdrive Overview

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by Huw Evans  More from Author

A Complete Guide to Overdrive Transmissions

Photos by Huw Evans and courtesy of TCI Automotive and TREMEC transmissions

 

By its very nature, hot rodding has always encouraged new ideas and developments. Although in the early days it was primarily about going fast and making an individual statement, as time went on, it was also about making cars drive, handle and even stop better than they did originally. Today, many hot rod enthusiasts are looking for ways to combine the traditional looks and style of a 1930s-through-1950s car, without sacrificing modern conveniences such as fuel injection, disc brakes and independent suspension. Considering that many hot rodders and custom car fans like to drive their rides extensively – to car shows, cruise-ins, conventions, even cross country road trips or tours – more and more are adopting modern overdrive transmissions for use in their rods and customs. In this article, we’ll take a look at why overdrive units are becoming the norm, as well as some popular options available for rodders today, both for those who like to shift gears themselves and those who prefer to let the transmission do the work.

 

Manual Overdrive Transmissions

Essentially, an overdrive transmission is a gearbox that uses an additional, tall top gear ratio (or ratios) to drop engine rpm at a given speed in order to save fuel and motor wear without compromising performance. Overdrive gears are designed to be operable primarily at cruising speeds and over longer distances, where without them, an engine would be working hard and burning a considerable amount of fuel – for example, during freeway driving. On most latter-day manual gearboxes, an overdrive gear is integral to the transmission. For example, Borg Warner’s popular T-5 five-speed, found in many light trucks and American passenger cars, uses four gears, plus an overdrive. For illustration purposes, a typical T-5 will have ratios of 3.35:1 for first, 1.99:1 for second, 1.33:1 for third and 1:1 for fourth gear. This last gear is also known as direct drive, because it doesn’t require any additional ratios to transmit power to the vehicle’s wheels, reducing parasitic loss.

However, because the input and output shaft of the transmission are turning at the same speed in fourth, it’s not easy to provide quick acceleration (using a short numerical axle ratio) without having a significant impact on top speed and fuel economy. (Installing a numerically tall ratio would improve fuel economy but result in gutless acceleration).

By adding an overdrive ratio that is taller than the 1:1 direct drive, the output shaft is able to turn at a faster revolution than the input shaft linked to the engine, reducing engine rpm to the driveshaft and wheels when it reaches a particular speed and the driver shifts into high gear. So on a T-5, a fifth or overdrive gear, usually with a .67:1 ratio, is added. In a car like a V-8 powered Camaro or Mustang with a 3.08:1 rear axle ratio, this allows for quick acceleration from stoplights thanks to a low first gear, but also enables the car to cruise along at 70 mph in fifth with the engine turning at well below 3000 rpm, maximizing fuel economy and minimizing stress.

 

Origins

Like many automotive inventions, the overdrive transmission has been around for a fairly long time. Prior to the 1950s, most cars used manual gearboxes, normally with a maximum of three forward gears. In order to achieve some semblance of drivability, the ratios in the gearbox were widely spaced, but even top gear had to be a fairly low ratio in order to prevent the engine from bogging. However, this also meant that the vehicle could be a chore to drive at higher speeds due to excessive engine revs and noise. The solution was to install an overdrive box with an additional gear connected to the input and output shaft between the transmission and driveshaft, which used a solenoid, usually activated by a button by the accelerator. The driver could then ease off the throttle and this extra “overdrive” gear would spin the output shaft at a faster speed, reducing engine rpm. The overdrive could then be disengaged by the button-operated solenoid when not needed, i.e. city driving, traversing inclines or towing. In North America, Borg Warner supplied many factory installed overdrive units on production cars in the 1930s and 1940s, while aftermarket versions were later offered through companies like Gear Vendors, who are still in business today, providing bolt-on overdrive kits for older cars (generally from the 1920s through the 1970s).

As fuel economy concerns became more prevalent during the 1970s and the US Federal Government introduced its CAFE (Corporate Average Fuel Economy) standards, automakers had to achieve specific gas consumption targets across their entire fleet or face fines (the initial standard was 18 mpg, later rising to 27.5 mpg and more recently 35 mpg). One of the easiest solutions was to add overdrive ratios to manual gearboxes, resulting in five- and, more recently, six-speed manual gearboxes.

For example, TREMEC Transmissions’ new T-56 Magnum, designed for late-model high performance cars and street rods, incorporates a 2.66:1 first gear, but .80:1 and .63:1 fifth and sixth overdrive gear ratios. Says TREMEC’s Nate Tovey, “The Magnum is our newest product and is designed to provide a high torque capacity (700 lb-ft), while making the car it’s bolted into exceedingly easy to live with every day. In the past, we built tough five-speed transmissions, like the TKO and TKO 600, but when you shifted into fifth gear, especially if you were running a low axle ratio like a 3.73:1 and you had a mildly modified pushrod engine like a traditional small-block Chevy or Ford Windsor V-8, you’d be driving near the limit of its operating range virtually all the time.

With the Magnum, by having two overdrive gears, it allows you to drop the rpm significantly, so in sixth gear, where you’d be doing 3500 rpm before, now you’re doing 2200, but at the same time, you’ve got a low 2.66:1 first gear ratio so you can really apply the effects of torque multiplication for quick acceleration.”

Although the Magnum comes with a standard gear package, you can opt for an even shorter first gear (2.74:1) and taller overdrive cogs .74:1 and .50:1. You can even specify pairs of custom drive and driven gears, for a price, giving hot-rodders a wide range of options when it comes to achieving optimum gearing for their particular engine and rear axle ratios.

 

Further Choices

As refined as the Magnum is (we actually drove a Windsor-engined car equipped with one), it’s still bulky and fairly heavy. If your tastes lean toward a 1930s era Ford where saving weight is part of achieving a desired level of performance, but you still want to shift gears yourself, TREMEC still offers a range of five-speed units designed to handle different torque capacities – including the T-5 and TKO models.

Some renowned rod builders, including Hot Rod Chassis & Cycle in Addison, Illinois, frequently specify TKOs when building cars for customers. “We like building traditional style hot rods and customs, but want to add modern features to make the cars faster and more drivable, without it being obvious,” says proprietor Kevin Tully. “In order to give the car an authentic feel, I like to use traditional style engines, say an original Chrysler Hemi or 1950s/1960s Buick Nailhead, Oldsmobile or Cadillac OHV V-8s, but build them into high performance engines, often using multiple carburetors.

“But whereas in the old days you’d use a three-speed manual behind one of these, I like to team them with a late-model five-speed gearbox like a TKO. You just can’t beat the combination. The TKO is extremely adaptable, plus you’ve got a transmission that’s far stronger than anything available in the 1950s. And unless you look underneath the car, you won’t know its there. At the same time, having five speeds means that you can jump in the car and drive it long distances without it feeling like it’s going to shake itself apart. And for many of our customers, that’s a very big deal.”

But what if you’re on a budget and can’t afford a brand new five or six speed transmission? “There are tons of used ones out there,” says Nate Tovey, “particularly T-5s and even T-56s, but there are a couple of things you have to consider. One of them is strength – most factory T-5s have a maximum torque capacity of just 265 or 300 lb-ft, so anything more than a lightly modified engine and you’re likely to have problems, including damaging the input and output shafts, unless the gearbox is strengthened to cope. Also, when you source a used manual transmission, you’ll also have to contend with the speedometer gear design on the donor car, so you need to choose carefully. For example, if you choose a used T-56 from a ’90s Camaro Z28 and want to install it in your Ford-powered Deuce, it’s not going to work without some serious modification, because the speedometer gear is designed for the Chevy. You also need to think about shifter location – one thing we learned at TREMEC is that different guys want different shifter locations depending on the type of car they’re putting the gearbox into. That’s why on the TKO and T-56 Magnum we offer different shifter mounting locations – you’ll be surprised what a difference this makes.”

 

Automatic Overdrive Transmissions

Although there’s a strong case for manual gearboxes, for many enthusiasts, particularly those who drive their cars long distances on rod runs or to big national shows, an automatic can have greater appeal.

Since Oldsmobile introduced the first true mass-produced automatic transmission in 1939 (the Hydra-Matic), self-shift units have become the norm in passenger vehicles in North America. Unlike a manual gearbox, where the driver essentially locks and unlocks different sets of gears on the output shaft in response to engine speed via a clutch, an automatic transmission uses single clusters of gears (sun, planet and ring carrier) mounted in sequence, to change the engine speed automatically via hydraulic pressure generated from a torque converter, pump, valve body and multiple clutch packs and bands.

 

How They Work

Although automatic transmissions are very complex devices, and we won’t delve into them in great detail here, it’s a good idea to grasp the fundamentals of how they operate, as this has a huge impact on deciding whether a particular automatic overdrive will work for your application.

In an automatic, the planetary gear set, in which the planet gears have more than double the number of teeth of the sun gear, allow for multiple different speeds on the same set of gears. By locking the sun and planetary gears together, you get a 1:1 gear reduction, allowing the input shaft to turn faster than the output shaft for a shorter ratio. Locking the ring and planetary gears together has the opposite effect – achieving a greater than 1:1 ratio, hence overdrive. By combining two sets of planetary gears together, with the first set engaging the second set, you can have four different forward speeds and a reverse gear (more sets equals more ratios). However, in order to make the car actually move, the engine is linked to the transmission via a torque converter. The torque converter is a fluid coupling which works like a clutch in a manual car, engaging and disengaging power from the engine to the driven wheels via the transmission.

Inside the torque converter are three components – a pump, turbine and stator. Transmission fluid is drawn into the converter via the pump, which flings it to the outside. It then enters the turbine, which draws it toward the center. The effect of this fluid transfer causes the turbine (linked to the input shaft on the transmission) to spin in the opposite direction of the pump to achieve gear reduction. To help efficiency, a stator, located between the two, uses specially angled blades to reverse the flow of fluid from the turbine back in the direction of the pump. However, as vehicle velocity increases, the pump and turbine start to turn at almost the same speed, so the fluid is flung faster in the direction of the turbine than the pump, which actually causes the stator to freewheel.

Because the turbine and pump aren’t turning at exactly the same speed on a conventional automatic, slippage results, wasting energy and causing heat – this is a major reason why automatic transmissions aren’t as efficient as manual gearboxes.

 

Lock Up and Shift

To help combat the problem, many modern automatics use a clutch inside the torque converter that “locks up” the turbine and pump, usually above speeds of 40 mph, when the transmission achieves 1:1 direct drive (usually third gear in most automatics) and any ratio taller than that (overdrive).

The selection of gear ratios in an automatic is controlled by clutch packs located in the planetary gears that are controlled by bands and pistons activated by hydraulic pressure through a valve body and oil pump linked to the torque converter (not to be confused with the pump inside the T-C). Transmission fluid is drawn up into the oil pump from a filter located on the bottom of the transmission, and the pump supplies fluid to the pressure regulator, valve body and other components of the transmission when the engine is running and/or the vehicle is at speed.

The valve body contains multiple channels that direct fluid to the valves and pistons that operate the individual clutch packs and bands for each gear, with selection based on the amount of fluid pressure from the pump. This pressure is regulated by a governor mounted on the output shaft of the transmission and usually a throttle cable that monitors engine load, via tension, which tells the transmission when to shift gear. (Some automatics use vacuum pressure instead of a cable to monitor engine speed – high pressure at low speeds for gentle shifting, low vacuum at higher engine speed for firmer, more aggressive shifts.)

 

Intelligent Automatics

Since the late 1980s, electronic overdrive automatics, which use solenoids to tell the transmission when to shift gears, have become increasing popular. Linked directly to the engine’s electronic brain, not only do they allow for more precise shift timing and control related to engine speed, but some also enable “manual” shifting of the individual gears in an automatic, with an override feature to prevent the driver from over-revving the engine and potentially causing damage.

 

Benefits of Automatic Overdrives

Although the general consensus is that manual gearboxes are preferable for high performance applications, modern overdrive automatics are more than up to the task. Because there is no direct link between the engine and gears – actuation is done by fluid and or/electronics – many automatics can be made into much stronger high performance transmissions, because there is less direct stress on the gears. In addition, the shifts can be dialed in for maximum consistency – a major reason why automatics prevail in circles such as drag racing. But even on the street in your hot rod or custom, they have multiple benefits. They generally allow for smoother shifting between gears, and although some will say they offer less car control (specifically when it comes to engine braking), with modern piggy back electronic controls, a late-model automatic overdrive allows you to shift gears when you want and then drop the lever into automatic “drive” for cruising or stop-and-go traffic. (Stop and go is hard on clutches in manual transmission cars, not to mention your left foot.) 

Also, thanks to the advent of lock-up torque converters, the disparity in efficiency and fuel economy between manuals and automatics is far less than it was in the past, so you can still drop in short rear axle ratios and maintain decent fuel economy. For example, GM’s current 6L80E six-speed automatic features an ultra short first gear ratio and aggressive second (4.02:1 and 2.36:1, respectively) for maximum acceleration, with third and fourth ratios of 1.53:1 and 1.15:1 that allow for responsive, flexible power delivery either in city driving or the open road. In addition, by featuring .85 and .67 fifth and sixth overdrive gears, it also allows for great highway cruising and gas mileage, without having to resort to tall rear axle ratios. The 6L80E, as installed in the current Chevy Corvette, allows it to accelerate from 0-60 mph in almost four seconds flat, yet achieves more than 25 miles per gallon on the highway from its 430 horsepower, 6.2-liter LS3 V-8 engine.

 

Fluid for Thought

Although it’s important in a manual gearbox, fluid is even more critical in an automatic because it controls operation as well as lubricating the moving parts. And the biggest enemy in any automatic transmission is friction and heat build-up, usually caused by old or defective fluid. As the fluid heats up, it begins to burn and will eventually evaporate. With less fluid inside the transmission, there’s a greater risk of friction and reduced hydraulic pressure, which further amplifies heat and will ultimately damage the moving parts. As a result, anything you can do to keep the fluid temperature down will be beneficial. A high performance transmission fluid, available from companies like TCI Automotive, Amsoil and Royal Purple, with a greater resistance to thermal breakdown is recommended, along with a high flow transmission filter for improved fluid circulation.

In addition, a transmission cooler is a must when building a high performance automatic, particularly for older cars (like many hot rods and customs), which tend to run hotter. A transmission cooler acts like a radiator, allowing the fluid to be cooled as it passes through it before being routed back into the transmission.

Today, there are numerous aftermarket transmission cooler kits available, the most popular being offered from companies like Earl’s, Hayden Auto and TCI Automotive. For optimum results, the best place to mount a transmission cooler is usually in front of the car’s main radiator so it can draw in as much air as possible. Today, many modern coolers use a sturdy plate and fin design, which means they can be both efficient and small, allowing them to be mounted inside the tall, narrow grille shells on most 1930s style hot rods, while still providing adequate cooling capacity for a beefy automatic overdrive transmission.

 

How to Get the Best Results

When choosing an automatic overdrive transmission for your hot rod or custom car, there are a number of aspects you need to consider. To help provide a better understanding of these, we spoke with Gary Bruce of TCI, whose been working on and developing automatic transmissions and parts for the better part of two decades.

“The primary reason why you want to go with an automatic overdrive in your hot rod or custom is to improve fuel economy and reduce engine life, while maintaining performance. In the hot rodding world today, an overdrive automatic often goes hand in hand with electronic fuel injection, which improves engine cold starting, throttle response and reliability. For a long time, GM’s Turbo-Hydramatic 350 was a popular choice for rod and custom cars, because it was a durable automatic transmission and used ones were widely available. However it only had three gears, with third being a 1:1, which meant it was often a serious compromise between finding the right axle ratio and maintaining a level of drivability on the street. As fuel economy became more of a concern in the late 1970s, GM developed the 700R4, which used a .70:1 overdrive fourth gear and a lock-up torque converter. This not only allowed for lower engine rpm at cruising speed, but also improved efficiency, resulting in significant gains in fuel economy. This transmission, along with its successors, the 4L60 and later 4L60E, are the most popular automatics for use in street rods today, primarily because there are so many parts and upgrades available for them. Ford’s AOD (Automatic Over Drive) and later wide ratio 4R70W are not quite as popular, but they can still be built into strong, dependable high performance units.”

But although he says that the 700R4 and 4L60 are generally good transmissions, Gary has some suggestions for rodders considering using one. “The 700R4 came out in the early 1980s, but the initial transmissions were full of problems. The valve body and case design were major issues, and the circuitry was badly configured on them, which resulted in frequent oil pump failure. GM made some major improvements to the 700R4, including a revised pump design and stronger, 30-spline input shaft instead of the 27-spline unit on early 1982-84 units, and these transmissions, particularly 1987 and up, are much more dependable. When searching for 700R4 cores to rebuild for a performance overdrive automatic, we won’t even look at anything that’s 1984 or older.”

He also suggests that you need to choose the right torque converter for your particular application, and stall speed is critical.

“The idea behind stall speed is to optimize torque multiplication just under peak torque delivery from the engine (usually between 500 and 750 rpm below it) which allows the car to move forward with the maximum amount of traction. A major problem is a lot of people don’t understand this and choose a converter with too high a stall speed. They’ll think that because a racecar or gasser uses a 3500 rpm stall converter they can use one in their street machine. The problem is that racecars are designed to make maximum torque at higher rpm, which is why you’ll see them revving their engines at the drag strip before they launch. If you put a high stall speed converter in a street car it will actually make the car slower, because the converter isn’t matched to the engine’s torque output and all it’s doing is slipping at lower rpm, absorbing heat and transferring it into the transmission, which affects fuel economy and shortens transmission life.”

And because heat can kill a transmission, Gary says you need to take special care, especially with a new or recently rebuilt automatic. “With a newly built performance automatic, there’s a break-in period of 500 miles. This is necessary to allow the bushings and clutches to seat. Once the 500 miles are up, change the fluid and transmission filter, otherwise the clutches will start to wear away, causing particles to get caught in the fluid, which will cause friction, heat and ultimately component failure.”

 

Cam ’n Stall

Getting back to torque converters, another prime factor in choosing a particular stall speed concerns engine camshaft specs. “The camshaft’s duration will affect its peak torque output, and that in turn will dictate the optimum stall speed for the transmission’s torque converter,” says TCI’s Bruce. “On many modern, fuel-injected street engines, like GM’s Tune Port Injected V-8s or Ford’s 302 V-8 with Sequential EFI, you’ll need a lower stall speed, because these engines make maximum torque at around 3000 rpm. Let’s say you’re running a 3.08:1 rear axle ratio, and you choose a cam that peaks out at 5000 rpm. Adding a converter with a stall speed of around 2400 rpm will give you a nice combination for the street, with good off-the-line acceleration and decent drivability and fuel economy. At TCI, because Comp Cams is one of our sister companies, we’ve spent a lot of time matching converters to cam specs, resulting in proven combinations, so if you want to, you can order a cam plus a converter specifically matched to it, and start putting it all together.”

But in the case of overdrives and lock-up torque converters, Gary also has a few suggestions in the interests of optimum reliability. “In a traditional three-speed automatic like the TH350, the non-lock up converter has a slippage rate of about 4-6 percent. In an overdrive unit like the 700R4, by locking the turbine and pump inside the converter, you’ve virtually eliminated that slippage, which also allows for greater torque multiplication to the wheels at a given engine speed. However, locked-up torque converters are happiest at lower rpm – i.e., when cruising in overdrive. If you apply too much torque to them directly when they’re locked up, you can burn the clutch inside the converter. So to get around the problem on a four-speed automatic overdrive, for higher horsepower engines we recommend multi-plate clutch torque converters – these are still lock up units, but better able to handle additional power than single clutch converters. That way, for the person who has a fairly high power engine but still wants good drivability, you’ve essentially got the best of both worlds.”

 

A Final Note

Hopefully, we’ve been able to provide a little understanding about overdrive transmissions and some of the options currently out there. Although both manual and automatic overdrive units have their own strengths and weaknesses, for those who intend to drive their hot rods and customs and want the most enjoyment out of them, there’s arguably no better upgrade than installing one. The cost may be significant up front, but the long-term benefits far outweigh the drawbacks – what other component can you think of that allows for rapid acceleration, quiet highway cruising and decent fuel economy, even with a big thumping V-8? Certainly not a two-speed Powerglide or old fashioned “three-on-the-tree.”

Acknowledgements: The author would like to extend a very big thank you to Gary Bruce of TCI automotive and Nate Tovey of TREMEC Transmissions for their assistance with this article.
 


Useful Sources

TCI Automotive
888-776-9824
www.tciauto.com

TREMEC Transmissions
877-4-TREMEC
www.tremec.com

Hayden Automotive
www.haydenauto.com

Hot Rod Chassis & Cycle
630-458-0808
www.hotrodchassisandcycle.com

 

 

 

 

 

There are many aspects to consider when building a hot rod, and one of the most important is the driveline, including the engine and transmission. Increasingly, many rodders are seeing the benefits of adopting modern overdrive gearboxes, both in terms of performance and efficiency.

 

In terms of manual overdrive gearboxes, one of the most popular and affordable is the Borg Warner T-5, first introduced in the early 1980s. These were built in huge numbers and today represent one of the easiest ways to install an overdrive unit in a hot rod. However, they weren’t designed to handle high torque capacities and are best reserved for cars with stock or lightly modified engines.

 

Here’s one of the vehicles originally equipped with a T-5 – a 1988 Chevrolet Camaro IROC-Z. Combined with a tuned port injected V-8, the adoption of a manual five-speed with a .67:1 overdrive gear in a car like this allowed for snappy acceleration with decent cruising refinement and respectable gas mileage, without the need to use a tall rear axle ratio. The same principles the factory engineers used when developing this car can be applied to your hot rod today.

 

TREMEC transmissions eventually took over production of the T-5, and from it developed a much stronger version aimed at high performance applications – the T3350 and this, the TKO. Able to handle up to 490 lb-ft of torque, it employs a much stronger casing and stouter input shaft, among other things. These transmissions have proven particularly popular with the street/strip crowd and make a good choice for a high horsepower/high torque, lightweight hot rod.

 

Even beefier than the standard TKO is the five-speed TKO 600, so named because of its maximum torque rating. Besides its considerable strength, this gearbox incorporates provisions for both mechanical and electronic speedometer drives, as well as eight different shift lever mounting points. This makes it adaptable to a wide variety of different vehicles, unlike some other heavy-duty manual transmissions on the market.

 

A key aspect to the strength of any manual transmission is the gears themselves. On stock T-5s, third gear was especially considered a weak link. These cogs belong to TREMEC’s new T-56 six-speed Magnum – its highest torque capacity unit ever. The gears and clutching teeth are actually formed in two parts and welded together, not only resulting in wide, strong individual cogs, but also allowing for a finer pitch on the clutch teeth, which results in relatively smooth shifting action, something that the TKO and TKO 600 aren’t generally known for.

 

In modern American performance cars like the Chevrolet Corvette, six-speed manual gearboxes offer the best blend of performance and fuel economy. The 2005 model (shown here in chassis cutaway form) uses a 6.0-liter, 400hp V-8 and TREMEC T-56 six-speed manual. A fairly short 2.66 first gear provides great off-the-line bite, while tall fifth and sixth speed overdrive gears (.74 and .50) allow for relaxed highway cruising – enabling this Corvette to achieve more than 25 mpg on the highway. Also note the rear- mounted transaxle on this particular application, designed to improve weight distribution and handling.

 

When looking at transmissions, gearing is affected considerably by the rear axle ratio that you run. Part of the beauty of an overdrive transmission is that you can employ a numerically short rear end ratio in the differential housing without compromising streetability.

 

Here’s TREMEC’s current big dog – the T-56 Magnum. It’s primarily designed for use with high performance small block Chevrolet and Ford engines and is rated at a whopping 700 lb-ft maximum torque capacity. However, it’s been designed not only as an incredibly robust six-speed overdrive manual, but one that incorporates a number of features to make it easier to use on a wider variety of vehicles, something the original T-56 didn’t do.

 

The Magnum allows provisions for both mechanical and electronic speedometer pickups, so you can bolt it in your late model Mustang or adapt it for something like a hot-rodded 1953 Ford F-100 pickup.

 

Along with multiple shifter mounts, adjustable offset short throw shifters like these from TREMEC allow drivers to place the shifter at virtually any angle for individual comfort and better reach.

 

Here, you can see the available mounting locations for shift handles on the T-56 Magnum – fore, center and rear. For hot rodders, it’s stuff like this which makes things a little easier when putting a project car together.

 

One of the most crucial components of any transmission is the input shaft. It’s the first line of defense when it comes to absorbing stresses. Many hot rodders and enthusiasts have grenaded their gearbox by not having an input shaft up to the task of handling the engine’s torque output. Here you can see a 10-spline input shaft from an original T-56 on the left and a shaft from the Magnum on the right. Note how much thicker the Magnum unit is, both the shaft itself and the ring gear. This is a major contributor to the Magnum’s overall durability.

 

Automatics have long been the most popular choice in North America, spurred on by the development of General Motors’ original Hydra-Matic. Today, descendents of that original unit, like this 700R4 four-speed auto, are very popular with hot rodders, because they provide smooth, progressive shifts and the added benefits of an overdrive fourth gear and lock-up torque converter to aide drivability and fuel economy. They’re also widely served by the performance aftermarket.

 

Compared to manual units, automatic transmissions are seen as incredibly complex units. Operation is largely controlled by fluid pressure, which operates planetary gears mounted in series. The torque converter (at the front of the transmission) is a fluid coupling which enables power to be transmitted from the engine to the drive wheels via the transmission.

 

This cutaway of a torque converter shows the main components inside: the pump, turbine, stator and one-way lock-up clutch. A crucial factor in selecting a torque converter for any performance automatic transmission is its stall speed, that is, the rpm at which it overcomes any resistance applied to it (like the car’s braking system). For most street cars, this is around 2000-2500 rpm.

 

Lock-up torque converters like this one (shown in an exploded view), became popular in the 1980s. By using a clutch inside the converter, they were able to lock the turbine and pump together at the same speed, reducing slippage and energy loss – long considered a major issue on automatic transmissions.

 

Because it relies on fluid to not only lubricate it but essentially to operate it as well, keeping temperatures cool inside an automatic is absolutely crucial. Therefore, when building a performance automatic, a transmission cooler like this one is an essential part of the package.

 

Another essential step in cooling your automatic transmission is using heavy-duty automatic transmission fluid, which is more resistant to higher temperatures and extreme use, especially if you plan to add “a valve body improvement kit.” More frequently known as a shift kit, this boosts fluid pressure through the valve body to allow for firmer, faster shifts.

 

With a little ingenuity, it’s amazing what you can achieve by installing a late-model overdrive transmission, even in a “traditional” style car like this 1932 Ford Roadster. You’d be hard pressed to tell that below that old fashioned looking shift handle is a modern gearbox.

 

For cruising long distances, modern overdrive automatics are often the way to go. Dan Bartley, who owns this super rare 1953 Meteor (a Canadian market Ford product), drives it to shows and events all over North America. Having a Windsor engine and a Ford Automatic Overdrive four-speed transmission makes such trips a little bit more pleasurable.

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