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Maximum Overdrive

Maximum Overdrive

Just as the Chevy 350 small-block has been the de-facto engine of choice for street rodders during the last 30 years, so too has been the transmission hooked to it – the GM Turbo 350. Increasingly, rodders are turning to later-model overdrive transmissions to provide better fuel economy and lower engine speed on the highway – an attribute that also supports fuel efficiency, enables a higher cruising speed and generally lowers the noise level in the passenger compartment.

In some cases, an overdrive transmission can lower highway rpm by 1,500 revs or more and deliver approximately 10-20 percent better fuel economy. That’s a smart option to consider for street rods that are driven frequently or over long distances.

Of course, incorporating overdrive capability is nothing new in the street rodding world. It’s been done for years with add-on overdrive units, such as those available from Gear Vendors, as well as the use of production transmissions like GM’s 200-4R and later 700-R4. None of those options were electronically controlled, which allows for greater control of shift speed and overall performance.

Within the last few years, GM’s family of production-based, electronically controlled transmissions – known as the 4L60, 4L80, 6L80, etc. – have filtered into more and more rods. For the most part, they are based on older GM automatic transmissions, like the classic Turbo 400 and 700-R4, but updated with electronic controls to work with the computer-controlled powertrain systems of modern vehicles. The street rod aftermarket industry has embraced these smooth-shifting, efficient transmissions with stand-alone controllers that allow them to be used with non-computer-controlled vehicles. In fact, GM Performance Parts offers a controller, too (part number 12497316).

If there’s a downside to these production transmissions, it’s their limited torque capacity. Simply put, they’re just not designed to handle the output of high-performance engines. And when an engine overpowers an automatic transmission, the clutches start to burn, slip and generally cause expensive problems.

A street rod with a mild to relatively high-power V-8 engine should work well with a production transmission if the car isn’t raced or often subjected to the engine’s full torque load. In other words, your basic crate engine powering your weekend cruiser should be just fine. But if you’re running a big-torque stroker engine, a supercharger or another combination that’s making more than 400-500 horsepower, a production transmission probably won’t give you the long-term durability required to match the engine’s power.

Some specialty transmission shops offer upgrade kits and rebuilds for GM’s electronically controlled transmissions, but GM Performance Parts has recently introduced a line of “Supermatic” transmissions that offer the strength and durability of a performance-minded rebuild, but in a brand-new package that was tested and validated like any other GM production part.

The first Supermatic transmission was introduced at the SEMA show in 2008. It was the Supermatic 4L85-E (P/N 19154550), and it was developed for one thing: handling big, big power from the big-block engine family.

“The Supermatic 4L85-E is based on the GM Hydra-Matic 4L80, and was designed specifically for GMPP’s powerful big-block family of engines,” says Dr. Jamie Meyer, product marketing manager for GM Performance Parts. “It was meticulously tested and validated using both a dynamometer and real-world, in-car testing. In fact, our mighty ZZ572/720 crate engine put this transmission through its paces on the drag strip during testing, so we’re confident it will stand up to just about anything on the street or strip.”

While the Supermatic 4L85-E handles the highest-performing big-block engines, GMPP just announced the Supermatic 4L70-E transmission, too. It doesn’t have quite the torque rating of the 4L85-E, but is smaller and better-suited to less radical, street-based high-performance engines.

“It still has all the heavy-duty components and engineering in it, but the Supermatic 4L70-E is designed for milder applications,” says Dr. Meyer.

The Supermatic 4L70-E is based on the popular 4L60-E production transmission – itself an admirably strong, electronically-controlled transmission that is suitable for street rod applications. For small-block engines and rods using moderately-powered, late-model LS engines, it is an excellent choice that delivers a good balance between performance and fuel economy.

Inside the transmission case

Although the names 4L85, 4L65 and 6L80 may be unfamiliar to many rodders, the transmissions’ designs should be very recognizable. They’re largely based on previous GM Hydra-Matic designs.

Below is a quick scorecard on GM’s overdrive transmissions, their origins and application notes:

200-4R – It was introduced in 1981 and used on a variety of 1980s performance cars, including the Buick Grand National. The 200-4R (also known as TH-200) is a four-speed with the same length and overall size as the old Turbo-Hydra-Matic 350 three-speed automatic. Basically, where a TH-350 fits, a 200-4R fits. Some prefer the less-abrupt takeoff feel with its 2.74 first gear over the 700-R4’s lower, 3.06 gear. It’s a non-electronic transmission, but requires precise setup of the throttle valve (TV) cable between it and the carburetor’s throttle linkage.

Note: Swapping the 200-4R into a vehicle previously fitted with the ubiquitous Turbo 400 may require a dual-pattern flexplate, because the 200-4R uses a torque converter with the same bolt pattern as the Turbo 350.

700-R4 – The 700-R4 (also known as TH-700) debuted in 1982 and was a clean-sheet transmission design, whereas the 200-4R had its roots in the Turbo 350. It was a lockup-style transmission. Unfortunately, early models were pretty weak and prone to failure. It took GM several years to refine and strengthen the 700-R4, and they replaced plenty in customer vehicles. Like the 200-4R, the 700-R4 is non-electronic and requires precise “tuning” of the TV cable. When properly built, the 700-R4 is a strong, durable overdrive transmission.

4L60-E – The 4L60-E arrived in GM trucks starting in 1993. It is essentially an electronically-controlled version of the 700-R4, with a different name that did much to mitigate the bad feelings customers had about those earlier transmissions. Over the years, the 4L60-E has seen numerous refinements that have bolstered its strength, smoothness and kick-down performance, including improved lockup function. Stronger versions carried 4L65-E and 4L70-E names, but they were, basically, upgraded versions of the 4L60-E. GM Performance Parts’ new Supermatic 4L70-E transmission is the strongest of the lot. Compared to the 4L80-E (see below), the comparatively compact 4L60-E is the easiest electronically-controlled transmission to package in a street rod, but that relative ease comes with a lower maximum torque capacity.

4L80-E – In a very real way, the 4L80-E is merely an electronically controlled overdrive version of the original Turbo 400 three-speed transmission that was introduced in 1964. The Turbo 400 has been a popular complement to high-horsepower engines, because it has a very high torque capacity rating – 450 lb.-ft. or more. The 4L80-E offers tremendous torque capacity, too, but like its three-speed forerunner, it’s big and it’s heavy. That can mean installation and vehicle balance issues in cars like T-buckets and 1920s-30s rods. But if you’re running a big-inch engine and your street rod is at home on the drag strip as much as a cruise night parking spot, the 4L80-E is probably the late-model transmission for you.

6L80-E – The newest GM overdrive transmissions have six gears, which deliver comparatively great fuel economy for a variety of the company’s full-size trucks and SUVs. Adapting one to your street rod is possible, particularly if you’re running a late-model LS engine, but if you thought the 4L80-E/Turbo 400 was big, bulky and heavy, be prepared to pick your jaw off the floor when you lay eyes on the 6L80-E. It’s huge. But it’s also surprisingly efficient; and with a low, 2.06 first gear, it enable’s neck-snapping takeoff. Its size makes it pretty impractical for most pre-1949 cars – and even then, it’ll be a squeeze. If you really want the fuel economy benefits of a six-speed in your street rod, adapting a later-model six-speed manual from, say, a 1993-2002 Camaro/Firebird is probably a better idea. For an automatic, stick with the 4L60-E or 4L80-E.

Lockup and TV cable basics

Whether electronically controlled or not, a common feature among all GM overdrive transmissions is torque converter lockup, which couples the transmission’s output shaft with the engine in high gear to maximize fuel economy. It’s a smart design, but maximum oil pressure within the transmission is only achieved when lockup occurs. When used with higher-powered engine combinations, the transmission must be built to accommodate the greater load when it’s not in lockup mode.

The other quirk with 200-4R and 700-R4 overdrives that enthusiasts must contend with is the throttle valve cable. If it is not connected and/or adjusted properly, the transmission won’t shift correctly, leading to quick – and possibly terminal – damage. There are a variety of aftermarket brackets and components that help set up the TV cable correctly with the throttle linkage, but the installer must follow the instructions precisely. There’s no room for flexibility with the cable.

One of the benefits of using an electronically-controlled transmission like the 4L60-E or 4L80-E is that its computer handles the equivalent of the TV cable internally. As long as the transmission controller is connected properly, there should be no problems.

Speedometer converter

Often forgotten in the swap to an electronically-controlled transmission is the effect on the speedometer. GM’s electronically-controlled transmissions also signal the speedometer electronically, rather than with a conventional mechanical cable. Unless your street rod has the electronically-actuated gauges from a late-model car blended into its dashboard, an aftermarket converter is required to translate the electronic signal from the trans to the mechanical cable of the gauge.

Arkansas-based Abbott Enterprises has a popular “Cable X” converter, which is a three-wire installation that sandwiches between the transmission’s vehicle speed sensor and the speedometer. Another, more expensive and more time-consuming alternative is installing conventional speedometer drive gears in the transmission, but the converter box is the recommended method.

For more information:

Gear Vendors

GM Performance Parts

Abbott Enterprises

GM’s Hydra-Matic electronically controlled overdrive transmissions are excellent choices for street rods intended for plenty of road miles. The overdriven top gear significantly reduces engine rpm on the highway, delivering greater fuel economy and higher cruising speed, less load on the engine (which promotes longer engine life) and reduced engine noise. Shown here are the 4L80 series (foreground) and the 4L60 series (background).

The big daddy of GM’s overdrive transmissions is the new Supermatic 4L85-E. It’s a beefed-up version of the production 4L80 transmission that’s used in full-size trucks and SUVs. It is rated to 720 horsepower and 685 lb.-ft. of torque. While it was designed specifically to be used with a big-block engine, the high torque capacity makes it a perfect complement to high-power LS and small-block engines.

Like all versions of the 4L80 series, the Supermatic 4L85-E is based on the old Turbo 400 three-speed automatic from the 1970s. It’s a large, heavy transmission, but one that has proven very strong and durable.

The bell housing on the late-model 4L80/4L85-series transmissions may not match older small- and big-block engines without an adapter kit. The installer should consult GM Performance Parts or a knowledgeable aftermarket transmission company before attempting to marry the engine and transmission.

GM Performance Parts supplies a torque converter with its transmissions, but it should be considered a “placeholder” converter, as it typically has an 1,800-2,000 stall speed that may not be the best-suited to the engine. Again, consultation with an expert regarding the engine, driveline components and other details dictate the best converter for the combination. Selecting the correct converter will have a huge impact on the performance feel and driving experience of the car.

Smaller and, ultimately, with a lower maximum torque capacity than the 4L80 series is the Hydra-Matic 4L60 series four-speed. It is based on the 700-R4 non-electronic overdrive transmission from the 1980s, but significantly improved and strengthened over the years. The strongest, with the greatest torque capacity, is GM Performance Parts’ new Supermatic 4L70-E.

An inside look at a performance-strengthened 4L60/4L70-series transmission reveals a number of enhanced components, including a heat-treated stator shaft spline, an induction-hardened turbine shaft, a heavier-duty low/reverse roller clutch, an additional friction plate for the 3-4 clutch and a shot-peened output shaft.

Another important design factor of the stronger 4L60/4L70 series transmission is the use of five-pinion gears for the input and reaction gear sets. That spreads the torque load, so that less of the load is held by a single gear.

This side-by-side comparison shows the 4L80 series (left) and the 4L60 series. At a glance, they appear similarly sized, but the 4L80 is considerably larger in the center “barrel” section, while the 4L60 has a longer tail shaft. The 4L80 is also much heavier than the 4L60 series.

In this side shot, the larger barrel section of the 4L80 trans (foreground) is very apparent. Its bulk can make fitting it in older street rods a problem, particularly those built before 1949, where interior floor/transmission tunnel space is limited.

Larger cars from the 1950s are better-suited to adapting the 4L80-series transmission. Besides having more room to accommodate it, the chassis layout better spreads the weight of the transmission rearward. On earlier cars, with small and lightweight frames, a 4L80 transmission adds a considerable amount of weight on the front half of the chassis.

Because tail shaft lengths can vary among production applications, the important measurements for sizing up a transmission to install in your car is the length from the bell housing to the point where the transmission case meets the tail shaft. For 700-R4/4L60-series transmissions built through 1995, that length is 23-3/8 inches; for 1996-later electronically controlled versions, the length is 21-3/4 inches. The 4L80 series is 26 inches between the bell housing and tail shaft mounting flange.

The slimmer profile and lower weight of the 4L60-series transmission makes it a better choice for early street rods, where interior space and the effects of mass have a larger impact on the overall balance and comfort of the vehicle.

The tail shaft section varies with different production applications, but doesn’t really impact the overall “installability” of the transmission. The shifter location isn’t affected by the tail shaft length, so the only major consideration is driveshaft length – and for most rods, there’s no such thing as a standard driveshaft anyway. Mount the transmission and rear axle to measure the length required for the driveshaft.

An electronically controlled transmission won’t function without its complementing controller. GM Performance Parts offers one that works on both the 4L60/4L70-series and 4L80 series transmissions. The part number is 12497316.

Because the computer-controlled electronic transmission is designed for electronically operated instruments, the conventional, cable-driven speedometers of early cars aren’t compatible without a converter of some type. Several are available from aftermarket companies, such as Abbott Enterprises.

With a modern overdrive transmission behind your street rod’s engine, you get the best of both worlds: classic styling and modern drivability.

Published Dec 7th, 2015

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