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Everything You Need to Know About Overdrives - Built-In Or Bolt-On?

Posted in How To on May 1, 2009
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It's been said that the only reason transmissions exist is because engines aren't good enough. A typical V-8 can rev up to about 6,000 rpm. If you didn't have a transmission, you'd need an engine that could spin up to three times that high in order to reach top highway speeds. Trimmed down to the basics, the function of a transmission is to take engine rpm and create the wheel rpm that a given situation requires. Low gears allow the vehicle to pull away from a dead stop, the mid-range gears allow acceleration to freeway speeds, and the high gears keep the momentum going.

In the past, what was called "high gear" was a straight-through gear with a 1:1 ratio between engine rpm and transmission rpm. No gear reduction or multiplication was involved. Back when 60 mph was considered fast, a 1:1 high gear was sufficient to move things along at highway speeds. Now, running in 1:1 cruising along at 60 mph is likely to get you high-beamed or rear-ended by the driver behind you who was previously cruising at 70. You want to go faster, but you don't want to scream your engine until it pukes its guts out. You need another gear.

Adding gears for the highway is advantageous for a couple of reasons. First, if your engine isn't spinning as fast, it won't wear out as quickly. Second, if your engine isn't spinning as fast, it will consume less fuel. In a nutshell, increased longevity and fuel economy are the reasons for getting good highway gears.

There are several options for getting that needed gear. Four come most readily to mind.

Option one: you can swap your tires for a larger size.

Option two: you can change the ring-and-pinion ratio in your axle(s).

Option three: you can swap your three-speed automatic tranny for a newer model with a built-in overdrive gear.

Option four: you can bolt a self-contained overdrive to your existing three-speed automatic transmission.

What's best? The option that's right for you. That's not a cop-out. One size does not fit all, and one solution is not the best for everyone and every vehicle. We've got our opinions, but before we let them out of the bag, we'll give you enough information to form your own. Sound good? We thought so.

For the sake of simplicity, we'll focus on automatic transmissions this time.

Option One: Tall Rubber
Most of us want a set of bigger donuts on our trucks. Most of the time, changing to bigger tires mandates a change of differential gearing to offset the effect of the larger tires. If you retain your stock differential gearing and increase your tire size, it has the same effect as keeping your current tires size and changing the differential gears to a higher ratio. Bigger tires will increase your overall gearing, meaning that every gear will be higher. If you tow heavy loads, adding bigger tires may not be the best way to go because your low gear may not be low enough to pull the bigger tires and your towing load effectively. On the flip side, if you have a 4x4 and your transfer case has a respectable low-range ratio (about 2.6 to 1 or lower) then you'll still have the ability to crawl through tight trails with your big tires and stock differential gearing.

How about a hypothetical situation? Say you have a three-speed automatic, 4.10 differential gears, and 33-inch tires. According to the calculators on the Randy's Ring and Pinion website ( your engine rpm at 60 mph will be 2,572 rpm. Now try a set of 35's on for size. Your engine rpm at 60 mph will drop to 2,425 rpm.

Option Two: Tall Gears
Taller tires may not be the best way to go for you. Taller tires may require a suspension or body lift, or fender trimming or replacement fiberglass fenders with larger wheel openings. If you're happy with your current tire size, you might be better off changing your differential gearing. Keep in mind that just like going with taller tires, changing your differential gearing will affect your low gear as well as your high gear. Again, if your low gear is low enough, or if your low range in your transfer case is low enough, then swapping differential gears won't produce any drawbacks.

Let's go back to our hypothetical situation. We'll start with a three-speed automatic, 4.10 differential gears, and 33-inch tires, which will produce 2,572 engine rpm at 60 mph. If we change our differential gears to 3.73, we'll be cruising along at 60 with the engine turning at 2,340 rpm.

Option Three: Swap In an O.D. Transmission
Unbolting your three-speed automatic and putting an automatic overdrive transmission behind your engine is fairly straightforward. Many times, the block-to-bellhousing bolt patterns are the same.

Not all automatic overdrive transmissions are created equal.

The GM TH-700R4 and the Ford AOD automatic overdrive transmissions are two of the first popular overdrive automatics. When first introduced, the TH-700R4 quickly gained a reputation for being unreliable. The 700R4 was upgraded at the factory in 1987 to eliminate the early problems that plagued it. In the same vein, the Ford AOD had its own set of problems. These two transmissions can be upgraded with aftermarket internal parts to help them perform better and live longer. These transmissions use non-lockup torque convertors, and have built-in overdrive gears. Third gear is the 1:1 gear also found in three-speed non-overdrive automatics. Fourth gear is the overdrive gear (usually a 0.78:1).

The latest generation of overdrive automatics from GM and Ford include the GM 4L80-E, the GM 4L60-E, and the Ford 4R100, which is also called the E4OD. These automatics are technologically advanced and benefit from electronically controlled shifting points and lockup torque convertors. Check out the sidebar "What's in a Torque Convertor?" for more details

Modern overdrive automatics are close to the same lengths as their older three-speed counterparts. Swapping in an overdrive automatic will not significantly change your drivetrain length. Drivetrain length is a non-issue in full-size trucks, but it's a huge stumbling block in short wheelbase vehicles like Jeep CJ's and Wranglers.

Electronically-controlled overdrive automatics come from the factory with their control modules integrated with the engine computer, but available aftermarket stand-alone controllers let these new-style automatics be installed behind carbureted or stand-alone EFI engine control systems.

Option Four: Keep Your Three-speed Automatic and Add a Bolt-on Overdrive
If you like your three-speed enough to keep it, there's a way to get the extra gear you need for the highway. Gear Vendors makes a bolt-on overdrive unit that can be adapted to most American automatics, 2WD, or 4WD.

The Gear Vendors overdrive is a self-contained unit that's capable of handling 2,000 hp (yes, that's two thousand horsepower) and tow up to 25,000 pounds. The Gear Vendors overdrive unit provides a 22-percent overdrive and is actuated by an electronically-controlled solenoid. You can engage the overdrive at any time for true shift-on-the-fly capability.

Recently, we shot an F-150 Supercrew prerunner. This particular Supercrew is infused with long-travel suspension, a full roll cage, a mean high-horsepower V-8, and a C-6 three-speed automatic. Behind the C-6, there's a Gear Vendors overdrive. The owner loves to play in the sands of Glamis, where the Gear Vendors overdrive lets him use the optimal engine rpm for a given situation. We've got a feature of this truck in the works. In the mean time, trust us that the Supercrew launches off of the dunes in commanding style.

The Gear Vendors overdrive can be added to the back of the transfer case in 4x4's for use in 2WD only, or placed in between the tranny and transfer case for gear splitting capabilities in 2WD, 4-Hi or 4-Lo. When the Gear Vendors overdrive is added to the back of the transfer case, the electronic control will disengage the overdrive when the transfer case is shifted into 4WD.

Even though we're concentrating on automatic transmissions this time, it's noteworthy that the Gear Vendors overdrive can be added to most heavy-duty American four-speed manual transmissions. These old-school four-speeds are extremely strong. In fact, the old-school four-speeds are stronger than most of the new-generation five-speeds. With this in mind, perhaps the ultimate manual transmission would be a heavy-duty four-speed with a Gear Vendors overdrive. Since the overdrive can be engaged in any gear, a four-speed manual suddenly becomes an eight-speed. Cool!

If there's a single disadvantage to adding an overdrive to a three-speed automatic, it's that the three-speed auto will still have to use a non-lockup torque convertor. The extra heat can be dealt with via a large tranny cooler or two, but the non-lockup convertor will always slip. There's no way around it.

Drivetrain length can also be a concern, as the Gear Vendors unit can add several inches to the back of the transmission. The extra length is not a problem if you've got a full-size pickup truck even if it's a short bed standard-cab, but if you're working on a short wheelbase rig like a Jeep Wrangler or even a Chevy Blazer, you'd best do some measuring before taking the plunge. Gear Vendors has a multitude of adapters available to accommodate different needs.

It's also noteworthy that you can add a Gear Vendors unit to most new-school automatic overdrive transmissions. In that case you'll have an over-overdrive.

What's the best option?
We dig the new-school automatics with the lockup torque convertors, and if our vehicle came equipped with one, we'd beef it up and stay with it. If we had a stout motor under the hood and a Turbo 400 or a C-6 to back up the motor, we'd happily bolt up a Gear Vendors overdrive and tow or `wheel our little hearts out. If we were building a prerunner or sand toy, we'd go for the simplicity of the three-speed auto with a Gear Vendors overdrive. We always add larger tires, and we usually mess with our differential gearing. It's just the way we are. What should you do for your situation? Your homework. We're still leaving this up to you.

What's in a Torque Convertor?
They look like giant metallic donuts, but torque convertors actually have moving parts inside. A torque convertor is a fluid coupler that connects the engine's output with the transmission's input. One half of the torque convertor connects to the engine's crankshaft. This is called the impeller. The other half connects to the transmission's input shaft. This is called the turbine. A device called a stator rests between the impeller and the turbine. All three components, impeller, turbine, and stator, resemble fans because they have multiple blades built in. Automatic transmission fluid fills the torque convertor. When the impeller spins, it forces fluid against the stator and the turbine. At a critical rpm, called the stall speed, the force of the fluid causes the turbine to start spinning. Although the impeller and the turbine spin close to the same speed, there will always be some slippage because they cannot mechanically lock together. The slippage creates heat. Heat is the biggest enemy of the automatic transmission. This is where a lockup torque convertor becomes significant.

A lockup torque convertor has all the same components of a standard torque convertor, impeller, stator, turbine, but adds a lockup mechanism to the mix. The lockup mechanism hard-couples the torque convertor into a single unit where there is no slippage. The lockup mechanism has several benefits. The lockup is electronically controlled and can be engaged and disengaged at optimum rpm. This optimum rpm can be adjusted by changing the programming in the transmission computer module. This means easy adjustability. Next, when the torque convertor is locked up, the heat that would be generated by slippage is eliminated. The transmission runs cooler and lasts longer.

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Advance Adapters
Paso Robles, CA 93446
Orange County Transmissions
Costa Mesa, CA 92626
Gear Vendors
El Cajon, CA 92020

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