If you’ve covered some serious highway miles in your street rod or custom—on Americruise, Power Tour, or other long-distance events—and you’ve got a two- or three-speed auto trans, you know how it feels to spend hours on the road taching three grand or more. You can just imagine all those moving engine parts slowly wearing themselves out, sending small metal shavings into your oil pan, or worse. You also swear you can watch your gas gauge needle move as it falls from F to E. At times like these, you’ve probably considered swapping that slushbox for an automatic overdrive.
We’d guess, though, that the fantasy is quickly shot down by other thoughts: An overdrive automatic won’t fit in my car. It won’t handle the power my engine is making. It’s too complicated to hook up. I don’t want to mess with electronics. It’s too expensive.
Well, let’s go back to the fun part of this day-dream, the wishing-for-an-overdrive part. Let’s examine those arguments against an overdrive and see if we can shoot a few holes in your negative thinking. If we can, maybe the next time you find yourself getting your kicks on Route 66 (or 40, or 80, or 10) your engine will be happily loping along closer to 2,000 rpm and you’ll have to tap on the gas gauge to make sure the needle’s not stuck on F.
An Overdrive Will Fit, Depending
You may be surprised to learn that many of the commonly used overdrive automatics aren’t all that much bigger than comparable three-speeds. For example, a 200-4R overdrive transmission is almost exactly the same size as a TH350 with a short tailshaft. A 700-R4 is just 1/8 inch longer than a TH350 with a long tailshaft. A Ford AOD is only ¼ inch longer than a C4. Where you run into exceptions to this rule is with the heavy-duty trannies. A 4L80-E is more than 5 inches longer than a TH400; a Ford E4OD is 4 inches longer than a C6; and a Mopar 518 is 2½ inches longer than a 727. As you’re thinking of adding an overdrive, consider what you want and how to make it fit in your chassis. Be sure to carefully measure the distance between the end of the new trans case and your rear end to make sure the driveshaft you’re currently using will fit the new application.
How about mounting the OD trans in your frame? Read the fine print in most chassis manufacturers’ catalogs and you’ll find they make a number of transmission crossmembers to fit particular trans applications. Some frame makers, like So-Cal Speed Shop, offer a universal trans crossmember that’s shaped and drilled so that it can be installed to suit the transmission being used.
Mating the tranny to your engine isn’t all that tough, either. The easiest way to do it is to keep your transmission in the same family as your engine—use Turbo Hydramatic transmissions for Chevy engines, Fords for Fords, and so on—so the bolt pattern on the bellhousing will match the motor. A tip for those of you running non-Chevy GM V-8s: The bellhousing bolt pattern on some 200-4R transmissions is known as a BOP pattern, meaning it will fit Buick, Oldsmobile, and Pontiac engines. However, many 200-4R cases have a universal bellhousing pattern, so they fit Chevy applications as well as BOP.
If you have to mix engines and trannies, many transmission outfitters offer adapter plates that’ll allow you to bolt mismatched housings together. But you may face some additional complexity when hooking up the transmission to accessories like the shifter, speedometer, and so on. Like anything else with a street rod buildup, be sure to think through the details and plan, plan, plan.
An Overdrive Can Handle Power
It all depends on which transmission you choose. Each of these overdrive transmissions was originally designed to withstand a certain torque load. While tranny remanufacturers have made improvements to some of the vulnerable parts—clutch packs, planetary gearsets, input shafts—they still should operate within certain power thresholds.
Most transmission specialists refer to horsepower when talking about a transmission’s limits. What’s really going to wreak havoc with a tranny is too much torque, since that twisting force from the engine is going to shock-load the internals. A transmission is especially vulnerable during a hard launch or a firm One-Two shift, especially if there’s a huge gap in ratios between First and Second gear. Once the car is underway and headed towards peak horsepower output, there’s less risk of damage to the transmission. Yet, as one of our transmission experts put it, “Most people are more familiar with horsepower numbers than torque, so we use them.” This horsepower/torque difference should only be an issue if you’re building an engine with far more torque than horsepower. Since few of you are building tractors, the horsepower numbers should work fine.
What are those numbers? For typical remanufactured (but not extensively modified) overdrive transmissions, most of our experts agreed that the 200-4R, while a great candidate for a swap because of its compact size and universal bellhousing bolt pattern, can handle no more than 250 to 350 horsepower. The 700-R4, as long as it’s an ’88 or newer version (with stouter internals from the factory) can handle 350 to 450 horses, as can the computer-controlled 4L60-E. Ford’s AOD overdrive can handle as much power as the 700-R4, though some we spoke with bumped the AOD’s top number to 500 horses, especially if it’s used without the lockup function (see the sidebar “To Lockup or Not”).
Once you pass the 500-horse threshold, you’re in the market for heavy-duty truck-oriented transmissions like the 4L80-E on the GM side, the E4OD from the Ford camp, or a modified aftermarket unit like the 4STB from FB Performance. These transmissions are built with clutch packs, line pressures, gears, and other components that will stand up to anywhere from 600 to 850 horsepower, depending on the trans and how well it’s been beefed.
A third option is to find a well-built three-speed, like a TH400 or C6, and mount a Gear Vendors overdrive unit to it. Gear Vendors has customers that pump in excess of 1,200 horsepower through the OD units, so they should work with even the most powerful street rod applications (for more info on the Gear Vendors unit, see the sidebar “Bolt-On Overdrive”).
Okay, Some ODs Have Computers
No doubt you’ve spotted the letter E attached to some of these transmissions. Invariably, E stands for “electronic,” and that means the transmission needs a computer to control its shift points, shift quality, and torque converter lockup.
Now, for some of you, a computer is no big deal. If you’re running a late-model EFI engine in your project, you probably scavenged the donor car’s computer to run the fuel and ignition properly. In this case, you can easily retain the donor’s computer-controlled tranny as well.
But what about the rest of you? What if you’re running a thumping big-block and you’d love a 4L80-E behind it? No problem. Transmission manufacturers that offer 4L80-E kits—Jet Performance, Phoenix Transmission Products, TCI Automotive, Hughes Performance, and others—also have stand-alone computer control systems that allow you to operate the transmission behind just about any motor, even good, old-fashioned carbureted engines, thanks to sophisticated throttle-position sensors. Some manufacturers will even custom-tune the computer’s software to provide shift points and shift feel that’s tailored to your wants and driving style.
The computer does add complexity (and cost) to the transmission installation, so be sure you really need it. If you’re trying to decide between a 700-R4 and the 4L80-E’s little cousin, the 4L60-E, for example, keep in mind that they both can handle similar power loads, but the 4L60-E requires a computer while the 700-R4 doesn’t. The 60’s computer doesn’t magically make it stronger, it just makes it more complex—and about double the price of the 700.
Cost is Relative
Retail prices for overdrive transmissions vary quite a bit, depending on which OD unit you’re buying and who you’re buying it from. At the low end are the 200-4Rs and 700-R4s, which can range from $1,100 to about $1,400. At the high end, a properly setup 4L80-E, with custom-tuned software and all the right cables, will dent your wallet to the tune of $4,000 or more.
Each of the experts we spoke with offered this advice when considering transmission price: You probably paid a fair amount of money to get your engine just the way you wanted it, right? Then why scrimp on the single piece of equipment that’s going to do the most to deliver your engine’s power to the ground?
Plus, keep in mind the reason you’re considering an OD trans in the first place: Adding the overdrive gear ratio will give your rod longer legs. The engine will be turning fewer rpms at cruising speed, which reduces engine wear and increases fuel economy. So you’re spending money to save money over the long haul—or cruise, as the case may be.
