What Makes For a Good Hot Rod Overdrive And How To Install One

Hot rods are all about personality. Whether it’s a classic small block Chevy, a 302 Ford or the ubiquitous Nailhead Buick, half of the personality in any hot rod is the engine; but what about the transmission? Meh, whatever. The transmission rarely gets much more than a single thought: “stick or auto.” Yet, how that engine puts the power to the ground is just as important than the engine itself, and when done right, can add to that carefully constructed personality.

For the purpose of this story, we are going to throw out the whole stick-shift thing; this story is all about high-performance overdrive automatics. Modern automatic transmissions go far beyond the old slushbox technology inside the typical TH350 or C4 you find in most hot rods. With great power comes great responsibility…oh, wait, wrong story.

We talked to some of the best in the business when it comes to high-performance overdrive transmissions; Gearstar Performance Transmissions, ATI, and TCI. Knowing that these guys have more overdrive gearboxes running around some seriously heavy-hitting powerplants, we knew these were the guys to hit up.

Gearstar uses top-shelf components in their performance transmissions. Red oxide clutches, performance steels and billet servos make a big difference in the longevity and power handling of a transmission.

Avoiding Parasitic Loss

It is a well-known fact that transmissions eat up power; it is just a fact of gearhead life. The less efficient a transmission is, the more you have to put into the engine to make up for it. This can be a real drag (pun intended). Take the GM TH400 for instance, considered the strongest 3-speed GM auto, it’s used behind countless big-blocks in everything from land-barge Buicks to trucks.

The clutches are separated by steel rings, called “steels.” This group represents 3rd and 4th gear in this 4L60E.

The problem is that it eats up so much power because of those heavy duty internals. In terms of 1960s technology, it was great, but 40 years later, it is far cry from “cutting edge.” Modern transmissions, such as the GM 4L80E have the same or better power handling capability with far less parasitic loss. Plus you get that ever important extra gear.

Upgrading from a 3-speed auto to an overdrive can transform any hot rod from a cool little street cruiser into an exhilarating speed demon, without losing any of its character. Not only do you gain more than a small amount of fuel economy (which means you can afford to drive it more), it will be faster, quicker off the line, and more reliable.

A typical 3-speed auto has a final drive ratio of 1:1. Let’s break that down for a minute. Take a typical small block Chevy with a 27.5” tall rear tires, 3:73 rear gears and 1:1 3rd gear TH350 auto.  At 2500 RPM, the vehicle will travel roughly 55 MPH, that little small-block is going to be churning 3500 in order to cruise the highway at 75. Drop in a highway-friendly .75 4th-gear overdrive, and that cruising speed RPM drops all the way down to 2500, that is a substantial difference.

With an overdrive, you can keep your tire-shredding deep gears AND get low-RPM top speed cruising performance at the same time. Add in a lock up torque convertor, and you can assume an additional 500-RPM drop. That means you can cruise the highway at 75 miles an hour while only spinning the motor at a lazy 2,000 RPM.

This shows the differences between a stock servo and a Sonnax Super Servo, which applies holding power for fourth gear. The Sonnax Super Servo is used in most Gearstar transmissions. Patrick, one of the Gearstar techs applies the torque on the pump drive assembly on a 4L60E.

This GM 2004R has been treated to a TCI constant-pressure valve body to eliminate the risks of damaging the transmission with an errant TV cable. With this, the TV cable is essentially turned into a kick-down cable.

The Difference From Slop To Slammin’

We spent some time with Patrick, a Master Builder for Gearstar, talking about what to look for in a performance overdrive. “Each Gearstar transmission is fully remanufactured by one Master Tech Builder,” he told us. “From tear down to prep and even selecting the parts that go in each Gearstar transmission.”

In order to get quality and performance out of the transmission, you need quality parts. Gearstar uses only the best for their transmissions.“We use Koline steels, Torrington bearings, Raybesos Stage1 clutches and Sonnax hydraulics to ensure the customer ends up with a transmission that will last and perform” Patrick said.

The TCI 6X 6-speed overdrive is based on a GM 4L80E, but that only serves on market. By shaving off the GM bellhousing and mounting custom bellhousings from Reid, the 6X fits just about any major GM, Ford, or Mopar engine.

The biggest problem with modern overdrives is the reliability. “If the car ever gets to a racetrack and hooks, it will break something.  The biggest issue with 700, 4L65E style trans is 3.08 low gear is a big shock when it goes to second gear ratio of 1.65. They are a weak trans and have trouble handling much power without expensive builds,” said JC Beattie of ATI.

“You need to put the right components inside the transmission to keep them together. You can’t just stab in a stock overdrive, load it with 600 horsepower and expect it to hold, it just won’t happen.”

But why stop there? The performance aftermarket certainly is not content to stay with stock configurations. Oh, no, not even close. The aftermarket knows a winning idea when they think of one. If four gears are good, why not six?

TCI released their 6X transmission about a year ago, and it is pretty dang cool. Just about the same time the OEMS were bringing out stock 6- and 7-speed transmissions, TCI was right there, but they did it with a universal platform. The wizards at TCI took the heavy-duty 4-speed 4L80E case, sawed off the bellhousing and stuffed it full of awesome high performance clutches, billet servos, top-quality steels and, oh yeah, six gears.

Scott Miller of TCI told us, “The TCI 6x Six-Speed Automatic Transmission is a fully programmable transmission that gives you six forward gears for quicker acceleration and increased fuel economy at cruising speeds and is capable of handling up to 850 horsepower.” That means it can handle just about anything under the typical hot rod hood.

More gears mean a larger case. Most hot rods need a little massage work to get an overdrive automatic to fit. We used a plasma torch to open up the tunnel on this ’51 Ford Deluxe to fit a Ford 4R70W. You can see where most of the fitment issues are the connections for the electronics.

The gearing is set at 2.97, 2.23, 1.57, 1.18, 1.00, and 0.75. Close ratios mean less RPM drop between shifts, keeping the engine in the power band for better performance on the track, whether it is straight or twisty. And because it is based on the 4L80E, it is stout, capable of handling big power.

The Ford 4R70W has a lot of potential, and it fits behind just about any Ford 5.0, 4.6 or 5.4 V8 engine. This swap mount is made by JBA headers and fits early Mustangs.

You don’t have a Chevy engine? No problem. TCI literally cuts off the bellhousing and adds an adapter ring that couples to new bellhousings that fit all the popular GM, Ford, and Chrysler bolt patterns. If you have an oddball, you can buy it sans bellhousing and make your own. It doesn’t get much cooler than that.

Debunking Old Myths And Fixing Old Habits

One thing that has kept so many hot rodders out of the overdrive business is the TV cable. When GM and Ford released their factory overdrive automatics (2004R, 700R4 for GM and the Ford AOD), they required a throttle-valve cable connected to the carburetor or throttle body. If this key component was not set up correctly or God forbid left off altogether, the transmission would literally burn up within just a few miles.

The valve body inside the transmission uses the TV cable input to control the hydraulic pressure inside the transmission. Not enough pressure and the clutches slip. The overdrive is always the first to go. This can be remedied with constant pressure valve bodies and TV cable geometry calibrators, but it is a hassle in a swap situation.

The aftermarket knows a winning idea when they think of one. If four gears are good, why not six?

Many aftermarket carbs don’t even have a provision for a TV cable connection, hence the geometry calibrators. Modern overdrives are different, they are electronically controlled. Gearstar recommends using one the geometry corrector kits from Sonnax Industries.

At first look, electronic controls may seem more of the same, just with wires, but it isn’t. The aftermarket tranny controller is quite simple to hook up, usually just a few wires and you are ready to go. What you lose in brackets and cables and weird little adjustments, you gain in computerized tuning. Everything is controlled by the computer; shift timing, shift firmness, convertor lock up, everything you need to tune your transmission is right there one click away. Then there are paddle shifters.

If you get lucky, there are some aftermarket solutions for transmission swaps. This kit is built by Hanksville Hotrods in Littleton, Colorado to swap Ford 4.6 engines and 4R70W transmissions into 49-51 Shoebox Fords.

The rest of the hot rodders out there have to make mounts to fit late model trannys. This is a ’62 Mercury Comet wagon, ready for a TCI 6X behind a Ford 347 stroker. Careful measurements are the key. We mocked up a crossmember from 1x2 steel tubing, set it in place, just in front of the mount and measured for the attachment point. Next, we made a boxed mounting plate for the transmission mounting stud. The mount is slotted to allow for final adjustment.

Just like the Corvette, Ferraris, and half of the new performance cars on the market, your hot rod can have paddle shifters. That isn’t going to appeal to everyone, but the beauty of it is that you don’t have to have big paddles hanging off the steering column. You can set it up with a couple of buttons on the dash, console or whatever. Cool factor abounds in an old Mopar with push-button shifting; this is the same thing, just new and improved.

The final installation, with the painted crossmember mounted to the car. By design, the crossmember can be shimmed up or down to ensure the optimum transmission down angle.

Tools of The Trade

So now that you are sold on the benefits of modern overdrive transmissions, you want to bolt one into your sled. OK, let’s do it. To get started, you are going to need a few tools. Depending on the chassis you are working with, some will be easier than others. The older it gets, the more you are probably going to have to modify.

First on our list is a welder. Whether its a MIG or TIG welder, it doesn’t really matter as long you can make two pieces of metal stick together without falling apart.

Chances are you will need to weld something, specifically the trans mount and/or the floor you had to cut out. You don’t NEED a plasma torch, but having one on this type of job is so much easier and faster and anything else, especially cutting out the floor if necessary.

To ensure the tubing does not crush under torque and torsional stress, we added some round tubing where the bolts mount the crossmember to the frame. The bolts slide through and you can’t crush the crossmember. The round tubing was welded in place.

If you haven’t guessed by now, you may have to modify the floor pan in order to fit a late model overdrive transmission. Once the floor is cut out and the transmission in, you can make a simple hand-bent panel or get really trick, that is up to you. Welding the floor is nice, but you can also use sheet metal screws and seam sealer. Many factory trucks (and all full-size vans too) came like this from the factory. If the tranny hump is removable, you have easy access to the trans.

Sometimes, you have to make adapter brackets to secure the shift cables. This is for a 4R70W.

Other than these tools you really don’t need anything special. This is just a transmission swap, only slightly more complicated. Air tools will make it go faster.

The transmission itself is only part of the equation. You will need some accessories to go along with it. There are several options when it comes to shifters, depending on your vehicle and the transmission you choose. Most 4-speed autos can be shifted with a stock column-shift, the only thing you need is a pointer. Floor shifters get a little more complicated. Early GM muscle car shifters have detent plates that can be removed and swapped out or modified.

Shiftworks.com makes conversion kits for 1964-81 GM console-shifted cars, which work quite well. Aftermarket shifters are certainly easy to find; Lokar, TCI, B&M, there are all different flavors to suit your needs. Keep in mind that if you go the push-button or paddle-shift route, you will still need a mechanical shifter to put the car in park, reverse, neutral, and drive, the buttons only control the forward gears.

Should you decide to go with a non-computer controlled overdrive, such as a 2004R, 700R4 or Ford AOD, you will need a TV cable adapter kit like this one from Bowtie Overdrives. The geometry corrector provides the optimum angle for the TV cable and it allows for tuning the shift points, an important aspect of proper TV cable function.

If you need a new driveshaft, Dynotech Engineering is the premier manufacturer of custom driveshafts in the US. They make over 90% of all the shafts used in NASCAR. Each shaft is balanced using a computerized balancer. The end result is this 4” aluminum driveshaft. What type and size of shaft you need depends on the length, power potential and use of the car.

Thankfully there is not a lot of plumbing involved with an automatic transmission, but what plumbing there is needs to be done well. Just like the engine, the transmission has a certain heat range it likes to run at. You need to have the following- a quality cooler (either internal (radiator) or externally mounted), quality lines connecting the transmission to the cooler. AN fittings along with braided lines always look great and do the trick. You could bend up some custom hard lines as well.

New overdrives are computer controlled. Unless you want to deal with the factory computer, you will need an aftermarket computer like this one from TCI. The TCU shown here can control any transmission; it just needs the right connectors and software.

Before your swap is complete, it bears nothing that the driveshaft can be a bit of a pickle in these kinds of swaps. There are several factors that come into play here—spline count and driveshaft length are the main issues here. It is simple enough to swap out a slip yoke for a driveshaft of the correct length, but shortening a driveshaft takes a little more effort.

In high-performance applications, a custom driveshaft will yield better drivability and last longer to boot. Companies such as Dynotech Engineering can build any driveshaft for any car from steel, aluminum, even carbon fiber, all to spec for your measurements.

Pay attention to the driveline angle, which is the most important aspect of a custom transmission. The transmission needs to run at a slight downward (towards the rear of the car) angle between 1 and 5 degrees, 2-3 being optimum. This helps achieve the proper driveline angle, reducing vibration and harmonics.

Here’s another final tip, if you need to fabricate a transmission crossmember, make sure it is removable and adjustable. Don’t just weld it in place. Adding slotted holes for front-rear alignment as well as the ability to shim the tail mount will ensure you can fine the tune the installation. If you get the angle too shallow or too deep and have no way to adjust it, you will be sorry.

Shifting a 4-speed (or more) with a stock 3-speed just isn’t going to work. There are two options- replace it with an aftermarket shifter or modify it. This is a conversion kit from Shiftworks for GM horseshoe shifter. The main part of the kit is the detent plate. The new one (silver) provides the correct stops where you need them. The new detent simply bolts in place of the original. If you want to get real trick, you can go for this set-up from TCI. This is the shift and control package for the TCI 6X transmission in the Comet wagon. The Outlaw ratchet shifter features two buttons for quick shifts on the track, or you can keep your hands on the wheel and still have control over the shifts with the paddle shifters that mount between the steering wheel and the column.

Installing an overdrive in your rod is a fairly standard process, but there are some tips and tricks that make it easier.

Choosing Overdrives By Horsepower

We spoke to all three tranny builders, Gearstar, ATI, and TCI about what they suggest for three common power ranges. To keep it simple and not brand-specific, we are talking about generic power ranges and the comparable build levels from each manufacturer.

When throwing over 600-horsepower at an automatic overdrive transmission, only the best and strongest materials will hold up to the torture of hard launches as well as street driving.

In our experience, having a local builder gear up your transmission is a real shot in the dark. Most tranny shops are not performance builders, even though they may claim it, they are overhaulers that make grandma’s Lincoln shift again.

Even when you do find a good one, they can be inconsistent and often don’t warranty performance transmission work, so buying from a performance specialty manufacturer has its benefits.

Up to 400HP

This is where most hot rods live. As much as any of us love to daydream that the junkyard 350 we pulled out of a smashed ’74 G10 van is pumping out 600 ponies with that stock Quadra-Jet, in reality it is wheezing to make 275.

Unlike most performance option, the transmission is not going to hamper your performance if it is built for more power than you can make (i.e. too much carb or too much cam), but it will hamper your pocketbook, so buy accordingly.

  • TCI suggests their “Street Rodder” package for this power level, specifically the 700R4 for GM applications. This transmission is designed with the typical rodder in mind, hence the name. You get an overdrive trans that will last years under the floor boards without beating your budget with a 24k gold-plated hammer. Of course if you had one of those you wouldn’t be concerned with prices…
  • ATI maintains that their 700R4 is the optimum transmission for an sub-3500 lb car, and it does require a computer.
  • Gearstar classifies all of their performance builds in levels. Level 1 is basically stock, Level 2 is typically good to about 450 hp, depending on the transmission itself.

Not all overdrive transmissions are created equal. Depending on how much power you plant to throw it may drastically change which O/D transmission you choose.

400-600HP

Stepping up to the plate on the power plan, 401-600 hp, the need for better internal components such as high-friction clutches and steels, modified hydraulic circuits for more pressure and holding power becomes paramount to the performance of the transmission.

  • TCI offers the “Street Fighter” for up to 775 hp engines. The Street Fighter trannys comes with a manual/automatic valve body, meaning you can drive it in full auto mode for cruising and then go to full manual mode for stoplight to stoplight action. The valve body is also of the “constant-pressure” variety, which means it delivers the full pressure of the pump at all times, unlike the OE unit that varies the pressure by RPM and load. This provides firmer shifts with more consistent performance.
  • ATI suggests either the 4L65E or 4L70E, which requires a computer.
  • Gearstar’s Level 3 builds are a good choice for the mid-level power range. A Level 3 4L65E is good to 600 hp, while the same level Ford 4R70W is only good to 450 hp.

600-Plus-HP

For the big power crowd, pushing 601-plus-horsepower, getting the most out of an automatic transmission requires a lot of extra tricks.

Everything has to be perfect inside the transmission and up to the task. Certain transmissions need bigger planetary gear sets (such as the 4L60E, which has 4 gears, but can be upgraded to 5).

When you get into the heavy-duty parts, sometimes there is an increase in parasitic loss, because the guts are heavier. It takes more effort to move them.

This is an easy trade off when you have 1000 hp, but when you strap a monster trans to a 340-hp small block, it can choke off more than you need. It will still function, but there is no

  • TCI’s Scott Miller told us that the Super Street Fighter is the ticket for engines up to 1025 horsepower. TCI takes the stock case and adds all the custom heavy-duty internals, along with top-shelf clutches and steels to make these transmissions last under severe use. The biggest difference between the Street Figther and the Super Street Fighter is the beefed up internals.
  • ATI gets a little off the norm with their high-power suggestion, TH400 with a Gear Vendor. There is no computer needed, no lock up (which means the torque convertor is lighter) “but still a good Overdrive” says JC Beattie of ATI. The TH400  is a little more parasitic than modern transmissions, but they do hold power. The overdrive in this case is a Gear Vendors add-on overdrive unit that bolts onto the tailshaft of the transmission case.   For a modern transmission, ATI suggests the 4L85E.
  • Gearstar suggests the Level 4 for all-out performance engines. Capable of handling over 1,000 hp, the Level 4 transmission from Gearstar has all the top-quality components you would expect for a race-ready transmission.

In the end, the goal is to put the power to the ground and make the most of it. How many times have you come home from work, hopped in that old jalopy to meet some friends and found yourself grabbing for another gear that isn’t there? A modern overdrive brings new life to any rod, whether it was built 30 years or 30 days ago. After all isn’t that the point of hot rodding?

About the author

Jefferson Bryant

It is almost terrifying the breadth of Jefferson's technical abilities. A fabricator, master technician, engine builder, paint and body guy, dirt track racer, road course driver, or a glossy magazine reporter, Jefferson can do it all. Oh yeah, he's also a YouTube hero.
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