There’s never a bad reason to do an LS swap. Inexpensive, powerful, and plentiful, the Gen III and Gen IV small-block Chevy has changed the landscape forever for anyone looking to restore a classic muscle car or build an ’80s beater into a world-beater. One question you’re going to have to answer when planning your swap, though, is “what transmission do I want?”

For some, only a manual gearbox will do — they want to row through the gears themselves, and for projects destined for track days on road courses, a stick shift is the way to go. But for the vast majority of would-be swappers, an automatic transmission is the preferred gearbox. Fortunately, there are a lot of good choices available, whether you’re looking for a bulletproof three-speed for the dragstrip, a long-legged overdrive for the highway, or something in between.

To get some expert advice on the topic, we reached out to both the OEM world and the aftermarket, with Rusty Sampsel from Chevrolet Performance and Kevin Winstead from TCI fielding our questions. Here’s what we learned:

The Family Tree

While the small-block Chevy was evolving from the “traditional” Gen I and Gen II to the LS family, the automatic transmission offerings were evolving, too. From the original two-speed Powerglide, to the three-speed Turbo-Hydramatic 350 and 400, to the 700R4 and 4L60E, and finally to the six-speed electronically-controlled transmissions in late model GM RWD cars, many critical dimensions carried over.

That’s not by accident, of course; giving the engine and transmission designers a stable interface point rather than reinventing the wheel with every new generation is just good practice. As a result, a wide range of automatics ended up being physically compatible with Gen III and Gen IV engines.

“The TH350, TH400, and Powerglide are a direct bolt-on for an LS engine,” explains TCI’s Winstead. “The block dowel pins are in the correct location, and five out of the six bellhousing bolts align. However, the torque converter bolt pattern and pilot length is different on the LS transmission applications.” Fortunately, the solution to this problem is a simple one. Per Winstead, “You can have a converter custom-built for the application, or you can change the flexplate to one that has the correct bolt pattern and accommodates the different pilot length of the older type torque converter.”

For owners of classic musclecars already equipped with a serviceable, original automatic, this will be the simplest (and least expensive) solution when doing an LS swap. Between the two options, a flexplate swap using the original converter will be cheaper, but considering that the new engine is likely going to have a very different powerband than the one coming out, a torque converter with a more aggressive stall speed is likely to be the more desirable way to go.

Even if you’re not intent on keeping the original transmission as part of the swap, there are good reasons to consider a new non-overdrive transmission to back your LS. Winstead points out, “The TH400 and Powerglide can be built to be significantly stronger than a 700R4 or 4L60E transmission. A stock 700R4/4L60E will be limited to around 450 horsepower. A properly built version can handle 25 percent more than that.” 

Maximum Overdrive

Unless their project is going to be a dedicated drag car, most swappers will want to consider a four-speed overdrive automatic to reap the benefits of both the tall highway gearing and lockup torque converter. Moving to an overdrive gearbox does have some special considerations to keep in mind in terms of space and controlling lockup and line pressure, though, and in the case of a fully electronic transmission, overall control.

One of GM’s first steps into the world of overdrive automatics was the 700R4, making its debut in the 1982 model year, and getting a new “4L60” moniker for 1990 to square up with GM’s new naming scheme — “4” for the number of forward gears, “L” for longitudinal mounting (in other words, for rear wheel drive applications), and “60” to indicate its relative torque handling capacity. It brought both overdrive gearing (0.696:1 for factory transmissions) and lockup converter capability.

Chevy Performance’s Rusty Sampsel explains the lineage of GM’s overdrive automatics: “The 4L60/65/70 are basically a TH350 with the overdrive added, and the 4L80/85 are a TH400 with the overdrive added,” he explains. “The overdrive transmissions are slightly heavier and larger than the non-overdrive TH350 and TH400 units. If an older vehicle came with a TH350 originally, most times a 4L60/65/70 will likely fit without too much effort.  The same can be said for TH400 and 4L80/85. If you are wanting to go from a car that originally came with a TH350 and replace it with a 4L80/85, you may have to make some floor/cross member modifications.”

Generally speaking, a 700R4/4L60E transmission will fit most applications that originally contained a TH350/TH400,” concurs Winstead, adding,”The 4L80E will not require many modifications in early A-body and F-body applications.” Samples says, “The 4L60 and 4L80 families of transmissions are considered the ‘small-block and big-block Chevys of the transmission world’ because of their simplistic and durable design and the ease of installation.”

You may be wondering about the 6L80E family that came strapped to the back of a lot of LS engines over the last 10 years, but the general advice we got from our experts tends to make us recommend against one, except in some specific cases. Per Winstead, “As far as the 6L80E, it’s a much larger transmission and will sometimes require some clearancing in musclecar applications.” In its favor is the fact that even with factory internals, it’s a pretty sturdy gearbox. “A stock 6L80E transmission with a proper tune can typically hold up to 750 horsepower,” says Winstead. Unfortunately, the changeover to internal control electronics means that unless you are transplanting an entire drivetrain, complete with the matching factory powertrain control module and harness, you’re going to run into problems getting it to run right. “The 6L80E is a different animal, Winstead said, “It requires an OE PCM to make it shift properly.”

As for what’s being offered from Chevy Performance, for the moment they’re sticking to the 4L60 and 4L80 architecture. “We are always looking at ways to introduce current production components into our performance parts portfolio that make sense and that customers want,” Sampsel explains. “Some things have more underlying challenges than it may seem on the surface.  The six- and eight-speed autos are a totally different technology and have unique challenges that would have to be overcome to make them a good ‘hot rodding’ product.”

Getting It Under Control

Your chosen transmission and engine combination will also determine what you need for transmission control. For the TH400, the solution is the simplest (if also the least flexible in terms of transmission tuning) — a single vacuum connection to the modulator on the transmission will do, making the interface with either a carbureted LS or one running factory-style multi-point EFI straightforward.

The TH350 utilizes a kickdown cable, while the 700R4 uses what’s called a TV cable; for carbureted LS engines, appropriate hardware to adapt the carburetor’s throttle lever and mount the cable to the intake manifold is available from a number of sources, including Holley and TCI. On the 700R4, TV cable adjustment is critical to transmission performance, and many a 700R4 has died an early, ugly death because the owner wasn’t aware of the necessity of setting it correctly.

So, what if you’re doing an EFI LS swap? Factory-style LS throttle bodies have no provision for a kickdown or TV cable, so some degree of finagling and fabrication will be required to use a TH350 or 700R4. If you’re doing something like an LS1 install, and have the complete engine, 4L60/4L80 transmission, and factory ECU and harness package, using the OEM computer to control the transmission is the obvious choice. Once you start mixing and matching engines, computers, and electronically controlled transmissions, though, you may end up wishing for a turn-key solution.

“With the 4L60E and 4L80E applications, there are some very easy solutions available nowadays,” asserts Winstead. “TCI offers the EZ-TCU that includes a handheld interface to allow programming without a laptop. It’s a two-minute job to program the EZ-TCU.” The EZ-TCU handles parameters like shift RPM, firmness, and a full manual mode;  it can also interface with optional paddle or pushbutton tap shifters. While EFI swaps can share the signal from the throttle position sensor with the ECU via CAN bus or a simple Y-harness connection, carbureted LS engines will need a remote throttle position sensor added to the carb.

As the company who created these particular electronically-controlled transmissions in the first place, it’s no surprise that Chevy Performance offers a number of control solutions for them as well, covering both factory-style EFI and carbureted LS applications. Per Sampsel, “For 2016, we are launching two new controller kits that are carbureted crate engine specific. They are designed just like our LS fuel injected controller kits in that they are pre-calibrated to function after they are hooked up and plugged in.”

Like the TCI EZ-TCU, for non-EFI applications, the Chevy controllers need a way to get the load/throttle position data that the vacuum signal, kickdown lever, or TV cable provided in the earlier all-mechanical transmissions. “The major difference is that there is a throttle position sensor signal required to operate the controller, so a sensor has to be added to the carburetor,” Sampsel explains. “We include a sensor that mounts to the Holley carbs we currently sell with our crate engines, and it is simple to install and does not need in-depth set up. Just bolt it on and make sure the sensor travel is visually nearly centered and the controller will self-calibrate zero to 100 percent throttle automatically once you get it all up and running.”

Chevrolet Performance has also gone out of the way to take the ‘set-up anxiety’ out of introducing electronics into the mix, making its transmission controllers plug-and-play (but with tuning options built in as well). Sampsel says, “You can use a laptop to optimize your particular vehicle once you drive it a bit, but it will shift and have torque converter [lockup] clutch operation without hooking up a laptop right out of the box. The software (if you decide to use it) is very easy to navigate and most people are able to make the changes they want without having to refer to the help section.”

Bits And Pieces

Of course, anyone contemplating an LS swap probably already knows that the devil is in the details; as we’ve seen, there aren’t any real show-stoppers when it comes to running either a carbureted or EFI LS engine with any of GM’s popular three- or four-speed automatics, but every installation is going to be different in its particulars. Though aftermarket support for all aspects of doing an LS swap in popular vehicles is good and growing (Holley is a good example), you can expect some level of fabrication and adaptation will be necessary.

Be sure to consider things like the shifter, transmission cooler (if you’re not using one integral to the radiator), cooler lines, and miscellaneous hardware in addition to the top-of-mind components like the crossmember, driveshaft, flexplate, and torque converter. And if you’re going to re-use an existing cooler, whether built in to the radiator or separate, make sure you’re not giving your new transmission a fatal dose of grit from the old one.

“It’s very important to flush the transmission cooling system when swapping from one transmission to the next,” warns Winstead. “Make sure there are no contaminants that will impair the new transmission and torque converter.  The fluid level is critical as well: make sure that the transmission is not underfilled.”

Lost In Transmission(s)

Whether you’re re-using an existing three-speed or getting a freshly-built one, choosing a mechanical overdrive transmission or going all in with an electronically-controlled gearbox, the task of strapping the right automatic to an LS is easier than it’s ever been. “Some folks feel transmissions are nothing more than the rear engine mount,” concludes Sampsel. “Even though you can’t ‘pop the floor’ and show off your transmission at the car show [like you can pop your hood to show off the engine], picking the right transmission package for your hot rod is key to enjoying the drive to and from the car show.