ATI’s 4L85 Offers Overdrive With Near Turbo 400 Durability


When it comes to a performance-oriented automatic transmission primarily used in modified, vintage  streetcars, the Turbo 400 is usually the first unit that comes to mind. It’s reliable, nearly bullet proof, and has earned a reputation as a go-to transmission in high horsepower applications. But is there something just as strong and reliable? Is there a transmission that can deliver nearly the strength of a Turbo 400 but improve driveability? Whether you are building a classic or late-model ride that can turn the tires into asphalt crayons at will, we’re here to tell you that there is, and it’s a late-model overdrive.

We currently offer a 4L85E package that is rated to 1,000 horsepower… – Dave Caine

A Better Box

We contacted Dave Caine, general manager of ATI Performance Products, and he was quick to let us know, “The average guy these days has a late-model hot rod that makes between 800 and 1,000 horsepower at the tires. We currently offer a 4L85E package that is rated to 1,000 horsepower (has been tested higher). We are working on one that is rated to at least 1,500, and even maybe as high as 2,000 horsepower.” That means that for the average street guy and the guy that hits the track occasionally, the 4L85E is definitely an option worth checking out.

On paper, a stock 4L85E is conservatively rated to handle up to 460 lb-ft of torque. They are the strongest GM overdrive transmission ever produced, and easily accept upgrades to make them a great choice for extreme power applications. In stock applications, they lack any real problem areas, and they are a perfect choice for most mildly-hot-rodded Chevys. Dave continued, “One thing I need to clarify is that we make a line of Turbo 400s that will handle over 3,000 horsepower, and the 4L85 will never get close to that. So, to be factual, the 4L85 cannot be built “just as durable” as the Turbo 400.”


These upgraded parts make the 4L85 “nearly” as strong as a Turbo 400.

When it comes to the torque converters, they are entirely different. A Turbo 400 has no lock up feaature, and although the lock up feature in the 4L85 is controlled by the transmission, it is contained in the converter. Also, the converter range that ATI can offer for a Turbo 400 is much greater. Dave said, “While we do offer a wide variety of torque converters for the 4L85, we still cannot make a converter as loose or as tight as we can for a Turbo 400. The 4L85 stuff is going to be limited in size and stall.”

That caused us to wonder if that would allow more aggressive stall availability in a 4L85, and Dave affirmed, “Yes, you can get more aggressive with the converter knowing you have a lock up feature. Honestly though, the overdrive will load the converter more, so a looser converter is not always the best decision. But, each application varies. While the lock up helps drivability in the efficiency department, it hinders performance a little due to the added weight and drag it adds to the transmission. While that might not be great for the race-only guy, it’s a homerun for the street/strip guy.”

Tubo 400 SHORT

The Turbo 400 has earned the reputation of a tough, reliable transmission that can survive harsh duty, but is there a better alternative?

Although not often considered as such, much like the Turbo 400, the 4L85E is a solid contender for use in heavy, high-horsepower vehicles, and has the added benefit of an overdrive. There is a drawback however to the 4L85, and that has to do with the fact that it’s an electronically-controlled transmission. This presents additional complexity and expense during installation when compared to the Turbo 400. Also, the 4L85E’s case is 1 1⁄2 inches longer, and has a slightly larger circumference than a Turbo 400. This additional size could result in clearance issues with the car’s floor – especially in smaller cars like the early Nova.

Finally, since older vehicles use a speedometer cable and the 4L85 uses an electronic vehicle speed sensor (VSS), you will need a speedometer cable-to-wire adaptor to make the speedometer work.

Cable X

The Cable X is the easy way to operate your stock cable-driven speedometer when using a transmission equipped with vehicle speed sensor. The Cable X is a universal adapter that will pick up the electronic speedometer signal and will operate the cable driven speedometer to the corresponding speed. It’s a snap to install with only three wires: hot, ground, and signal.

Dimensional Dichotomy

When trying to decide whether you want a traditional Turbo 400, or a 4L85 that gives you the overdrive capability, there are a few things to consider, like vehicle usage and transmission fitment.

Dave confirmed that there are dimensional differences between the two, “The 4L85 is going to be bigger around the middle, longer, and even heavier; no question! The gear sets are interchangeable with little effort, so dropping to a 2.10-ratio is no big deal, and that’s what the most common option would be. We also make a special tailhousing and output shaft to shorten the 4L85 to get as close as possible to the length of a Turbo 400, but the transmission mount will still be in a slightly rearward location.”

The 4L85 (left) is 50 pounds heavier than the Turbo 400 (right), but the overdrive is a benefit we feel overshadows the extra weight issue.

Speaking of weight and dimensional differences, a Turbo 400 with no fluid or torque converter weighs between 135 and 140 pounds. Conversely, a 4L85 weighs 185 to 190 pounds when weighed with the same parameters. That means the 4L85 brings an extra 50 pounds to carry around.

What's Needed

Mechanical speedometer connection:

  • Because older vehicles use a gear-driven speedometer cable and the 4L85 uses an electronic Vehicle Speed Sensor (VSS), you will need a speedometer cable-to-wire adaptor.


  • Unlike the Turbo 400, the 4L85 does require a computer to control shifts. If you are running a carbureted engine, you will also need an aftermarket Throttle Position Sensor (TPS)


  • You will need to have it shortened


  • The 4L85 is longer, and while you might be able to relocate your factory crossmember, this is not a guaranteed option. Fabricating a new one, or locating an aftermarket crossmember might be required.
But as Dave said, “Yes, the unit is heavier, but you can also hit Fourth gear, lock up the converter, and cruise down the highway with your 4.10 gears and not need to stop at every gas station.” Dave followed that with, “For example, if you had 4.10 rear gears and a 28-inch tall tire and drove down the highway at 70 mph, you’d be at about 3,800 rpm with a Turbo 400. By only switching to the 4L85E, you would be running about 2,900 rpm because of the .75 overdrive. Now, lock up the converter, and that drops to 2,600 rpm.” By adding the 4L85 to the car, fuel mileage will obviously increase for two reasons; the Overdrive and the lock-up converter. It is hard to say how much of  difference, because that will vary depending on the engine combination and driver’s habits.

We also asked Dave if switching from a three-speed automatic to the four-speed version would hurt track performance and he replied, “Yes. Again, the unit is heavier and takes a little more power to turn, so you will add a few numbers to the time slip.”

Another consideration for you track junkies concerns whether a transbrake and manual valve body are available, and the answer is yes they are. We found they are available from Jake’s Performance, who makes them in several versions.

Stock Gear Ratios

First Gear Second Gear Overdrive
Turbo 400 2.48:1 1.48:1  —–
4L85E 2.48:1 1.48:1 .750:1

Apples To Oranges
We were curious about what ATI does to this new-age transmission to increase its strength, and Dave said, “Some of the items we change for increased strength are going to be the input and main shafts, aluminum direct drum, and heavy-duty center support assembly. We can also put any number of gear sets in there as well, depending on what the combination dictates. I’m guessing that most of your readers will be fine with the standard 2.48 and 1.48 gear set, but a common replacement is the 2.10 and 1.40 gears.”

That last part prompted us to ask Dave why anyone would need to change the gearing, and he replied, “As you make more power, the car can get more violent when launching at the starting line. In the past, it was very common for people to install a Powerglide to ‘calm the car down’. This was achieved due to the ‘Glide’s 1.80 first-gear ratio versus the 2.48 ratio.”


ATI’s Severe Duty direct drum assembly is an aluminum drum with a severe duty sprag. It is twice the size of the OE sprag.

Dave continued, “Remember, you need to multiply the rearend gear by the transmission’s first gear to get your SLR (Starting Line Ratio). If the SLR is too aggressive, the car can be very violent when you begin to make real power.

If a car is making more than 1,000 horsepower (depending on vehicle weight and rearend gearing), a standard 2.48-ratio transmission first-gear can be too aggressive.” That being said, those of you with late-model cars that weigh 4,000 pounds or more, the 2.48 ratio is probably going to be the gear of choice unless you are making some really big horsepower, or have increased the rearend gear ratio.

Size Is All That Matters

Although we focused on the 4L85E in this article, Dave also felt that their 4L65 might be of interest to readers. ATI can build a 4L65 transmission that is conservatively rated for 800 horsepower, but is a smaller and lighter unit than the 4L85, and may be more appealing to guys dealing with space limitations.

Controlling The Changes

The “E” in 4L85E stands for electronic, and as such, this transmission does require a computer to control its functions. With a stock Turbo 400, part-throttle shifts are controlled via the modulator and the governor. Full-throttle shifts are controlled solely by the governor. You can alter the shift points of full-throttle shifts by installing a shift-improver kit, as well as a tuning kit for for the governor. Because of these “mechanical controls, it is nearly impossible to achieve the shift-point accuracy of an electronically-controlled transmission. Even adding a manually-shifted valvebody still relies on driver input. Also, with a manual-shift valvebody, you run the risk of burning up the transmission if you forget to start out in a low gear each time.

For some enthusiasts, the idea of having a transmission that needs a controller is not something that interests them, but Dave thinks just the opposite, “There are a lot of pros, and I’m not sure there are any cons. Writing a shift program correctly provides repeatable and dependable shift points, as well as programmable shift pressures. This is important so you can drive to the store at part throttle without spilling your coffee in your lap every time the transmission shifts. But, as you are accelerating and apply more throttle, the ‘program’ will accurately increase the pressure to ensure you get just the right amount for the power being applied. A properly written program also provides perfect downshift points, which can make all the difference in the way a car feels.”

There are several companies like Chevrolet Performance, TCI, Powertrain Control Solutions, Baumann Electronics, and MSD that offer stand-alone controllers. If you are running a carbureted engine, you will need a stand-alone transmission controller. If you happen to have a late-model LS engine with factory-style EFI under your hood, and you are running the factory engine controller, it’s a good bet that your engine controller already has the capability to control your transmission. If you are utilizing an aftermarket engine controller, consult the manufacturer to see if it is also capable of controlling the transmission – some are not. Likewise, if you’re running a classic, small or big-block with aftermarket EFI, that doesn’t mean that your EFI controller can also control your transmission.

CP Controller

Get One

With so many benefits, there is no reason why a 4L85E transmission should not be given some serious consideration when upgrading your high-horsepower ride. In fact, ATI even offers a bellhousing adapter that allows the use of a 4L85E behind many other brand engines as well. The versatility of ATI Performance makes them one place you can call to get all your questions answered, and get the perfect transmission for your application.


Article Sources

About the author

Randy Bolig

Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. His passion for performance got him noticed by many locals, and he began helping them modify their vehicles.
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