The Torque Converter Buyer's Guide

Start here

The torque converter is one of the most misunderstood parts of an automatic-transmission build, and one of the most important. Get it right and your car launches hard, cruises comfortably, and puts power to the ground. Get it wrong and it feels sluggish off the line, runs hot, or drones on the highway.

This guide walks through every decision that goes into choosing a converter: what it actually does, how stall speed works, whether you need lockup, how diameter and construction change the way it behaves, and how to match all of it to your engine, transmission, gearing, and how you drive. If you read one thing before you buy, read this.

What a torque converter does

A torque converter is a fluid coupling that sits between your engine and your automatic transmission. It does the same job a clutch does in a manual (it lets the engine keep running while the car sits still) but it does it with transmission fluid instead of a friction disc, and it does something a clutch can't: it multiplies torque.

Inside the housing are three main pieces. The impeller is driven by the engine and flings fluid outward. The turbine is connected to the transmission input shaft and is pushed by that fluid. Between them sits the stator, which redirects fluid coming off the turbine back into the impeller, and that redirection is what multiplies torque at low speed, giving you that strong initial pull off the line.

At a stop, the impeller spins but the turbine can stay still, so the car doesn't stall. As engine speed climbs, the fluid coupling tightens up and the turbine catches up to the impeller. For a deeper walk-through of the internals, read How a Torque Converter Works.

Stall speed, explained

Stall speed is the single most important number when you're buying a converter. In plain terms, it's the RPM at which the converter stops slipping freely and starts efficiently transferring power to the transmission, the RPM the engine will "flash" to when you launch.

Why does that matter? Because engines make torque in a specific RPM band. A stock engine makes good torque down low, so a low stall works fine. But once you add a bigger cam, heads, or a power adder, the torque curve moves up the RPM range. If your stall speed is too low, the engine is stuck "below the power" when you launch. It bogs. Raising the stall lets the engine spin up into the range where it's actually making torque before it hooks up.

There's an important distinction here: rated stall vs. true (flash) stall. A converter's advertised stall is measured against a specific amount of engine torque. A higher-torque engine will flash the same converter to a higher RPM; a lower-torque engine, lower. That's why two people can buy the identical converter and see different stall numbers, and why telling your builder your real combination matters. Our full breakdown is in What Stall Speed Do I Need?

Choosing your stall speed

The right stall speed comes from your camshaft first, then gets fine-tuned by your gearing, weight, tire size, and how the car is used. Here's a realistic starting-point range by build type. Treat these as ballpark figures. Your exact cam specs and combination move the number.

Notice the ranges overlap. That's because stall alone doesn't tell the whole story. A numerically higher rear gear (like a 4.10) and a lighter car let you run a lower stall for the same cam, because the engine gets into its powerband faster. A heavy car, tall tires, or a highway-geared rear end push you toward more stall. This is exactly the kind of trade-off worth talking through before you order. See our worked example for a cammed engine in Best Converter for a Cammed LS.

Lockup vs. non-lockup

A non-lockup converter always drives through the fluid coupling. It's simple and proven, but there's always a little slip at cruise, which makes heat and costs a bit of highway mileage. A lockup converter adds an internal clutch that mechanically links the engine to the transmission once you're up to speed, eliminating that slip.

For anything that sees real road miles (a street car, a daily, a tow rig, a street/strip build), lockup is usually worth it: cooler transmission temps, better mileage, and less slip on long drives, while still giving you the stall you need off the line. For a dedicated strip car, non-lockup keeps things simple. Plenty of street/strip cars run lockup on the street and unlock at the track.

The catch with lockup is that the clutch needs a way to be controlled: the factory computer, a standalone lockup controller, or a manual switch. If your transmission was originally non-lockup, converting can add wiring or a controller to the project. We cover the full pros and cons in Lockup vs. Non-Lockup Torque Converters.

Diameter & construction

Two converters can carry the same stall rating and still behave differently, because diameter and construction change how the converter makes that stall and how efficiently it couples.

Diameter

As a general rule, a smaller-diameter converter raises stall speed and tends to be more efficient once coupled, which is why high-stall race converters are often physically smaller (for example, stepping from a ~12" to a ~10" or ~9" unit). A larger diameter gives a lower, softer stall that's easy to live with on the street. Diameter also has to physically fit your transmission and flexplate bolt pattern, so it isn't a free choice. It's part of the package your builder balances.

Construction & quality

Not all converters are built to the same standard. Things that separate a quality performance converter from a bargain unit:

• Furnace-brazed fins instead of only stamped/tabbed, so the impeller and turbine survive higher power and RPM.
• An upgraded or billet stator and hardened components for consistent, repeatable stall.
• A billet front cover on higher-horsepower and lockup applications to resist flexing (ballooning) under load.
• Needle bearings in place of plastic thrust washers for durability.
• True balancing and pressure testing before it ships.

You don't need a full billet race converter for a mild street car, but you do want a unit built for your power level, not under it. A converter that comes apart or balloons takes the transmission with it.

Matching to your combination

Stall speed is the headline, but a converter is really matched to your whole combination. When you're ready to spec one, have these numbers ready. They're what a good builder will ask for:

• Engine & power level: displacement, power adder (if any), and a rough horsepower/torque figure.
• Camshaft specs: duration and lobe separation matter more than the "stage" name; they tell the builder where your torque actually starts.
• Transmission: exact model, and whether it's lockup or non-lockup.
• Rear gear ratio: a 3.08 and a 4.10 want very different stall for the same engine.
• Tire diameter: tall tires act like taller gearing and change effective stall.
• Vehicle weight: heavier cars generally want a bit more stall.
• How it's used: daily, weekend cruiser, tow rig, street/strip, or track-only.

Not sure where your combination lands? Send us the details through the Converter Finder and we'll spec it for you.

By platform: GM, Ford & Mopar

Converter selection follows your transmission. Here are the most common automatics we build for and what to keep in mind for each. Diameter and spline/pilot details vary within each family, so always confirm the exact model.

GM

The big ones are the TH350 and TH400 (non-lockup classics), the 700R4 / 4L60E and 4L80E (lockup overdrives), and the newer 6L80/6L90 and 8L90. The 4L60E is the most common street build, great with a lockup converter for a cammed LS or small-block. The 4L80E and TH400 are the go-to behind big power. Match the converter to the exact case (a 4L60E converter is not a 4L80E converter).

Ford

Common Ford automatics include the C4 and C6 (non-lockup), the AOD and AODE / 4R70W (overdrive, lockup on the electronic versions), the E4OD / 4R100 truck units, and modern 6R80 / 10R80 automatics. The AOD has a unique input setup, so converter choice is model-specific, worth confirming before you order.

Mopar

The classic TorqueFlite 727 and 904 are tough non-lockup units still popular in performance builds. The A518 / 46RH-RE and later 545RFE / 68RFE add overdrive and lockup. As with the others, confirm the exact transmission so the converter's pilot, bolt pattern, and spline match.

Don't see your transmission here? We build for far more than this short list. Tell us what you have and we'll confirm the right unit.

Common buying mistakes

Most converter regret comes down to a handful of avoidable mistakes:

• Buying too much stall. A super-high stall in a street car slips constantly, makes heat, kills mileage, and feels lazy in normal driving. Match the stall to the build, not to a number that sounds impressive.
• Buying too little stall. The opposite problem: the engine bogs off the line because it can't reach its powerband. Common when a converter is chosen off the transmission alone without accounting for the cam.
• Ignoring gearing and tire size. Stall doesn't live in a vacuum. The same converter behaves differently behind a 3.08 versus a 4.10, or with 26" versus 33" tires.
• Under-building for the power level. A budget converter behind a power adder can come apart. Buy a unit rated for where you're going, not just where you are today.
• Forgetting lockup control. Adding a lockup converter without a plan to control the clutch leads to driveability problems. Sort the controller or wiring up front.
• Not sharing the full combination. The single best thing you can do is describe your build accurately. A cam card, gear ratio, tire size, weight, and use case turn a guess into a match.

Worried your current converter is already wrong, or failing? Learn the warning signs in Signs of a Bad Torque Converter.

Frequently asked questions

Will a higher-stall converter hurt my gas mileage?

A non-lockup high-stall converter slips more at cruise, which does cost some mileage and adds heat. A lockup converter largely solves this by locking out the slip once you're up to speed, which is why lockup is the popular choice for street cars that also want performance.

Do I need a transmission cooler with a performance converter?

It's strongly recommended, especially with higher stall or any towing. More slip means more heat, and heat is the number-one killer of automatic transmissions. A good cooler is cheap insurance.

Can I reuse my old converter after a transmission rebuild?

Generally, no. Reusing an old converter can send debris and contamination into a freshly rebuilt transmission and undo the work. A new or properly rebuilt converter is part of doing the job right.

What's the difference between a rebuilt and a new performance converter?

A quality rebuilt converter is fully disassembled, inspected, upgraded as needed, rebalanced, and tested. A new performance unit is built from the ground up to a spec. Both can be excellent when done by a real converter shop. The key is who's doing the work and to what standard.

How do I know what stall I'll actually get?

Because true stall depends on how much torque your engine makes, the most reliable path is to give your builder your full combination and let them spec it. That's what turns an advertised rating into the stall you actually feel.