What Stall Converter Do You Actually Need? How to Choose One for Your Cam and Build

How to Choose a Stall Converter: Start With the Combination, Not the Number

Most stall converter shopping starts with a number. Somebody on a forum runs a 3,000 stall, a buddy says his cam "needs" 3,500, and a catalog lists converters by stall range. The trouble is that a stall speed printed on a box is only true behind one engine in one car. Put the same converter behind a different combination and it stalls somewhere else.

So the better question is not "what stall do I buy?" It is "what does my combination need?" This guide walks through what stall speed actually is, the inputs that decide it, the mistakes we see most, and the exact information we ask for before we spec a converter for a customer.

What Stall Speed Actually Means

A torque converter is a fluid coupling. The engine spins the impeller, the impeller throws fluid at the turbine, and the turbine drives the transmission. Stall speed is the engine rpm the converter allows before the turbine starts turning the car under load. Below that rpm the converter slips. Above it, the coupling tightens up and power goes to the tires.

You will hear two terms:

• Flash stall is the rpm the engine flashes to when you stomp the throttle from a roll or a launch. It is the number that matters most for how the car leaves.
• Foot-brake stall is the rpm the engine reaches with the brakes held and the throttle pinned. It reads lower than flash stall, it depends on how strong the brakes are, and holding it for long builds heat fast.

The key point: stall speed is not a fixed property of the converter. It moves with the torque the engine makes. More torque pushes the same converter to a higher stall. A converter that flashes to a certain rpm behind a mild small block will flash noticeably higher behind a big-inch or boosted engine. That is why a stall number with no engine attached tells you very little.

The Inputs That Decide It

When we spec a converter, we look at the whole application, not one part of it.

The Cam Card

We read three numbers off the cam card: duration at .050, valve lift, and lobe separation angle. Together they tell us how aggressive the idle will be and where in the rpm range the cam makes power. A long-duration cam with a tight lobe separation idles rough and makes little torque down low, so it needs the converter to let the engine climb into its power before the car is asked to move. A short-duration street cam makes useful torque early and does not need much stall at all.

Too little stall behind a big cam shows up quickly. The car pulls hard against the brakes in Drive at idle, or the converter loads the engine enough to stall it at a light.

The Engine's Torque Curve

Peak horsepower gets the attention, but peak torque and where it arrives matter more for converter selection. An engine that makes its torque low needs less stall to launch well. One that makes it high in the rpm range needs more.

Vehicle Weight

A heavy car or truck puts more load on the converter at launch and on every shift. The same converter behaves differently in a 3,200-pound street car and a 5,000-pound truck.

Rear Gear and Tire Size

Gearing changes how hard the engine has to work to move the car. A steep rear gear and a short tire multiply torque at the wheels, so the converter sees the load differently than it would with a tall gear and a tall tire. If the gear or tire is changing, tell your builder before the converter is ordered.

Power Adders

A turbo, supercharger or nitrous changes everything above. Boost raises the torque the converter sees, which raises its stall. A turbo combination also needs enough stall to get the turbo spooled at the line. Nitrous adds a large torque spike the converter and the transmission both have to handle. These combinations always need to be spec'd as a package.

Intended Use

A car that cruises the highway, a car that tows, and a car that runs the track on weekends can share an engine and still need different converters. How the vehicle will actually be driven is part of the spec.

The Most Common Mistake: Too Much Stall

The mistake we see most is choosing more stall than the application needs. It feels like more stall should always mean a harder launch, but past the point the combination needs, it works against you:

• Heat. A loose converter slips more, and slip turns into heat in the fluid. Heat is the leading cause of transmission failure.
• Stack shifting at wide-open throttle. With an overly loose converter, the rpm barely drops on each shift, and the transmission runs through the gears too quickly. The engine never pulls back down into its power band.
• Lost drivability. Part-throttle driving gets soft and vague, and the car feels like it is slipping around town because it is.

Our target is an 800 to 1,000 rpm drop between shifts. That drop gives proper shift extension, so each gear pulls the engine back into the range where it makes power. If the rpm hardly falls on the shift, the converter is too loose for the combination.

Too Little Stall

The opposite problem is real too, and it is usually easier to spot:

• The car pulls hard in gear at idle, and you are standing on the brakes at every light.
• The engine stalls or nearly stalls when you put it in gear or come to a stop.
• The launch bogs because the engine is being asked to move the car below where the cam makes power.

Behind a big cam, too little stall can make the car miserable to drive on the street. The fix is not to guess higher. It is to match the converter to the cam and the rest of the combination.

Why We Don't Pick From a Range Chart

You will find plenty of charts that list a stall range for "street," "street-strip" or "towing." We don't recommend converters that way. Every build and every customer is different, and two cars that fit the same category can need very different converters because of cam, weight, gearing and power adders. A converter has to be spec'd for that application.

That is also why our Turbo 350 stall converter guide no longer carries range charts. The method is the same no matter which transmission sits behind the engine.

Lockup, Diameter and Overdrive Units

Lockup Converters

Overdrive automatics like the 700R4, 4L60E, 4L80E and the newer six- and eight-speeds use a lockup converter. At cruise, a clutch inside the converter locks the turbine to the cover so there is no slip. That cuts heat and brings highway rpm down. A higher-stall converter can still be very livable on the highway if it locks up, because once it is locked the stall speed no longer matters at cruise. How and when the converter clutch applies is controlled by the transmission's controller or tune on electronic units, so the converter and the calibration need to agree. Our lockup converter explainer covers the basics.

Converter Diameter

Converter diameter is one of the tools a builder uses to move stall. A smaller-diameter converter generally stalls higher than a larger one built for the same application. It is one more reason converters are not interchangeable just because they bolt in. Even within one transmission family, the converter has to match the specific configuration. We cover that in will a 4L60E torque converter fit a 4L65E or 4L80E.

A Simple Method You Can Follow

1. Write down the combination. Engine, cam card, peak horsepower and torque, power adders, weight, gear, tire and how the car will be used.
2. Be honest about use. Most of the time on the street and the occasional track day is a street car. Spec for how it is driven most.
3. Find where the engine makes power. The cam card and the torque curve tell you where the converter needs to let the engine get to.
4. Account for load. Weight, gearing and power adders change how the converter behaves. Factor them in rather than adjusting the number afterward.
5. Check the shift. On a running car, watch the rpm drop between gears at wide-open throttle. If it barely falls, the converter is too loose. Our target is 800 to 1,000 rpm.
6. Have the converter spec'd, not picked. Send the full combination to the builder and let the converter be matched to it.

What We Need to Spec Your Converter

Have this ready when you call or email us. It is the same information our tech staff works from.

• Year, make and model of the vehicle
• Engine size and type
• Cam card: duration at .050, valve lift and lobe separation angle
• Peak horsepower and torque, and the rpm where each happens if you have a dyno sheet
• Power adders: turbo, supercharger or nitrous, now or planned
• Vehicle weight
• Rear gear ratio and tire diameter
• Transmission model
• Intended use: street, towing, street and strip, or drag racing

If any of that is about to change, tell us what it is changing to. A converter spec'd for next season's combination beats buying one twice.

The Takeaway

Stall speed is a match, not a rating. The right converter comes from the cam, the torque curve, the weight, the gearing, any power adders and how you drive the car. Too much stall costs you heat and shift extension. Too little makes the car fight you at every light. Getting it right takes the whole combination.

Every Gearstar transmission is assembled start to finish by one technician, matched to your engine, gearing and use, and dyno tested before it ships, with the converter spec'd for the same combination. Send us your build details and we'll spec the converter around it.

FAQs

How do I choose the right stall converter?
Start with the full combination: cam card (duration at .050, lift and lobe separation), peak horsepower and torque, power adders, vehicle weight, rear gear, tire size and intended use. The converter should be spec'd for that application rather than picked from a generic range.

What happens if my stall converter is too loose?
Too much stall creates extra slip and heat, makes part-throttle driving feel soft, and can cause stack shifting at wide-open throttle, where the rpm drops very little and the transmission runs through the gears too quickly.

How much should rpm drop between shifts?
Our target is an 800 to 1,000 rpm drop between shifts. That gives proper shift extension so each gear pulls the engine back into its power band.

What happens if my stall converter is too tight?
The car can pull hard in gear at idle, load the engine enough to stall it, and bog at launch because the engine is asked to move the car below where the cam makes power.

Does a stall converter change stall speed behind a different engine?
Yes. Stall speed moves with engine torque. The same converter stalls higher behind an engine that makes more torque, which is why a stall number with no engine attached tells you very little.

What do I need to read on my cam card?
Duration at .050, valve lift and lobe separation angle. Together they show how aggressive the idle will be and where the cam makes power.