All About Stall (Part 3): How Your Camshaft Helps Determine Torque Converter Stall Speed

Our initial All About Stall post established the significance of engine setup in choosing torque converter stall. Nelson Gill, President and CEO of ACC Performance, then demonstrated how variables in engine setup — things like cylinder heads and exhaust choice — can change your ideal stall speed in Part 2 of the series.

So where do you start when trying to pinpoint the right stall speed for your vehicle?

As we mentioned in an earlier post, the tech experts at Summit Racing believe it all starts with your camshaft. Gill covers the importance of the camshaft powerband and how it ties the engine torque curves and horsepower together:

“You have to match the cam with the rest of the vehicle’s setup and how you want to use it,” Gill said. “An engine is a machine that has fixed parameters and is controlled by its components and the amount of fuel you give it. In order to have the best output, all of the components must match or complement each other. All of the parts’ powerbands or operating ranges must match each other. Just one thing can throw the entire setup out of whack. This can cause rough idle, the car bogging down, or insufficient vacuum to operate the vehicle.”

Let’s go back to basics.

Your camshaft raises and lowers the intake and exhaust valves as follows:

1. The intake valve opens as the piston moves down and sucks the air and fuel into the cylinder.
2. The intake valve closes, then the piston moves up, which squishes or compresses the air and fuel mixture.
3. As the piston rounds the top again, the spark plug fires and ignites the air/fuel mixture, forcing the piston down.
4. The piston then rounds the bottom again as the exhaust valve begins to open, releasing the exhaust gases out the exhaust valve.

Each cam comes with an ID card called a “cam card.” This will have all of the cam specifications on it, including cam lift and duration. Lift is the distance the valve travels to its maximum open position and is a key factor in torque production. Duration is the amount of time, measured in degrees of crankshaft rotation, that a camshaft holds a valve open. There are a couple of ways to denote duration; however, duration at .050-inch (also known as “at 50”) is the more accurate indicator of when the cam is going to begin its powerband. Your cam card will also include “lobe separation” and “centerline.” These two specs will tell you how choppy the cam is and how well it’s able to idle. A lower number means less vacuum and decreased idle quality. The need for a higher stall increases as those numbers decrease.

Lift, duration, lobe separation, and centerline all help determine the powerband of the camshaft. Again, it’s always best to match the camshaft powerband with the operating range of the rest of the components in your setup. As a rule of thumb, the Summit Racing tech department begins with the  starting rpm of the camshaft powerband and adds 500 rpm to create a starting point for choosing optimal stall speed. But it’s just that — a starting point.

Gill says ACC Performance factors in the powerband, but also considers individual cam specs like duration and lobe separation when matching stall speed to an application. To illustrate the point, Gill refers back to the 350-cubic-inch motor mentioned in the initial two posts. Remember, it’s a 9:1 compression engine with 750 cfm carburetor, 1,500-6,500 rpm Edelbrock Performer RPM dual plane manifold, 64cc aluminum cylinder heads, and 1 5/8-inch headers with 2 1/2-inch crossover pipe.

“We are going to add a cam that has good street manners for drivability,” Gill said. “At the same time, we are adding more power through a longer rpm range than factory. We’ll choose a camshaft with a .488-inch intake/.510-inch exhaust lift, 234-degree intake/244-degree exhaust duration at 50, and .112 separation. The operating range for this cam is 1,500-6,500 rpm, and it would be considered a mild cam or ‘a couple of steps over stock.’ This cam will directly affect the stall in the torque converter by its duration and lobe separation. In this case, we’d expect the 234-degree duration to draw down the stall by 200 rpm, and the lobe separation will further weaken the torque at idle, dropping the stall an additional 200 rpm. In this setup, we’d say a diameter change will be more than likely depending on the rest of the setup.”

As you can see, specific camshaft specifications can lead to changes in the optimal torque converter stall speed. That’s why Gill says ACC Performance/Boss Hog has its own specific method that allows the company to pinpoint the stall range of your application to within 99.5 percent. This method takes into account the camshaft specs mentioned here as well as the complete engine setup. It also factors in vehicle setup — a topic we’ll tackle in the next installment of All About Stall.

SOURCES: ACC Performance and Summit Racing.