All About Stall (Part 4): How Vehicle Weight and Gear Ratio Factor into Torque Converter Selection

So far, our All About Stall series has focused on engine setup and its effect on torque converter stall speed selection. We’ve covered the importance of things like camshaft selection, engine operating rpm, and even exhaust backpressure.

Another key part of the equation, according to ACC Performance president and CEO Nelson Gill, is vehicle setup. Vehicle weight, gear ratio, and tire height all play a part in choosing the ideal stall speed range for your vehicle.

We’ll look at vehicle weight first.

Gill says ACC Performance uses curb weight — or weight as the vehicle sits — when calculating ideal stall speed. He notes that people often mistake curb weight for gross vehicle weight rating. The gross vehicle weight is the weight of that vehicle plus the additional weight it’s designed to carry, like passengers, luggage, and fuel. These combine to equal gross vehicle weight.

“We must remember that the torque converters’ output comes from fluid pressure vs. resistance,” Gill said. “The heavier the vehicle weighs, the more resistance that there is in moving it. Conversely, a lighter vehicle takes less resistance to move. This brings us to the resistance against the pressure that tries to move the vehicle. The more resistance (weight) against the torque converter, the higher the stall speed to move that weight. The less resistance against moving the vehicle, the lower the stall will be in the torque converter.”

Now we’ll factor in gear ratio.

“As the ratio gets larger, the engine has to turn more rpm to make the axle rotate the same number of revolutions,” Gill said. “A 3.08:1 rear gear ratio would be considered a road gear for cruising or top end speed, as the engine turns the driveshaft 3.08 revolutions for every one revolution of the axle. Therefore, a 4.11:1 would be considered a pulling gear for 4 x 4 vehicles to gain low-end torque. It is also a usable gear ratio to compensate for tall tires, commonly used for drag racing.”

Along with gear ratio, the height of the tire makes a big difference on actual output and the performance of the vehicle.

“Most ring and pinion gears are designed for a 26-inch overall diameter (OD) tire,” Gill said. “That means for every 1.48 inches over 26-inches in diameter, you can take one whole gear set out (less) in order to get the final drive ratio. For our calculations and easier math, we round it up and use 1.5 inches over the 26-inch tire. As you can see in the picture below, the taller tire will travel further than the smaller tire with only one revolution.”

Case Study

To understand the effect of vehicle, gear ratio, and tire height, Gill offers up a case study based on a vehicle with a curb weight of 3,550 pounds:

“If our test vehicle has a 3.73 gear ratio with 26-inch tires, then the gear ratio will run true because of the tire height.

1. If we had changed the 3.73 gear to a 4.56 gear ratio, the torque converter would in fact stall less due to the lower resistance needed to move the vehicle.
2. With the 3.73 ratio, if we went from a 26-inch-tall tire to a 27.5-inch tire, the roll out of the tire will give us 3.55 rear gear output and performance. That is one whole gear ratio less. (Note: NOS and Superchargers/turbos like the longer or lower gears to give them the time needed to load up).
3. The same is true for the opposite, if you went to a 24.5-inch tire from the 26-inch tire, then your ratio will increase (that is one whole gear set more) to 3.90 output and performance.

Rolling Resistance

When considering gears and tire combinations, there is another factor that comes into play: rolling resistance.

“You can have too much tire,” Gill said. “An over-sized tire or even tires with not enough air in them can hinder the performance and speed of the vehicle, not to mention that the actual weight of the over-sized tire is higher. This is known as “unsprung” weight, which also includes the weight of the wheel, rotors, and other rotating parts. This increased rotating mass robs horsepower.”

With the gear ratio and the rollout of the tire, you can better see how much resistance there is against moving the vehicle. As we mentioned in the first post of the series, that resistance and the diameter of the torque converter dictate how the torque converter stalls.

The combined information of these All About Stall posts should help you understand that actual stall range is variable and depends on your engine and vehicle setup. You’ll need to provide your sales rep accurate information on your setup in order to get the right stall speed for your application.