Use our stall converter calculator to find converter K-Factor from your current stall speed and torque, then estimate new stall RPM after an engine torque increase or decrease.
Formulas & Notes
K = Stall Speed (RPM) / √(Engine Torque (lb-ft))
New Stall Speed:
New Stall = K × √(New Engine Torque (lb-ft))
Note: The K-Factor is a theoretical constant used to estimate changes in stall speed when engine torque increases or decreases, assuming the same torque converter is used.
When you modify your engine to produce more power, your existing torque converter will not behave the same way. An increase in engine torque pushes harder against the fluid inside the converter, causing it to slip further and stall at a higher RPM.
This calculator uses your baseline performance numbers to find your converter’s specific K-Factor and provides an estimated new stall speed after an engine upgrade. Because torque curves can vary significantly between engine builds, this K-factor method serves as a solid ballpark prediction rather than an exact result.
How to Use the Stall Converter Calculator
To get an accurate estimate of your future converter performance, you need your current baseline data and your projected engine output.
- Current Stall Speed: Enter the actual RPM where your current setup flashes or stalls. The tool requires a number strictly greater than zero.
- Current Engine Torque: Input the torque of your current engine setup. You can select pound-feet (lb-ft) or Newton-meters (Nm).
- New Engine Torque: Enter the expected torque of your upgraded engine. The calculator will automatically apply the K-Factor to this new number.
The calculator automatically handles unit conversions. If you input your torque in Newton-meters, the tool applies the standard conversion of $1 \text{ Nm} = 0.73756 \text{ lb-ft}$ in the background before calculating the results.
The K-Factor and Stall Speed Formulas
The relationship between stall speed and engine torque is defined by a constant known as the K-Factor. This factor represents the internal design of your specific torque converter—specifically the blade geometry of the impeller, stator, and turbine.
In the automotive industry, the standard convention calculates this specific K-Factor value using torque measured in pound-feet. Because the numeric output changes depending on the unit used, this calculator converts Newton-meters to pound-feet in the background to ensure consistency.
To find your converter’s specific K-Factor, the formula is:$$K = \frac{\text{Stall Speed}}{\sqrt{\text{Current Torque}}}$$
Once the K-Factor is established, you can predict the new stall speed based on your upgraded torque output:$$\text{New Stall Speed} = K \times \sqrt{\text{New Torque}}$$
Example Calculation for an Engine Upgrade
Consider a common scenario where you have a 3000 RPM stall converter bolted behind an engine making 400 lb-ft of torque. You are planning to add an upgraded intake and camshaft package that will increase your engine output to 500 lb-ft.
First, the calculator determines the K-Factor of your current setup:$$K = \frac{3000}{\sqrt{400}} = \frac{3000}{20} = 150$$
Next, it calculates the new stall speed using your upgraded torque number:$$\text{New Stall Speed} = 150 \times \sqrt{500} = 150 \times 22.36 = 3354 \text{ RPM}$$
Without replacing the physical torque converter, your new engine setup will cause the same converter to stall at an estimated 3354 RPM simply because of the added twisting force.
Stall Speed Shift Reference Table
This table illustrates how a single torque converter reacts to different engine torque levels. This specific example uses a baseline converter rated for 3000 RPM at 400 lb-ft of torque, which results in a K-Factor of 150.
| Engine Torque (lb-ft) | Converter K-Factor | Estimated Stall Speed (RPM) |
|---|---|---|
| 300 | 150 | 2598 |
| 350 | 150 | 2806 |
| 400 (Baseline) | 150 | 3000 |
| 450 | 150 | 3181 |
| 500 | 150 | 3354 |
| 550 | 150 | 3517 |
| 600 | 150 | 3674 |
Frequently Asked Questions
Why does my converter stall higher than the manufacturer rating?
Torque converters do not have a fixed mechanical stall speed. Manufacturers rate them based on a theoretical baseline engine. If your engine produces significantly more torque than the manufacturer’s test engine, the increased rotational force will cause the converter to slip more, resulting in a higher actual stall speed.
Should I use peak torque or torque at the stall RPM?
For the most precise calculation, you should use the exact torque your engine produces at the stall RPM. Since full dyno graphs are not always available, car builders frequently substitute peak torque numbers to get a rough estimate. Keep in mind that if your new engine has a vastly different torque curve than your old one, relying solely on peak torque can throw off this simple formula.
Do vehicle weight and gear ratio change the K-Factor?
It depends on whether you are measuring brake stall or flash stall. The actual K-Factor and true brake stall are determined purely by engine torque and the converter’s internal geometry. Vehicle weight, tire size, suspension setup, and gear ratios do not influence brake stall.
However, those external factors heavily influence flash stall, which is how the converter reacts under a dynamic launch. Heavier vehicles or lower numeric gear ratios create more drivetrain resistance, causing the converter to flash higher on the track than a stationary brake test might predict.
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