Stall Converter K Factor Calculator

The Stall Converter K Factor Calculator applies the engine’s observed stall RPM and torque figures to determine K-factor and project a new stall speed for the next projected torque

Torque Converter K-Factor
150.00 K-Factor
The mathematical constant representing this specific converter’s physical capacity to absorb engine torque.
Projected Stall Speed
3,354.10 RPM Prediction
Shift Variance +354.10 RPM
Load Differential +100.00 lb-ft
The predicted flash stall behavior based on the mathematically isolated difference between current and modeled load.
Torque Sensitivity Curve
+146.43 RPM at +10% Torque
Stall at 90% Torque 2,846.05 RPM
Stall at 110% Torque 3,146.43 RPM
The calculated reaction rate showing how the same converter responds to a 10% torque increase or decrease.
RPM Torque Absorption
277.78 lb-ft at 2500 RPM
Absorbed at 2000 RPM 177.78 lb-ft
Absorbed at 4000 RPM 711.11 lb-ft
The precise mathematical limits of torque this converter architecture can absorb at specific RPMs before slipping further.
Cross-System Equivalent
128.82 Metric K-Factor
Observed Torque Equivalent 542.33 Nm
Projected Torque Equivalent 677.91 Nm
The universally translated K-Factor mapping and torque outputs plotted across standard Imperial and Metric engineering benchmarks.
K-Factor Predictive Accuracy
The K-Factor assumes that the engine’s volumetric efficiency curve remains relatively constant. Drastic modifications such as adding nitrous oxide or turbocharging will alter how torque is delivered, meaning true flash stall may vary slightly from predictive math.

Predict Torque Converter Stall Speed with the Stall Converter K Factor Calculator

The Stall Converter K Factor Calculator derives a torque converter’s K-factor from an observed stall speed and torque figure, then projects how stall speed will shift if engine torque changes.

Engine builders and drag racers use it to predict how a converter will behave after a torque-changing modification, without having to guess or re-test at the track.

Entering Stall Speed and Torque Values in the Stall Converter K Factor Calculator

Select Imperial (lb-ft) or Metric (Nm), then enter the observed stall speed in RPM, the engine’s torque output at that stall RPM, and a projected new torque figure.

The calculator returns the converter’s K-factor, a projected new stall speed, torque sensitivity at ±10%, and torque-absorption figures at set RPM points.

Finding K-Factor With the Stall Converter K Factor Calculator

K-factor is defined consistently across both OEM transmission engineering and the aftermarket performance industry as stall RPM divided by the square root of torque at that RPM — GM’s own patent filings on torque converter control state this explicitly. $$K = \frac{RPM_{stall}}{\sqrt{Torque}}$$

Once K is known for a given converter, a new stall speed can be projected for a different torque figure by reversing the formula: $$RPM_{projected} = K \times \sqrt{Torque_{new}}$$

At the calculator’s own defaults — 3,000 RPM observed at 400 lb-ft — that’s a K-factor of 150, and Dragzine’s own published example (4,000 RPM at 500 lb-ft, K of 178.89, projecting to roughly 4,195 RPM at 550 lb-ft) walks through the identical math.

A common mistake here is entering an engine’s peak torque figure rather than the torque it actually produces at the specific stall RPM being measured, since K-factor is only meaningful at the RPM the torque figure was taken at.

A GM patent on stall-torque management notes that converters built to a nominal 180 K-factor commonly vary from about 145 to 215 due to manufacturing tolerance, so a calculated K-factor is specific to one physical converter, not a fixed spec for its model line.

K-factor stays essentially constant from stall through roughly the first 60% of the converter’s speed ratio range before rising sharply as the converter approaches coupling and lockup — so this calculator’s math describes brake and flash stall specifically, not how the converter behaves once it’s coupled up at road speed.

A non-obvious mechanical detail from converter builder Sonnax: tightening internal impeller-to-turbine clearance during a rebuild lowers K-factor and drops stall speed by roughly 100 to 300 RPM on its own, with no change to engine torque at all.

Common Stall Converter K Factor Calculator Input Mistakes

Using a torque figure from a dyno sheet or spec chart for a different engine combination than the one the observed stall speed was actually measured on.

Entering the projected post-modification torque into the “observed” field instead of the current baseline, which inverts the whole prediction.

Treating brake or flash stall speed as if it were the same thing as the RPM the engine actually hits at launch on the track, when vehicle weight, gear ratio, and camshaft timing all shift the real-world launch RPM independently of the converter’s own K-factor.

Stall Converter K Factor Calculator Questions From Drag Racers

What is a torque converter K factor?

It’s a fixed mathematical constant — stall RPM divided by the square root of torque — that characterizes one specific converter’s blade geometry independent of whatever engine it’s bolted to.

What’s a good K factor for a torque converter?

There’s no universal target: OEM converters are commonly built to a nominal K-factor around 180 (typically ranging 145–215 by manufacturing tolerance), and a lower K-factor generally means more efficient torque transfer at the cost of less low-speed torque multiplication.

Does gear ratio or vehicle weight change my converter’s K factor?

No — brake stall depends only on engine torque and the converter’s own blade geometry, though the RPM you actually see at launch in a real car is separately affected by weight, gear ratio, and camshaft profile.

Why doesn’t my calculated stall speed match what I see on the track?

Flash stall at launch and how a converter couples up at road speed are two different behaviors — K-factor math predicts brake stall specifically, not down-track lockup, which depends on car weight, gearing, and wind resistance instead.