Torque Converter Slip Calculator (Engine RPM vs Turbine RPM)
What torque converter slip means (and why you might measure it)
A torque converter is a fluid coupling between the engine and the transmission input. Because it transfers power through fluid flow (and often through a lockup clutch in some conditions), the turbine side of the converter usually rotates slightly slower than the engine/impeller side. That difference is called torque converter slip.
Slip is not automatically “bad.” At low vehicle speed and during launch, slip is part of how a converter multiplies torque and lets the engine rev into its power band. At steady cruise—especially when lockup is commanded—excess slip can mean wasted power and added heat, and it can be a clue when diagnosing efficiency or drivability concerns.
What this calculator does
- Inputs: engine speed (RPM) and turbine/input speed (RPM).
- Output: slip percentage (%), plus the RPM difference.
- Use case: quick checks during steady-state driving (e.g., cruise in a given gear) or controlled tests (dyno, fixed load) when you have reliable RPM signals.
The slip formula
Slip is the fractional difference between engine RPM and turbine RPM, expressed as a percentage of engine RPM:
Where Nengine is engine RPM and Nturbine is the transmission input/turbine RPM.
How to measure the RPMs (so the result is meaningful)
Engine RPM
Engine RPM typically comes from the tachometer, scan tool, or ECU data.
Turbine (input) RPM
The turbine RPM should represent the speed of the transmission input shaft / turbine speed sensor (often called TSS, Input Speed, or Turbine Speed in scan tools). This is important:
- If you use output shaft speed or wheel speed instead, you must account for gear ratio and final drive—otherwise the slip % will be wrong.
- Some vehicles infer turbine speed differently depending on the transmission; use the most direct input-speed PID available.
Interpreting your result
The calculator returns a percentage. Lower slip generally indicates more efficient coupling between engine and transmission input.
- Near 0%: typical when lockup is fully engaged and holding (exact values vary; many real systems show small non-zero values due to sensor resolution and control strategy).
- Moderate slip: common in light throttle without lockup, during gentle acceleration, or when the transmission is intentionally allowing some slip for NVH (noise/vibration/harshness) control.
- High slip at steady cruise: can mean lockup is off, lockup is commanded but slipping, the converter is mismatched for the application, or there is a control/mechanical issue. Always interpret alongside gear, throttle/load, fluid temperature, and whether lockup is commanded.
Typical slip ranges (rule-of-thumb)
The “right” slip depends on the converter design, transmission calibration, gear, throttle, load, and temperature. Use the table below as general guidance rather than strict pass/fail limits.
| Scenario | Common slip range | Notes |
|---|---|---|
| Idle in Drive (foot on brake) | ~10%–20% | Varies widely by idle speed, load, and converter stall characteristics. |
| Light cruise, lockup off | ~3%–10% | Lower is typically more efficient, but some calibrations allow more slip. |
| Steady cruise, lockup on | ~0%–1% | Sensor resolution and commanded micro-slip can show small non-zero values. |
| Moderate acceleration (non-lockup) | ~5%–20%+ | Higher slip can be normal during torque multiplication and shifting events. |
Worked example (step-by-step)
Imagine you log data while cruising in a steady gear:
- Engine RPM: 3000 RPM
- Turbine RPM: 2700 RPM
First compute the RPM difference: 3000 − 2700 = 300 RPM. Then compute slip:
Interpretation: 10% slip at steady cruise could be normal if lockup is off, but would be unusually high if lockup is commanded and should be holding. Check lockup command status, fluid temperature, and whether the RPMs were taken during a stable condition (no grade change, no throttle movement).
Common reasons slip changes
- Lockup clutch state: engaged vs. released vs. controlled “slip lockup.”
- Load and throttle: towing, headwind, uphill grade, or higher throttle typically increases slip if not locked.
- Fluid temperature: viscosity and control strategy can change with temperature.
- Converter design: stall speed and fin angles affect coupling efficiency.
- Transmission strategy: some modern transmissions intentionally allow small slip for smoothness.
Limitations & assumptions (read before diagnosing)
- Steady-state assumption: Best used when RPMs are stable. During shifts, rapid throttle changes, or traction events, slip % may jump and be less diagnostic.
- Turbine RPM must be true input speed: If you use output/wheel speed, you must convert using gear ratios; otherwise results are not meaningful.
- No gear ratio modeling: This calculator does not infer turbine RPM from vehicle speed, tire size, final drive, or transmission gear.
- Does not detect cause: The slip % indicates a difference in speeds, but not whether it’s due to commanded lockup behavior, calibration, clutch wear, or hydraulic issues.
- Sensor noise/resolution: Very low slip values can be dominated by sensor rounding or sampling lag.
- Not safety/repair advice: Persistent high slip plus overheating, shudder, or drivability symptoms warrants professional inspection.
FAQ
Is torque converter slip normal?
Yes. Slip is inherent to fluid coupling. Many vehicles also control slip intentionally. What matters is the operating condition (launch vs. cruise) and whether lockup is expected.
Should slip be zero with lockup?
Not always. Some systems command slight “micro-slip” for smoothness. Also, measurement resolution can show small non-zero values even when lockup is holding.
Why does slip increase under load?
More load demands more torque transfer. If lockup is off (or if lockup is slipping by design or due to an issue), the turbine can lag the engine more under heavier load.