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Understanding Torque Converter Slip
In high-performance automotive applications, especially drag racing, torque converter slip is a critical metric for determining how efficiently your transmission is transferring power from the engine to the wheels. A torque converter is a fluid coupling, and unless it is a "lock-up" style converter engaged in high gear, there will always be some difference between the engine speed and the transmission input shaft speed.
How to Use the Converter Slip Calculator
To get an accurate calculation, you need five specific data points from a pass or a data log:
- Engine RPM: The exact RPM the engine was turning at the moment the speed was recorded.
- Tire Diameter: The actual measured height of the rear tires in inches.
- Rear Axle Ratio: Your differential gear ratio (e.g., 3.73, 4.11).
- Transmission Gear Ratio: The ratio of the gear you were in. For most 3-speed automatics (TH350, TH400, C4, C6), high gear is 1.00.
- Actual Vehicle Speed (MPH): The speed measured by the timing lights or a GPS sensor.
The Converter Slip Formula
The math behind converter slip involves calculating your "theoretical speed" (the speed you would be going if there were a mechanical connection like a manual clutch) and comparing it to your "actual speed."
Step 1: Calculate Theoretical MPH
Theoretical MPH = (RPM * Tire Diameter) / (Axle Ratio * Trans Ratio * 336)
Step 2: Calculate Slip Percentage
Slip % = ((Theoretical MPH – Actual MPH) / Theoretical MPH) * 100
What is a "Good" Slip Percentage?
The ideal amount of slip depends on your vehicle's purpose:
- Street Vehicles: Typically see 8% to 15% slip under heavy load.
- Performance Street/Strip: Usually aim for 5% to 10% slip.
- Dedicated Drag Race Cars: Tight converters in high-horsepower applications often aim for 2% to 7% slip.
If your slip is over 15%, you are likely losing significant power to heat, which can damage the transmission and slow down your ET. If your slip is 0% or negative, check your tire growth or tachometer accuracy, as a non-lockup converter will always have some slip.
Example Calculation
If you cross the finish line at 6500 RPM with 28-inch tires, a 3.73 axle ratio, a 1.00 transmission ratio, and your speed is 125 MPH:
- Theoretical MPH = (6500 * 28) / (3.73 * 1.0 * 336) = 182,000 / 1253.28 = 145.22 MPH.
- Slip % = ((145.22 – 125) / 145.22) * 100 = 13.92%.
function calculateConverterSlip() {
var rpm = parseFloat(document.getElementById("engineRpm").value);
var tire = parseFloat(document.getElementById("tireDiameter").value);
var axle = parseFloat(document.getElementById("axleRatio").value);
var trans = parseFloat(document.getElementById("transRatio").value);
var mph = parseFloat(document.getElementById("vehicleMph").value);
var resultArea = document.getElementById("resultArea");
var slipDisplay = document.getElementById("slipDisplay");
var slipStatus = document.getElementById("slipStatus");
if (isNaN(rpm) || isNaN(tire) || isNaN(axle) || isNaN(trans) || isNaN(mph) || rpm <= 0 || tire <= 0 || axle <= 0 || trans <= 0 || mph <= 0) {
alert("Please enter valid positive numbers for all fields.");
return;
}
// Formula: Theoretical MPH = (RPM * Tire Diameter) / (Axle Ratio * Trans Ratio * 336)
var constant = 336;
var theoreticalMph = (rpm * tire) / (axle * trans * constant);
// Formula: Slip % = ((Theoretical – Actual) / Theoretical) * 100
var slipPercent = ((theoreticalMph – mph) / theoreticalMph) * 100;
slipDisplay.innerHTML = slipPercent.toFixed(2) + "%";
resultArea.style.display = "block";
var statusText = "";
if (slipPercent < 0) {
statusText = "Negative slip suggests tire growth or inaccurate RPM/MPH data.";
} else if (slipPercent <= 7) {
statusText = "Excellent efficiency. Typical for high-end race converters.";
} else if (slipPercent <= 12) {
statusText = "Good efficiency. Ideal for high-performance street/strip cars.";
} else if (slipPercent <= 18) {
statusText = "Moderate slip. Common for street converters with high stall speeds.";
} else {
statusText = "High slip detected. Your converter may be too loose for this setup, causing excess heat.";
}
slipStatus.innerHTML = statusText;
}