Displaying Torque Converter 'Slip' with the PowerLogger


Resident mad scientist
Staff member
May 24, 2001
The PowerLogger can calculate torque converter slip. Here is how to enable that feature.

Tech details:
To know converter slip you need to know the speed of the flywheel (RPM), and the speed of the input shaft of the transmission. We don't have a way to measure the input speed, but the speedo cable is connected to the output of the transmission, so if we factor in the speedo drive gears and the gear ratio of the transmission we have what we need...

In PLC, the F3 Config and Download page, click on the Adjust button in the Analog Input Settings group.

put the following numbers in the CS factor boxes.

Second Gear: 75.50
Third Gear: 46.40
Fourth Gear: 31.00

Note that the ECM/PLC can't tell first gear from second, so the CS in first gear will be wierd...

Once these values are entered, CS will appear on the Graph page. This will show up and work even on old logs.

(edit, replaced my wrong numbers with TurboDave's numbers)
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The PowerLogger can calculate torque converter slip. Here is how to enable that feature.

Second Gear: 28.95
Third Gear: 45.45
Fourth Gear: 67.84


Our numbers are fairly close, but I think you might have yours reversed. First, here are my numbers, and then I'll explain how I derived them.

Second Gear: 75.50
Third Gear: 46.40
Fourth Gear: 31.00

it took me a long time but wanted the values to be as exact as I could realistically make them. In each gear I locked the TCC manually. And I'm sure that when mine is locked it's locked solid with zero slip, you see, I have a vigilante 7 disc converter. So I would drive around in each gear with it locked and change my values until I saw a consistent CS% value of 0.0 with the tcc locked.
I will go back over my numbers, the number of speedo pulses per mile is a guess, the test you did would be "reality"

The CS factor is really just RPM/MPH for 0 slip. The software attempts to correct for differences in how the ECM calculates the RPM data vs the MPH data, so that the factor will work the same as steady speed vs hard accel. Once others post their findings we can tighten up the "default factors"

Note also, Tire size or tire growth at speed does not affect the calculation, since we are using transmission output shaft speed.

You can calculate using a 26" tall tire since that is what the speedometer is calibrated for, it should net the same result as using pulses per mile since your ECM MPH is based on pulses per mile based on a 26" tire. So if your reading ECM mph, tire size will not impact drive shaft RPM calculation.
Drive Shaft RPM = (5280/((26/12)*PI)*3.42*MPH)/60
Input shaft RPM = Drive Shaft RPM*Gear ratio
CF = Input Shaft RPM @ 1MPH

Calculated Numbers
CF 2nd 69.42
CF 3rd 44.21
CF 4th 29.62
Awhile back I came up with the CS factor of 45.87 for third gear.

This netted an approx. slip of 10% at the top of third, that I have been using as a reference point.

I am running a 28" tall DR, and my ECM speed is low by about 10%. PL shows it at 110MPH, while the track's timers have it closer to 120MPH.

How does this affect the CS factor we use, with regard to better accuracy?

I ASS-U-ME it would drive the factor's value down which will result in an increased slip being calculated. But by how much?

So how do we correct the factor in this case? Anyone care to elaborate?
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tire size does not affect the CS calculations.

Note that when you are comparing CS from run to run, you need to compare at the same MPH's

There is a tire size correction in PLC, correcting this does not affect the CS calculations.

Let me put the MPH corrections in a post.

Just tried this with a log I had on a jump drive. More current logs are on the laptop I keep in the car. I still have the factory torque converter. If I set this up correctly I am seeing slippage almost to 30% in the beginning of 3rd and it gradually drops down into single digits as it peaks 3rd. Is it because the engine is making more power at the lower rpm's because of the factory cam / heads, or am I understanding this whole torque converter thing wrong? I would assume a log showing slip will be nice for getting Dusty to spec me a converter (y)
what you are seeing is correct, but there are more things going on in the converter than just "slip".

Post the log and we can discuss the particulars.

awww your going to make me post a bad log lol. This was the first log I took after just installing a new LS1 sensor, intake set up, translator, and RJC boost controller. But Ill play cause I want to learn!

Really around frame 5575 is where I was looking

PLX wideband, 3bar MAP, pressure 4 is FP.


  • 6.1_RJC_LS1.dat
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Sorry, had stuff to do.....

Anyway, when guys first look at the converter slip right after the shift they are surprised, but the RPM differential is not all "slip", the converter is working like a hydraulic gear reduction, so the torque coming out of the converter is higher than the torque going in. Certainly there is some loss but most of it is torque multiplication.


Sorry I was just messing. So I want to learn here. Where would you look in the rpm band to determine what slip is too much, or what should I look at to compare performance with a different converter when I change it? When everybody posts these 5-6% slip numbers going through the traps what do the numbers look like earlier in 3rd?

Could someone post a log thats running like a 9.5" PTC NLU so I can learn? Would like to see all kinds of different logs.

Thanks Bob I find this interesting and formally thought this kind of info was limited to people with xfi and other sensors.
I plugged in Bob's numbers and got negative values towards the top of 3rd gear. Years back I changed out my driven speedo gear to a 40 tooth black one to correct the speedo for the 275/60's. I'm assuming that would skew the numbers. How would I go about getting corrected numbers?
I went to the TCI web site and they have a formula for calculating the required speedo gears. Using the stock 11 tooth drive, the required driven gear is 29.15 teeth. Our cars came with a 30 tooth gear. Error is roughly 3%. If I recalculate then the numbers are very close to Dave's experimentally determined numbers. The 3 % also gets my powerlogger MPH very close to my time slip trap speeds.

Calculated Numbers
CF 2nd 71.44
CF 3rd 45.50
CF 4th 30.49

Doug, if your speedo gears are dead nuts on for 28" tires you could try using:
CF 2nd 64.46
CF 3rd 41.06
CF 4th 27.51
Bob, is their anyway to read the raw pulse data from the VSS?
I can read the info in the chip, I don't believe that register is available to the powerlogger.

What did you want to do?