Torque Converter Slippage

Oct 14, 2008
8,248
113
Melville,Saskatchewan
Due to an argument on another forum, I want you fine people's input. I say a generic calculation isn't 100% accurate, it comes down to, two big variables. Number 1. Advertised tire height vs actual tire height. A few of us have found BFG Radial TA's advertised height taller than the actual height. The 245/60R14, I measured were a 1/2" short. Fun71 found one 1.85" short. That alone makes rpm nearly 100 rpm higher than calculated with the advertised height. I may have found my BFG 225/70R14 current tires short as well, I can't remember. Between shortage in height and brown letters, I can see why people are going with different tires. Who knows if other manufacturers aren't the same and I bet any out of spec, will be short on height, not taller, if they are wrong. Companies spending 5 cents less rubber on every tire makes the Shareholder's happy. Number 2 is slippage, it isn't 5% on every car made, it varies. The first article I found mentioned 3 to 5 percent as the norm but as high as 8% on a mild Street Converter. Step into a cheapy 4000 stall, is it going to be 5%? I think not. Are tachometers 100% accurate, probably not but how far off are they? On GM factory tachs to Autometer, I have seen a 250 rpm drop for most factory 1500 to 1600 flash stall lock up torque converters and around 300 rpm for mild, cheap mid 2000 flash stall converters, locked up. My tachometer may be not 100% accurate, is it that far off? What is the allowed variance? I don't 100% rely on the speedometer for speed, I always check with GPS, it eliminates one more variable, Speedometer inaccuracy. Even the calculator I use says the number calculated varies 100 to 200 rpm. I can hit that number bang on with my converter locked up, otherwise it is 250 to 300 rpm lower🤔.
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
4,965
113
Upstate NY
Without sounding sarcastic, what is the concern with coming up with a precise, rather than accurate, number for converter slip?
 

olds307 and 403

Geezer
Thread starter
Oct 14, 2008
8,248
113
Melville,Saskatchewan
Basically I thought his non lock up sounded low, by about 300 rpm. He says it math, he doesn't care what my tachometer or GPS says, his number is 100% accurate due to math. With a variable like slippage, how can a calculator be 100% accurate? What rpm drop do you see when your converter locks up? Is 300 rpm artificially high?
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
4,965
113
Upstate NY
200-4R about 220 rpm drop

4L80E about 290 rpm drop

This is off from logs, and has to be measured when the converter coupled in OD, NOT while it’s slipping under load.
 
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olds307 and 403

Geezer
Thread starter
Oct 14, 2008
8,248
113
Melville,Saskatchewan
200-4R about 220 rpm drop

4L80E about 290 rpm drop

This is off from logs, and has to be measured when the converter coupled in OD, NOT while it’s slipping under load.
What stall were both? That is close to what have seen locked up. On an analog dial, hard to tell exact numbers. Pretty sure the ECM logs will be as accurate as anything, probably more accurate. Good I am not losing my mind, not completely anyways😳.
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
4,965
113
Upstate NY
200 is 9.5” FTI that stalls to 2900 and the 4L80 is a Freakshow that stalls to 3800.

The 4L80 is very accurate because of the input shaft speed sensor - when the converter is locked the rpm’s and input shaft speed match. It takes all of the tire calculations out of the equation.
 
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Rt Jam

Master Mechanic
Mar 30, 2020
306
63
Ontario Canada
Not sure how far off a factory tach is but for sure they are not as responsive as a aftermarket Autometer or something like that.
I can go in an out of the throttle on a car like an 02 Camaro and the tach will not move. If you give it a second to stabilize, it's probably right on.

Another scenario is rpm cut off. This was only happening in the lower gears like 1st because the tach is sweeping up so fast we are hitting the limiter. Limiter at 6600 and tach showing 6000.
In 3rd, 6000 is 6000.
 
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