Recently I was reading up on torque converters and the exact meaning of "stall speed" as it relates to them. I read something that said that you usually need a transmission brake to really figure out stall speed because a vehicle's brakes typically could not hold with enough force to reach stall speed RPM before overcoming brakes and spinning tires... I don't know if this is true, but it's what I read.

So...... yesterday I stood on my brake pedal and began easing into the throttle to see at what point I overcame the rear brakes and began spinning the tires. To my surprise, I reached about 2100-2200 RPM or so and speed stopped increasing (I guess this is my stall speed?) even to WOT. I tried this again today and same thing. BTW, truck is 02 SuperCrew with 4.6, 4R70W, 3.55 LS rear end.

Does this seem right? Are the rear brakes on this truck actually able to hold back the torque put out by the engine at 2200, multiplied by TC ratio, and the 1st gear ratio, and the rear end ratio? I've seen the power and torque curve for this engine and I know it doesn't make much torque down low like the 5.4 does but this really surprises me. Is it possible that the computer control is recognizing in some way what I'm doing and is somehow limiting engine output so I don't break something?

Thoughts?

 

are you sure it was in first gear??????????many times modern auto transmissions may start off in a higher gear. some start off in 1st unless the tranny feels a need to move to first.

 

Well, as far as I know. I didn't specifically set the gear shift to 1st, I just left it set to drive but I tried this from a dead stop and the truck does always start out in 1st when I take off from a stop. And I never noticed anything that indicated that the transmission was shifting gears.

 

Just a thought, what you are doing is really BAD for the transmission. Builds up a ton of heat in a really short time. And, yes, the brakes can generally hold back the vehicle in the manner you have observed.

 

I agree with "ggarrahan" I would worry more about toasting my transmission that about some specs that will not make a difference one way or another. I would just assume the specs are what they are and save my trannie. Just my opinion because I'm no transmission mechanic or anything.

 

Thats about right for the stall speed on the converter. Just as said before however, too much and too long will burn it up it does build heat in a big hurry. But, it is how to check stall speed to see if the converter is working and not too much slip.

 

i added a tci stall 2400-2600 and if your stalling up to 2200 i must have wasted my money for the stall?

 

The stall speed of a converter is not found by power braking. It is a flash stall speed. This means, with the engine at an idle, let off the brake and floorboard it. The converter will flash to about 2600rpm. Fluid build-up when holding the brake will lower the actual stall speed. All a trans brake does is lock 1st and reverse together so that they are holding each other from moving, when you release the button, reverse is released and it launches.

 

kool so i dont have to worry about overheating my tranny..

 

Thanks for all the input. I knew that doing this would build up heat in the converter quickly so I certainly didn't hold it any longer than a second after engine speed topped out. I just never realized that full clamping force on rear calipers would be enough to hold back engine.

LxMan1, on your comment about a trans brake locking 1st and reverse together, what would be the difference here (from the engine and TC's point of view) compared with holding it with the brakes? Either way the output side of the TC is prevented from turning. And I'm not saying you're wrong because I just don't know (that's why I was reading up on it), but your definition of stall speed differs from what I had read elsewhere. Even when you floor it from idle, by the time engine speed reaches 2600 the vehicle has begun to move so the TC output shaft is now turning somewhat. If I had a bed full of stuff (more mass) then it would behave differently (take longer to accelerate) than if truck was an empty regular cab with a 100lb driver (not me!). My point is that trying to determine a stall speed as the output of TC is beginning to move seems like it would be in consistent.

Anyway, thanks for all the input.

 

A trans-brake isn't really even relevant here. It does lock 1st and reverse together, and it builds heat very quickly (think 20* per second), and is mainly used on drag cars. The difference between holding down the brakes and a trans brake is that a more powerful motor (like that of a drag car) will overcome the clamping force of the rear calipers and begin to spin the tires. On the right surfaces (blacktop, for example), my 5.4 will spin em all day long. A transbrake, however, locks the transmission itself up.