TangoRed

Well he was specifically talking about the E38 740i's high stall speed torque converter so the same principles apply.

Stroock639

someone please correct me if this is wrong, but basically stall speed can be easily determined by simply power-braking the car

hold the brake with left foot and floor it with right... when the rpms reach a point where they no longer climb, you've reached the stall speed

AJT123

RWD at least, all that's gonna do is burn the rear tires if the car has decent power.

mmarshall

That can also tear the hell out of the transmission.....but I guess that's the nature of drag-racing.

Felix

Lets throw a trans brake & bump box to this mix.......

Stroock639

any car with that much power probably has a torque converter who's stall speed can be looked up with more relative ease lol

Stroock639

doing that in the average car for just a second one time (or probably even many times) isn't gonna do anything... by nature torque converters are meant to slip

geko29

exactly--same principles, just a less extreme application. The purpose of that option (part of the sport package) is that the higher stall speed (if memory serves, 400 rpm higher than the non-sport version) allows the engine to rev quickly into its powerband with only modest drag from the TC. When the TC hits its stall speed, the car takes off faster than it would have with the "stock" converter, because the engine is making more power at that moment.

AJT123

Turned out to be a good thread!! Thanks for input.

So answer me this. Say I floor my LS430, wouldn't the torque converter be locked as it runs through the gears?

mcomer

Not locked to provide nice smooth shifts and jerk free acceleration, locked at steady speeds to provide much better economy.

geko29

No, it really can't be, because the input and output shafts are turning at different rates. Also the car would shudder horribly at every upshift (and you'd stand a pretty good chance of breaking something). Generally speaking, the torque converter only locks at steady speeds in higher gears.

The easiest way to tell when your TC locks is to accelerate moderately, and watch the tach. You will generally notice one more "shift" than should be possible, given the number of gears in your transmission. That last one, which often occurs about 10 seconds after reaching a relatively steady speed in a higher gear (often 4th and up) is your TC locking up. It will unlock immediately when a downshift is called for, and lock again once a steady state is reached.

AJT123

All of this is exactly what I originally thought going back years, I know exactly what you mean. Especially noticeable on older 4 speed automatics.

AJT123

Lol it will still burn tires. An LS430 will do one hellllll of a burnout, it's a cinch. I probably don't need to tell you as an E55 owner lol.

Tony2214

Interesting posts thanks guys. I tow moderately heavy trailers (4000 lbs) with my GS430, and in cruise will select 5th, rather than lugging in top. A couple of questions;
1. Will the lock-up occur in 4th & 5th?
2. I've never had any overheating issues, and the trans always works perfectly. I have considered installing an extra / aftermarket trans oil cooler, but haven't bothered because everything is working fine, with no signs of distress.
Comments appreciated.

AJT123

You tow 4k lbs with a GS430?????