rskrause

Based on popular request, an explanation of lockup torque converters. The description is fairly generic, but oriented toward the 4L60E and similar GM transmissions.

The term lockup as applied to automatic tranny torque converters means that the converter contains a clutch mechanism in addition to all the parts of a regular converter. The pressure plate and damper assembly of the lockup mechanism is essentially the same as a MT clutch. To achieve lockup, fluid pressure is released between the cover and the pressure plate allowing the pressure plate to move forward and lock to the machined surface on the cover. The fluid flow is controlled by a solenoid in the tranny. The solenoid receives a signal from the PCM and opens a passage in the valve body. Hydraulic fluid pressure generated by the pump applies the lockup clutch. When the signal stops, the solenoid closes the valve and pressure is relieved leading to disengagement. When the lockup clutch is engaged, the impeller and turbine are turning at the same speed - IOW, the converter is "locked up" and there is no slippage.

Without lockup, there is always some slippage in a torque converter. The amount of slip will vary with converter design and the amount of power being transmitted. At cruise, it may be as little as a couple of percent. At WOT, a lot more. Slip produces heat, decreases gas mileage and wastes power. So, if the converter can be locked up why wouldn't you want it locked up all the time except when the car was at a complete stop (to prevent stalling)? Well, first of all, performance would suffer. With lockup, the converter ceases to multiply torque. It also effectively makes the converters stall speed 0 rpm. This obviously puts you way out of the powerband at low speed. The cushioning effect of the converter on the tranny is also lost, which would decrease tranny life when shifting during hard acceleration. Overall, it would slow the car down.

The strategy in the computer that controls lockup takes these things into account. Essentially, it commands lockup under light load at higher speeds. You can feel the converter locking and unlocking and see the effects on the tach if you pay attention during cruise. Watch the rpm's and listen to the motor as you gradually open the throttle. At some point, the revs will jump up and the car will accelerate more rapidly, when the converter unlocks.

Why don't race converters have lockup mechanisms? First, it would be difficult to design and produce a clutch mechanism to fit into the available space that would handle the amount of power the would need to be transmitted. In that regard, keep in mind that high stall race converters are quite a bit smaller than street converters. Typically 8" or 9" diameter versus up to 12" for street converters. The extra rotating mass would also be a negative. But more to the point, at least for a drag car, it would probably slow the car down to lockup the converter. Maybe you could get a couple of more mph at the big end locking up just at the end of the track. But the gain would not be large.

As far as changing the lockup strategy goes in a street car, you could get LT_1 edit or Tunercat and go ahead and play with it. You may find a strategy that pleases you more than the stock programming. I have played with it a little on my Silverado. I prefer it locking up a bit sooner and using more throttle/less rpm in daily driving. OTOH, I have programmed the "tow/haul" mode (equivalent to "performance" mode in an F-body) to be more aggressive and disengage the converter sooner on acceleration. However, the difference from the stock programming I have settled on isn't large. You could also rig up a manual lockup switch, but I don't see the point of that. What you want to avoid is lockup under WOT as the stock converter was certainly not designed to do this, even behind a stock motor. Vigilante claims to have designed their converters to lockup reliably behind fairly stout motors. They make a hp claim that I can't recall at the moment. I can't verify this one way or the other. But except for bragging rights on the dyno or top speed runs, I am not sure there would be any advantage anyway to locking up at WOT.

Comments or questions welcome.

Rich

MyShibbyZ28

revtime

Thank You Rich.
I always wondered what made a lock up converter different, they look the same on the outside.

So the only drawback to cruising below the stall speed is increased heat? This won,t hurt the converter?
I plan on running a TH350 with a 3500 stall, my cruise RPM at 60 MPH should be around 3000 RPM with a 3.73 gear.
Thats a guesstimate.

rskrause

No, it won't hurt it IF you can control the heat. If you plan to do a lot cruising, maybe a tranny temp gauge would be a good idea. And get a really good cooler and run synthetic fluid. No matter what, your gas mileage will SUCK running at less than stall speed!

Rich

00cls1camaross

on converters wil a stall... do you flip a switch that will bring your motor to 3500 rpms? how do you engage the stall?

rskrause

Stall speed describes an operational characteristic of the converter. Any converter has a stall speed which will vary with the torque being applied. Read the other stickfied post and do a Google search, you will learn a lot.

Rich

30696bird

Maybe this has alot to do with why my gas mileage SUUUCKS so bad. I get about 10MPG with mods in sig (need to change the gears I broke the 3.73's and now have 3.42's). I always thought there must be something wrong with tune but I've changed to an OBDI PCM and have had it on tunercat and everything looks good. I knew a converter would affect the MPG but "this" much??


Maybe my converter is to loose........look at my MPH in the sig. That seems sub-par to me.

00cls1camaross

so... how does a unlocket TC mutiply torque even though it slips.... how does slip benifit a car's performance?

rskrause

Only an unlocked converter does multiply torque. When it's locked, there is a 1:1 relationship between the speed of the impeller and the turbine, hence no torque multiplication. An easy, though not really accurate way to think of it is like reduction gearing. When the converter is unlocked, the impeller (driven by the crank) is rotating faster than the turbine (connected to the tranny input shaft). There is thus more torque at the turbine than the impeller.

As far as benefitting performance, having a TC benefits in some ways and not others. It's a less efficient way of transmitting power than a clutch, but it allows the motor to operate in a narrower rpm range and does provide torque multiplication beyond the ratios in the tranny. This may allow for a harder launch if the available traction is sufficient and so on. There is no loss of power transmission during shifts with an automatic tranny, which is another advantage. A multispeed planetary transmission with clutchless shifting (such as the Lenco) may offer the "best of both worlds" but is quite bulky and expensive. G-force (and others) make a smaller, lighter unit that operates similarly to the Lenco. A Lenco can be combined with a torque converter. This really blurs the line between an automatic and a manual tranny.

Which is best depends on your budget, class rules, how much power you are making, and so on. With a 4th gen, the basic choices are the A4 and M6. The problem with the A4 is that in stock form it is relatively fragile, at leasr for drag racing use. The M6 is sturdier but cannot be (effectively) speed shifted. Both can be upgraded and there are fast cars running both of them. A very strong automatic can be installed, like a TH400, but this lacks OD which severely limits street use. People have swapped in the HD OD automatic 4L80E and also added external OD to a TH400 (like the Gear Vendors unit).

The choices are only limited by your budget, imagination, and intended use. I haven't seen a 4th gen with a Lenco yet though. A Lenco can be used both on the street and track and might represent the "ultimate" tranny for a street/strip car. The whole package adapted to a 4th gen would run in the $10,000 range however. I may go this route at some point.

Rich

viper03af

www.rosslertrans.com makes 4l60E's that will take about anything..... if you got the cash

rskrause

I have had a couple of Rossler trannys over the years. One of the best, but as mentioned far from cheap. I didn't know he was doing 4L60's though. Thanks for the info.

Rich

viper03af

http://www.rosslertrans.com/Sale4l60e1.htm

NVRLATE

What effect does a converter have on hp?

rskrause

A race converter (very high stall) will give chassis dyno numbers up to ~10-12% less than a manny tranny. A typical performance converter, maybe 5%. A stock converter, a couple of percent and if it's a lock-up converter, 0% when it's locked up.

Rich

Jameslt1TA

So if your on the dyno and do a pull in 3rd gear will the convertor stay locked up?