Answering A Member's Pm

[misterp]

I get this question at least once every 3 weeks; I am sure several other forum members and admins must as well. So I am posting the answer here in that others can get some help on the subject as well. Here is the original PM:

 

could u please explain stall speed. I haer its one of the most misunderstood topics involved with modifying. Im going to do my tranny and would like to understad whats going on...

 

OK Torque Converters For Dummies:

 

The "Torque Converter" is a major part of the vehicle powertrain; it is bolted between the engine and the transmission. Inside the torque converter are two "fans" facing each other, one is bolted to the engine (the driving fan, or 'pump') and the second fan is connected to the transmission (aka the 'turbine') - arranged like this, when the engine is running the driving fan is blowing transmission fluid against the second fan, which causes it to spin and move the vehicle. Because the two fans are separated by a small distance, it is possible to hold the vehicle still with the foot brake yet allow the engine to continue to spin freely.

 

"Stall" is a condition that happens when the driving fan (engine) is going faster than the driven fan (transmission), torque is literally multiplied inside the converter, and the greater the difference in fan speeds the more torque is created; in real numbers you can see torque be two to three times greater than engine torque at max stall depending on the internal construction of the converter.

 

"Stall Speed" is defined as the highest RPM the engine can achieve with the vehicle in gear and the wheels blocked; note that a converter achieving a stall speed of 2000-RPM in a small V6 engine may well give a 6000-RPM stall speed if installed behind a turbocharged 700+ cu. in. mountain motor. The engine's torque output must be considered when choosing a converter.

 

When a converter maker designs a torque converter, they can specialize the internals of the converter to not only "set" the desired stall speed but also how much torque multiplication is seen during stall; but nothing is a free lunch and the tradeoff with higher stall speed or higher torque multiplication is poor torque converter performance below stall speed - the car will no longer "creep" in gear and with more radical torque converter designs you may have to rev the engine as fast as if it were crusing on the freeway before it will even move, on an incline you may have to run the engine well over 3000-RPM or more. This increased slippage causes loss in mileage, increased torque converter heat (must run an external ATF cooler), and some re-learning how to drive the vehicle; but the upside is strong launches and good recovery after shifting gear.

 

Another way to think about it - water-skiing: if you go water skiing and use a 10-ft rope, you will be pulled out of the water as the boat begins to gain speed; you come up immediately because the rope was so short it was almost tight to begin with. OTOH, if you go water-skiing with a 100-ft rope then the boat has a 100-ft head start before the rope comes tight, but when it does *bang* you get your arms ripped out of socket! This is similar to a "tight" converter compared to a "loose" or high-stall converter, the tight converter moves the transmission (vehicle) almost immediately as the engine speeds up, the loose conveter stores this extra energy and when the stall speed is met you're on your way in a hurry.

 

IDEALLY (cost-no-object) you want to give your converter maker a dyno sheet from your motor so that the torque converter's behavior can be exactly tailored to the engine's output, so the converter will "kick-in" as the engine is gaining peak 'grunt' and after shifting gear the engine RPM is returned back down to the converter's "sweet-spot". Other things that will matter are the construction of the converter itself, because it is multiplying torque you don't want it failing (exploding) on you and converter failure will always carry collateral damage, usually damaging the transmission but in extreme cases will also cause destruction of the engine's crankshaft thrust bearing (i.e. lunching the short block).

 

Oh, and to be complete on the subject - installing an aftermarket torque converter will void any vehicle powertrain warranty; you will not be covered on engine, transmission, driveline, or axle issues.

 

Your best resources are first a expert converter manufacturer, and secondly feedback from others who have a vehicle very similar to your own who are using an aftermarket converter. So after a few years of listening here and my own experience on the subject, my recommendations:

* I myself would never buy, recommend, nor install a Yank product (go ahead, flame away);

* I am an open fan of TCS products, as full disclosure I resell their products and refer my business their way;

* I have not used a Precision product but I have ridden in a couple trucks with Vigilante 2800 & 3000 converters and like the way they drive;

* Fuddle and Phoenix Transmission's converters always stall far higher than advertised or desired;

* I have not seen a TCI or B&M converter but have it on good authority they just boil the ATF (are very inefficient inside);

* Do not buy a used converter unless you know -exactly- its life history, the last thing you want is to install is a converter filled with debris from a past transmission explosion, debris that will almost immediately chew up your transmission;

* DO install an external ATF cooler with an aftermarket converter - minimum design is a 12" x 12" stacked-plate cooler, best is the TruCool Max;

* a good torque converter manufacturer will rebuild/restall/remanufacture their converter good-as-new for a couple hundred bucks;

* a really good torque converter manufacturer will restall at least once for free;

* a great torque converter manufacturer will ask you every little detail of your vehicle (weight, gearing, daily use, tire size, power/engine specifics) and deliver you a product that is strong and stalls exactly where you & they have agreed it should.

* IMO there is NO converter on the market worth installing for under $800 (I've looked at them all); a full-competition racing converter easily costs a few grand, a quality torque converter will not be cheap but keep in mind there are a bunch of companies in the market that want to just take your money so make sure you get all the quality that you are laying your hard-earned dollar down for.

 

Guidelines for a STOCK late-model GM truck with LQ9 motor/cam/heads -

* 2600-2800 will make a good street truck combination, little effect on mileage and trans temp

* 2800-3000 will make a good hot-street combo, loose 1+ city mpg and ATF cooler required

* 3200-3600 I have seen used in trucks that see little/no off-track use, the higher stall speed converters used on shorter tracks (1/8-mi)

 

Guidelines for more advanced projects -

* if you are building a ton more torque down low (nitrous, radix, turbo) then you will want a tighter converter (2600-2800); don't worry the motor will still strong-arm that biatch to a stall speed of 3000 or higher.

* if you are building a heads/cam truck then you want to tailor the stall speed to be around the same speed as when the cam starts making serious power - the best advice is to discuss your converter specifics with your cam manufacturer and/or head porter and get them in agreement with your converter manufacturer on what stall speed is best for the given combo. All it takes is a couple more phone calls and these guys will actually be very happy you went to the trouble to research this to get the most from the combo.

* If you are building a centrifugal supercharged truck, generally speaking you want to help the motor "freewheel" up to an RPM where the blower is beginning to deliver noticeable boost and determining this point will require knowing your pulley sizes, compressor map, and some math; but the idea is that combination will have a sweet-spot stall speed where the blower is producing enough boost to lauch the truck hard yet the stall speed is not so high as to be a hindrance. The usual recommendation is 3000-RPM converter, but if you are "pullied up" or "pullied down" this will affect your decision.

 

Also my tranny guy recomended ... he would leave the stock bands. Why or why should I not replace the bands with raybestos bands. And what exactly does the difference in bands do???

The 4L60 has a series of clutches and a band that "grab" the desired gear; these are friction parts, think of them like special sandpaper adhered to steel plates that upon command grip spinning metal parts. I do agree with your transmission builder that the stock materials are superior to most of aftermarket stuff, and I also agree with him I would pass on the Raybestos products, that's a personal opinion as some guys do use the Raybestos Blue Plate products with success. In my own builds I prefer the Alto Red Eagle friction products in combination with kohlene treated steels, and on the band definitely use the Alto Red Wide band, mine gripped so hard it would instantly stall the engine with a corvette servo (!) and after tens of thousands of miles was still brand new on teardown; I have also heard that the carbon-graphite bands have been used with success, but the carbon-graphite clutches don't do so well. As a recommendation, DO get the 3-4 clutch stack to hold 9 friction clutches (instead of the OEM design of 7). Instead of a billet 2nd sevo, use the Corvette servo and plug the 3rd accumulator passage. With the Alto Red Eagle stuff there is a break-in period of about 600-800 miles, take it very easy during that period and you can really feel the frictions seat in. And last point, the trans setup (clearences, shift kit parts, etc) is everything to making it live.

 

Answering your exact question - difference in bands:

* The Alto Wide Band is 1/3rd wider than a stock band, and has reinforced construction and a better 'grippier' friction material. This band is fantastic and will not slip at all when setup correctly *however* if the servo pin length is wrong or the initial setup is not correct for whatever reason the band will be allowed to slip and once the Red High Energy material "burns" it will glaze over and forever continue slipping/burning like a cascading failure - once it goes, it'll be completely gone in a couple thousand miles.

* The Carbon-graphite band was developed by GM to address a specific problem, because carbon doesn't "burn" there is no way to "damage" it from slippage, the worn material just flakes off and exposes fresh carbon friction material so you get more mileage from the band. These bands are stock width, and several builders use them in supercharged applications under the philosophy that "slippage happens" and this band will take a ton of abuse. Encouraged by this several companies are selling carbon-graphite clutches, but in real life the clutches are not any more reliable than stock.

 

Mr. P.

Edited January 22, 2009 by Mr. P. (see edit history)

[F1SSS]

Thanks Mr. P, I am one that never fully understood this stuff as well.

[brobradh77]

very good write up as usual.

[Hankg42]

Another thorough and understandable write-up! Thanks, Steve.

[kcsilveradoss]

steve, i had papers in college that were shorter than that. great write up, and 50 extra credit points for this semester.

[lilmarv1117]

yet another great write...you should sit down and write a book about these trucks about modifying them...

[mcmahan]

very good write up as usual.

 

plus 1

[misterp]

yet another great write...you should sit down and write a book about these trucks about modifying them...

Thanks for the compliment - some stuff I know, but there's still a lot I'm learning! So we're learning together.

[sleepingcrna]

2

 

That'll save you some TM's from me. Thanks.

[Hrtbeat1]

Good write up Mr. P!

 

I've rebuilt several 700R4's and one 4L60, always using Alto frictions. One thing that I caught that confused me "plug the 3rd accumulator passage" I've never heard of this. What do you mean and how do you go about this?

 

Thanks again, learn something new every day.

 

[misterp]

Good write up Mr. P!

 

I've rebuilt several 700R4's and one 4L60, always using Alto frictions. One thing that I caught that confused me "plug the 3rd accumulator passage" I've never heard of this. What do you mean and how do you go about this?

 

Thanks again, learn something new every day.

 

The cavity around the 2nd servo is also the accumulator for 3rd gear - use a brass cup plug and block this passage from the valve body; the result is a 3rd upshift that hits as hard as the factory 1-2 upshift That way you can use the Vette servo for 2nd band engagement (the billet ones are too much) and still have that improved 3rd engagement.

 

Mr. P.

 

I tried to find a diagram of the underside of the case but cannot find what I am looking for...

Edited January 22, 2009 by Mr. P. (see edit history)

[sonickoolten]

nice i cant read at all but i read this 4 some reason lol tnx mr. p

[Fireman31]

nice Steve

[fox_forma]

I dont get it, could you please explain

[ANGRYGREG]