Guys,
Help me out a little with some very basic stuff. (I've searched back to 1999 and in the FAQ.s and tech articles). In the UK we have limited numbers of American cars, and access to knowledge can't come better than you guys. I've never driven a car with either a high stall converter and/or with a posi-traction rear. I generally understand that a high stall converter enables the flywheel to spin up to higher rev's before putting the power to the wheels, but when people talk about the stall speed does this mean the revs that the power goes down? (If my standard 'box starts to creep just off tickover, when does a 2200 TC do it?)

Also is posi-traction like a UK LImited Slip Differential (LSD)? In that it prevents an inside wheel spinning out of a sharp turn for example.

Replies in 5th grade level of understanding please.

 

I can't answer the stall speed question since I have a manual.

But Positraction = LSD....different terms, same meaning.

 

A high stall convertor does just what it says. Right now with your factory convertor, you can put your foot on the accelerator and the car will move as soon as you touch it. We'll say the factory has a stall speed of 800rpm for example. The car moves when revs hit 800rpm. A higher stall speed changes the rpm range where the car will actually move. So let's say now you have installed a 2800 stall convertor-your new stall speed might be 2000rpm. So if you step on the gas car won't move until 2000 rpm-these are nice for cars with lumpy cams and also for bringing motor into powerband quicker. Keep in mind that torque convertors are a misterious breed. What the stall speed is advertised as and what you actually get will more than likely be 2 different things unless you buy a custom built unit and they ask EVERY detail about your car.

 

Forgive me if I'm wrong, but of my knowledge of the TCC is not that it helpts to move it, but rather its a torque converter clutch that activates only when in 4th gear in standard operation. The activation of the TCC no matter the stall speed is a different matter. The TCC I'm sure does not engage in first gear due to fluid pressures being too low and the car will move regardless of TCC activation from as low as 500 RPM to whatever your stall speed and beyond. Now my understanding of the TCC is that the stall speed should be reflective of the highest peak in your torque curve. For TBI 305's its 2400, TPI 305's 3000, TPI 350 3200 is their respective peaks so a stall of 2000, 2500, 2800 is the norm for these makes. The TCC will engage around this level in 4th gear or 3rd as long as these conditions are met speed over 40 kph and operation has been consistent for a certain amount of time at a certain rpm. Now there are those of us who use TCC lockup switches and when mine's engaged a min throttle shift (press lightly on gas until it shifts to 2nd) from 1st to second will cause a stall every time at 1500 rpm down to 1200 rpm in 2nd, thus correlating with the specs of my car with a stall speed of 1200-1500 rpm. A min throttle shift from 2nd to 3rd produces the same effect. For me this is worth 2 tenths of a second in the 1/4 mile. Now the TCC can engage at any time when the criteria is met as determined by the ECM so its not to say 1500 is the time it always engages, it can engage anywhere after the stall speed in order to make the most of the torque produced by your engine. This makes a normal operating efficiency level of 75% jump up to 90% efficiency thus saving fuel and transferring more power to the rear wheels. The trade off is more strain on your tranny and increased temps (more torque being converted = more force = more heat, physics simply put) So anyone else want to correlate with this?

 

Slade, if I understand your post correctly, I think you've confused TC and TCC. TC = torque convertor, TCC = torque convertor clutch. All automatics have a TC, but not all automatics have a TCC.

The TCC is inside the TC. A TC with a TCC is a lockup convertor. Inside a lockup torque convertor, you've got an extra set of splines, and a friction material. When the TCC solenoid engages, the direction of fluid inside the TC changes. It forces the friction material against the rotor of the TC, just like a clutch, and that's how the "mechanical" coupling takes place.

When the TCC is disengaged, the friction material doesn't touch the rotor anymore. Then, the engine and the trans use "fluid" coupling.

This might be on the howstuffworks.com website.

 

ok, conventions is wrong, but the idea is sound at least I think. Thanks for info Tom. So what was IHI referring to when he said "doesn't engage till 800 rpm" and stall speed because as far as I know all automatics will move regardless of what rpm it is as long as its not stalling. That means when my tranny is engaged it'll go at its min 500 rpm. Now that's a far cry from not moving till the engine has 2000 rpm statement by IHI. I'm just trying to figure out what he means.

 

Hey Slade1, your right about your car moving as soon as you give it gas, I don't know what factory convertors are set at but they're low, I just threw 800 rpm out as an example. I come from a drag racing background where stall speed plays a big factor in lower E.T.s because your launching off the line at a higher rpm, so your into the power band sooner. For example, my buddies 78 Malibu has a Boss Hogg convertor (they're junk BTW)-he got this from his brother. In his brothers 70 Chevelle he was running a mild 396, 4.11 gear, and the car weighed 4000lbs. We put that same convertor in my buddies malibu, mild 350, 3.50 gear, and weighs 3100lbs. In the Chevelle, the car would not budge until 3500 rpm when gassing it slowly like you would everyday driving, then when he stood on it at the track it would flash to 4500rpm and leave with the tire hanging in mid air. Now in my buddies malibu (same convertor that just came out of the Chevelle) the car won't budge until 2200 rpm when taking off slowly like on the street and it only flashes to 3000 at the track. Same convertor, but reacts differently behind different weight and power combo's. I just installed a convertor in my 700r4 cause I'm running a big cam in my 355, I got screwed cause I bought off E-bay and the fella said it stalled to 2800. There is no differenc between a stock convertor and this one except the flash point. It only flashes to 2200 when I stand on it off the line, and when I put the car in gear at an idle, the motor damn near dies cause the convertor won't slip enough at idle so it's trying to pull the car right away. If you ever go to a drag race, listen once, most all the modified car will rev up before the car actually moves (when they're heading to the staging lane)-at what point the convertor engages is left to phyisics, but this is kind of the ******* explaination of a higher stall convertor.

 

Most 3rd gen 700r4 trannies came with a 1200 to 1600 rpm converter. Where it actually stalls depends on how much torque the engine has. More torque, higher rpm stall. Thus a 305 might stall it at 1200 rpm but a 350 at 1600 rpm.

It works by spinning the tranny fluid inside the torque converter--which makes sense because the converter is bolted to the engine flywheel. After a certain amount of engine power is applied, the rpms increase until the fluid starts to take on the properties of a solid. That is the point that your car starts moving forward. Below that point the car might creep a little but that's all. It's ingenious when you think about it.

Of course the more rpms it requires to stall, the hotter the tranny fluid will run due to increased friction within the fluid spinning in the converter. That is why it is IMPORTANT to always get a tranny cooler, ESPECIALLY if you are going for a high stall converter.

Heat is DEATH to an automatic transmission :nono:

Besides, what else will increase the lifespan of your tranny by around 100% for 50 measley bucks, installed?

I have also heard that the Iroc Zs had a 2000 rpm converter.

 

FWIW, the stock converter on my car was a 1654 RPM stall unit, which I believe can be found on all 305 and 350 TPI autos. It may be lower in earlier years, but I'm almost certain that for the later cars, this is the converter they got.

 

Thanks so far guys for keeping it simple (got a little heavy in the middle). However, if you have a high stall TC does the box just spin in neutral 'till it hits the revs then bang off you go (probably not except for drag cars maybe), or does it act a little more progressively than that?

 

It is always transmitting some power to the tranny, even at idle. With a higher "stall", it simply doesn't transmit full power until a higher engine RPM - at which point the engine is making more power. The net effect is incredible.

Think of it as slipping the clutch more as you give it more gas on a manual transmission, rather than just letting the clutch out all at once at an idle.

 

Ok I've done some research last nite and came up with some proper conventions and definitions.

A TC torque converter has essentially 2 blades, the theory being that if one blade flows air/fluid to another the other one with spin until it reaches the same speed as the first blade. Pretty simple and can see how it removes the need for a clutch. Spin slow and it'll just circulate the fluid around. Note they're not shaped like propeller blades, its just an example.

A TCC will take it up a notch and actually have a fluid pressure plate that when the ECM commands it, pressure build behind the plate and then locks the back blade to the front blade via a special clutch plate design. This connects the engine to the tranny and when in 4th, clutch gears are used which in essence provides a direct connection giving almost 100% power transfer. The only problem is the initial locking of the clutch creates a lot of heat from friction. Once going though is just like normal operation.

Now from what I understand you can increase the resistance of the clutch plate pressure to only lock in until a certain pressure is built up hence a high stall. This is far better than the old designs of TC's where you increase the resistance of the impeller with size in order to transfer more torque. this would explain why its hard to move those high stall drag TCC units, the rpm is diverted to building clutch pressure instead of flowing a impeller. You match it to the peak of your torque curve, if you have a peak at 3000 rpm, the clutch plate wouldn't engage until 3000 rpm. But this would smash a normal drivetrain to the ground.

 

Slade1 & Five7Kid,
Spot on chaps! Those last two posts did it!
Thanks.