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Old 08-07-2006, 05:49 PM
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Understanding Torque converters

I am having a hard time understanding how torque converters work or I just plain don't know

So say that you have a car with a 2800 stall converter and a th350 trans...and the car idles at say 1000rpm...so in gear are you not going to move until you rev it up past 2800? is their any acceleration below 2800 rpm? you does 2800 stall mean that you should be able to foot brake to that and lunch? so does making power below 2800rpm mean nothing to you?

Now say with a overdrive trans, 700r4, with a 2800 stall Lockup converter...and you have a high rear gear...3.08...so rpms cruising at 60-70mph are like 1700rpm. can the converter lockup at that speed(because its a 2800 stall? I guess what Im trying to say is can the converter lockup below its stall?



I think that all made sense hehe thanks

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Old 08-07-2006, 06:09 PM
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Every converter has a stall speed and this is the rpm at which the engine will stall against the transmission. If you have ever driven an automatic tranny you have experienced what happens with a converter at idle. If your stock converter has a 1500 rpm stall speed, you don't need to rev it to 1500 rpm to get the car to move, it will creep at idle. If you were to increase the torque of your engine and raise the stall speed of your stock converter to 2500 rpm, it would act the same as it always did, except you could rev the engine to 2500 rpm against the converter before it would stall.

A lockup converter will lock up any time you engage the lockup clutch.
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Old 08-07-2006, 06:33 PM
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I see, hmmm why do big cams have a recommend stall? - adding a huge cam generally reduces low speed torque right? so when you put it in gear it would kill the engine with a low stall? and why are big stall converters considered not really streetable? like 3500 stall? so any rpm above 3500 means that the converter isn't slipping and below 3500 rpm means that it is slipping right?(on a 3500 stall)

lol i still confused on how it all works or "feels" with relation to speed rpm engine power cam and rear gear

And just to be perfectly clear you could have a 3000 stall lock up converter and you could lock it up at 1500rpm. That makes for no slippage right...no heat...direct drive? That's pretty sweet

Also if your racing with a 700r4 do you want the converter to lock and do you want to go into overdrive, while going down the track?
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Old 08-07-2006, 06:58 PM
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Higher stall speed converters typically slip more at lower rpm's, which is good for engines that don't make much low rpm torque. The higher stall speed converters are usually smaller in diameter than their stock counterparts, hold less fluid and generate more heat. The stall speed depends on how much torque the engine generates, B&M rates theirs at about 230#ft @ 2500 rpm.

A quality high stall lockup converter is a reasonable compromise for a street performance car. You can have the stall speed for the launch and the lockup for efficiency. If you run an overdrive tranny, you really don't want to get into overdrive at the dragstrip, that puts a lot of strain on things. If you gear your car right, you can get to the top of third gear in the quarter. If you are able to lock the converter in third gear, you can increase the efficiency of the converter dramatically at the top end. In my Nova tow car, locking the converter in third gear was worth 2 tenths and 3 mph in the quarter, from 11.90's @ 114 to 11.75 @ 117.
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Old 08-08-2006, 06:18 AM
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Do a search on Google. TCI has a concise explanation and you will find a few others...
http://www.tciauto.com/tech_info/tor..._explained.htm
The biggest thing you have to remember with torque converters is the "advertised" stall speed is relative. The converters are generally rated with a certain HP/TQ input from the manufacturer and anything you do to change that input WILL change the stall speed. As an example; A TCI converter which is rated at 2500 stall for a 500 horse small block will have MORE stall if it is installed behind a 500 horse big block because the big block puts out more torque which will over-ride the factory rating. Behind that big block it may flash to 3000 before it starts to work.
This is why, when you contact a torque converter manufacturer, they ask you EVERYTHING about your motor/trans/rear end and tire combo which will affect the final stall numbers. Everything is relative and every change can change the actual stall numbers.
Mark
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Old 08-09-2006, 12:17 PM
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You get what you pay for, never forget that!

I removed a restalled (3000) 12" factory converter and now have a 9" diameter 3400 stall Precision "Vigilante" single disk lockup in my truck. This converter was $749.00 new, BUT it was worth SO much more than that. Considering that it single-handedly knocked 0.6 seconds off my 60ft time (2.2 -> 1.6) and 1.3 seconds off my 1/4 mile time (13.4 - > 12.1), it was the best spent money yet!

With the 12" converter, idling in gear, the truck would roll but I had to rev the pi$$ out of to get it moving. Then it was a slug until around 3500 or so (my engine's max is 5800 RPM) and would barely turn the tires off the line.

The 9" will idle in gear without moving, but any throttle input is instantly transmitted to the rear. It drives like a stocker but will blow the tires off if you don't perfect your launch technique. It locks like a stocker and my 70 mph cruise is 2200...

Russ
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