I heard torque convertors actually multiply torque whereas a clutch only puts out the same torque you put in (minus some loss of course).
How are torque convertors able to multiply torque?
You can't get something for nothing so if torque is multiplied what is lost?

 

I've never heard about torque multplication when referring to a converter. The primary purpose is conversion of torque. Some of the 'holeshot' and 'Saturday Night Special' converters are helpful in getting big-cammed engines into their power curve by stalling at higher speeds than stock but that isn't multiplication.

IMO, the only way to multiply torque once the power is applied to the crankshaft is via transmission gearing and final drive gearing (and different tire/wheel diameters).


[Modified by TedH, 1:33 PM 10/6/2003]

 

Yes, they do multiply torque: initially. The stator is the key.

From TCI's website

Stator --- the stator can be described as the "brain" of the torque converter, although the stator is by no means the sole determiner of converter function and characteristics. The stator, which changes fluid flow between the turbine and pump, is what makes a torque converter a torque converter (multiplier) and not a fluid coupler.

With the stator removed, however, it will retain none of its torque multiplying effect. In order for the stator to function properly the sprag must work as designed: (1) It must hold the stator perfectly still (locked in place) while the converter is still in stall mode (slow relative turbine speed to the impeller pump speed) and (2) allow the stator to spin with the rest of the converter after the turbine speed approaches the pump speed. This allows for more efficient and less restrictive fluid flow.

The sprag is a one-way mechanical clutch mounted on races and fits inside the stator while the inner race splines onto the stator support of the transmission. The torque multiplier effect means that a vehicle equipped with an automatic transmission and torque converter will output more torque to the drive wheels than the engine is actually producing. This occurs while the converter is in its "stall mode" (when the turbine is spinning considerably slower than the pump) and during vehicle acceleration. Torque multiplication rapidly decreases until it reaches a ratio of 1:1 (no torque increase over crankshaft torque.) A typical torque converter will have a torque multiplication ratio in the area of 2.5:1. The main point to remember is that all properly functioning torque converters do indeed multiply torque during initial acceleration. The more drastic the change in fluid path caused by the stator from its "natural" return path, the higher the torque multiplication ratio a given converter will have. Torque multiplication does not occur with a manual transmission clutch and pressure plate; hence the need for heavy flywheels, very high numerical gear ratios, and high launch rpm. A more detailed discussion of torque multiplication can get very confusing to the layman as high multiplication ratios can be easily considered the best choice when in fact more variables must be included in the decision. Remember, the ratio is still a factor of the engine torque in the relevant range of the torque converter stall speed, i.e.: a converter with a multiplication ratio of 2.5:1 that stalls 3000 rpm will produce 500 ft.-lbs. of torque at the instance of full throttle acceleration if its coupled to an engine producing 200 ft.-lbs. of torque at 3000 rpm. However, if this same engine produces 300 ft.-lbs. of torque at 4000 rpm, we would be better off with a converter that stalled 4000 rpm with only a 2.0:1 torque multiplication ratio, i.e.: 300 x 2.0 = 600 ft.-lbs. at initial acceleration. Of course it would be better yet to have a 2.5:1 ratio with the 4000 rpm in this example (provided his combination still allows the suspension to work and the tires don't spin.) This is just a brief overview as the actual scenarios are endless.

Here is the link on TCI's website to more good info:
http://www.tciauto.com/tech_info/tor..._explained.htm

:cheers:

 

VETDRMS,

Great info! I should have known better to respond to this question. I learned something today.

 

Rpm is lost. The engine initially revs much higher than it would with a clutch. In some sense the stator etc gears down the crank's rotation to give more torque until "stall speed" is reached.

 

And this doesen't offset the loss of HP due to stall (slipage) and the auto's hydraulics. Manuals are more efficient.

 

Go to this site: http://auto.howstuffworks.com/torque-converter.htm
How Stuff Works is one of the more informative sites on the web IMO.