ws6transam

Okay here we go: The difference between a clutch and a torque converter. Never before published, 'cause no one can explain it without math and physics.

First off, let's discuss the clutch. When the clutch is locked up, there is a 1:1 transfer of energy to the transmission. The drive wheels are essentially locked to the flywheel such that the only loss is from friction. The torque transfer is rated as Torque output divided by torque input. The best that a clutch can accomplish is a transfer factor of 1.

There are two kinds of friction coefficients: Static and dynamic. every type of material, be it concrete, aluminum, steel, diamond, rubber, or clutch, has two different coefficients of friction. Usually the static coefficient is greater than the dynamic. This is why a tire slows you down faster during braking if it isn't locked up. Once you begin to slide, the dynamic coefficient of friction applies. The same thing occurs in a clutch. Once you begin to slip a clutch, dynamic coefficient of friction applies, and there is less friction. Now, it doesn't matter how fast you slip the clutch: Friction is not determined by the relative speeds of the materials(otherwise known as slip). Friction is dependent only on the load you apply to the materials. Therefore, the harder you press the parts together, the more friction will exist. Now, a more powerful engine will allow you to increase slip at a given amount of friction, but it won't allow you to increase friction. That is entirely a function of the spring load inside the clutch! Because of this principle, a clutch will never output more torque than what is input. The torque multiplier will never exceed 1.

A torque converter does not use a clutch (except for locking at cruise conditions, which I will NOT discuss at this time). Instead, it uses a series of vanes and hydraulic fluid to transfer energy. One set of vanes, called the impeller, are attached to the outer housing of the torque converter, which is itself attached to the flexplate of the engine. As the engine spins, hydraulic fluid is picked up by the inner portion of the vanes. Centrifugal forces spin this fluid outwards where it picks up velocity. When it reaches the end of the vane, it is flung forwards at high velocity, right into another set of vanes. These vanes are attached to the input shaft of the transmission. This assembly is called the turbine. Think of them as a catcher's mit: They "catch" this high velocity fluid, and absorb the energy. Most of this energy is transformed into a torque onto the input shaft. Some of the energy is turned into heat, however.

In the case of a torque converter, the amount of energy received by the turbine is not a function of friction, but by the velocity (and momentum) of the fluid being thrown at it. Thus, output torque is a function of the velocity of the fluid being flung off the input vanes. Since the velocity of the fluid is related to the RPM at which the vanes rotate, output torque becomes a function of slip! In cases where the turbine and impeller turn at the same speed, slip equals zero, and torque multiplier equals 1. As slip increases, however, more energy is imparted into the turbine, resulting in an increase of torque. Thus, the torque multiplier can increase above a factor of one. What this means is that if you input 300 ft.lbs at 3000 RPM, and the stator is turning 2000 RPM, your output at 2000 RPM will exceed 300 ft.lbs.

This is obviously a simplified discussion of operation. Torque converters also include a stator, which is a device that uses a one-way clutch to redirect fluid from the impeller back at the impeller vanes. This in effect doubles the velocity of the fluid when it finally leaves the impeller and hits the turbine.

Whenever you transfer energy via fluid (be it air, or ATF), you can never transfer 100% of the energy. Therefore, a torque converter will "lose" energy during the process. In the case of a drag race, this is most evident at the top end of the track. The torque converter will let you launch hard due to torque multiplication, but as you approach zero slip, the advantage of torque multiplication fades to the advantage of having a 1:1 transfer of energy in a non-slipping clutch. This is why a clutch is an advantage for high MPH on the track. It's also why Precision Industries can charge so much for their converters: Their lockup-style converters are strong enough that you can actually lock them up under full throttle, making your Vigilante converter act like a clutch.

five7kid

iTrader: (14)

A couple of points:
1) The coefficient of friction will change with temperature as the properties of the materials change with temperature. That is why a hot clutch will slip more than a cool clutch.
2) The torque convertor does not multiply torque. It simply allows the engine to operate at a higher RPM, and therefore where it is putting out more power. The same thing is done with multiple disk racing clutches, which are set to engage at a particular RPM but allow slip below that (such as on top fuelers).

The only true way to multiply torque is by changing the fulcrum ratio between input and output. 3.73 gears will apply more torque to the wheels than will 2.73 gears, for instance. That is true torque multiplication.

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82 Berlinetta, orig V-6 car. Rescued w/86 LG4/TH700R with all harnesses, sensors, ECM, etc. 2.73 open. Cat-back from '91 GTA, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LB9 w/ZZ3 cam, ported heads, exhaust, paint, etc.).
57 Bel Air, my 1st car. Currently 396 .030 over, Weiand Action+, Edelbrock 1901 Q-Jet, Jacobs Omnipack, 1-3/4" headers, TH400 w/TCI Sat Night Special conv & shift kit, 3.08 10-bolt, AMSOIL syn lubes bumper-to-bumper. Best 15.1 @ 5800' Bandimere. Daily driver while Camaro was being put together.

ws6transam

Um, I don't know anything about dual-disk clutches, but in the case of the converter, if you were to apply a torque transducer on the crankshaft and another on the input shaft of the transmission, you would find that the torque applied to the input shaft would exceed the torque measured on the crank. This is torque multiplication. Typical multiplication fctors of 2 to 2.5 are not uncommon in most torque converter designs. In the case of a conventional slipping clutch, the torque on the crank would never exceed the torque measured at the transmission input shaft.