HP & Torque

[TheEngineer]

Have you ever wondered what the HP output and Torque curves for our engines looked like?
Torque is the force delivered by any rotating equipment or machine. When you tighten cylinder head bolts you apply torque. It is measured in force times lever-arm. Like kg x meter or lbs x feet. An electric motor puts out torque and our Mercedes engine puts out torque. A gear box is a torque multiplier: When there is a gear reduction of 3:1, the torque is multiplied tree times. On our W113 cars, fourth gear is 1:1 and the only set of reduction gears is in the rear axle. In the graphs, a rear axle ratio of 3.92 is shown. Therefore, engine torque is multiplied 3.92 times. If you want to know the force at which the tire pushes on the road, you divide the torque at the wheel by the rolling radius of the tire. This force propels the car forward. When you are in a lower gear, like first gear, you have an additional 4.05 reduction (in the G72 4-speed gearbox) and you get four times more force to the tires. This is where very high powered cars sometimes spin their tires: When the force available is greater than the friction between tire and road. Internal combustion engines need to turn up quite fast to develop significant torque. An electric motor does not: It develops it’s highest torque at standstill. This is why a combination of electric motor and gas engine, like in the Toyota Prius, is so successful.
The characteristic of the gas engine to have little torque at idle speed is successfully modified by adding a torque converter. The torque converter multiplies engine torque by about two at idle speed and locks up at about 2000 RPM. At lock-up it transmits engine torque without multiplication. There are losses that become heat: Gearboxes become hot and torque converters need cooling. As a result there is less force available at the rear wheels. This is where advertising comes in: SAE (Society of Automotive Engineers) calculates HP of the engine without losses from driving the oil pump, the alternator, the cooling fan, the distributor or the water pump. DIN (Deutsche Industrie Normen) includes these losses and measures the actual output of the engine at the flywheel. The tables are for a 280 SL/8 engine as sold in Europe 1968 - 1970, US spec. models came with emission control and deliver 160 HP DIN and 24.5 mkp at 4250 RPM.





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[Cees Klumper]

Quietly for years now I have been asking myself what 'torque' means and how it is relevant. I always thought that horsepower was the indicator of an engine's power, but then what does the torque value signify?

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[DaveB]

Torque is rotational force, or twisting force. Cars with a lot of torque, for example most diesels, are flexible to drive because you don't need to change down so often - they can comfortably drive at low speed in reasonably high gears.

DaveB

[rwmastel]

I may be way off, but I believe an over-simplification would be: Torque gets you going, HP keep you going.  Is that correct to say?

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420

[DaveB]

that's enlightening Engineer!
thanks

DaveB

[jimj]

Torque is normally thought of as the engines capacity to do work, and horsepower is a measure of how fast the engine does that work.  On all  dynos, torque is always measured and horsepower is always a calculated value.

(from my days as a hotrodder.)

[Cees Klumper]

What I gather from this is that torque is the better indicator of engine 'power'.

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[JimVillers]

I am not sure that the description of a "torque converter" is correct.  The torque converter in an automatic the transmission is a fluid coupling that allows the engine to turn at a different speed than the transmission.  This allows the engine to idle while the car is stopped.  As the engine speed is increased, the hydraulic forces in the torque converter turn the transmission shaft.  Unless there is a mechanical lock-up, when under power, the engine will always turn faster than the transmission.  The torque converter does not multiply torque; it just lets the engine run at an RPM where the engine has more torque than if it was running at the transmission RPM.

Torque is the primary parameter of "derivability"; it provides the "force" or feel of acceleration.  It is the force that allows you to drive up a hill.  Horsepower is speed of the force; it more horse power, the faster that you can drive up a hill.



Jim Villers
190SL, 230SL 5-Speed, 190E 2.3-16 Kompressor