Stall Converter's affect on Crawl Ratio

[WICKED]

How does a stall converter affect the final drive ratio?

I have around 36:1 with an Auto. Brother's is around 100-110:1 with a Manual.
They feel very similar in Low but with such a massive difference in numbers, how does the stall converter in the auto affect that?

[oldmate]

A torque converter increases torque by 2 to 3 times. That's why auto's can have such tall first gears.

[WICKED]

A torque converter increases torque by 2 to 3 times. That's why auto's can have such tall first gears.

So are you saying that the above crawl ratio of my auto setup is 72:1 - 108:1?

[oldmate]

No, the 'crawl ratio' is just the multiplication of the gear ratios. so your ratio is always going to be 36:1

The torque multiplication of a converter simply makes moving a higher geared vehcile much easier. up to 2 or 3 times torque multiplication is common on a modern convertor, but all depends on the 'stall ratio' of the converter

For example if your engine puts out 300 foot pound of torque, for a brief moment you would be getting 600 to 900 foot pound of torque. This only happens when the engine speed is much faster than the transmission speed, and it's primary advantage is to overcome the inertia of the stopped vehicle, that is the resistance of the vehicle to start rolling.

Only other way I can think to explain it is to take a manual car. Try starting off in second from a stand still, it will take a while and will be hard on the clutch. Get the car moving at 2-3kph and take off in second it will be considerably easier. That's because the initial inertia has been overcome.

[Gwagensteve]

It's fair to double the raw crawl ratio to get a feel for how an auto car's gearing will compare to a manual.

Most converter ratios are in the range from 1.8-2.4:1.

Bear in mind that once you above stall speed and there's far less slip, the converter ratio doesn't apply any more so if a car is badly undergeared it will still suffer.

Steve.

[brooksy]

A torque Convertor is basically a clutch for autos. It is in effect a fluid coupling that increases force to the transmission via speed increase from the flywheel. Fluid goes threw various fins from one half of the convertor to the other which allows the slippage & torque increase to occur.
The number & angle of the fins combined with the convertor housing determines the torque slippage limit before 100% drive.
This slippage is what makes you feel that the reduction/crawl is similar to Scotts. You are correct somewhat in thinking this as the crawl is actually a result of the convertor not allowing 100% drive & multiplying it but not ideal to use as a reduction device unless you have a locking up clutch. Positive reduction is definately best but the torque convertor does allow less ratio to produce the same when compared to a manual.





brooksy