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fredmeister 06-13-2013 01:21 PM

Quote:

Originally Posted by TurboKraft (Post 7495318)
Ideally, a 1:1 drive ratio. For a 930 4-sp, 3rd is closer to 1:1 than 4th is.

Why is it so important for a 1 to 1 drive ratio?

I know running a higher gear gives you more time to get load and high boost on the engine, but do not understand the importance of 1 to 1 ratio in the tranny.

Fred

WinRice 06-14-2013 11:25 AM

Quote:

Originally Posted by fredmeister (Post 7496798)
Why is it so important for a 1 to 1 drive ratio?

Because gear ratio can increase or decrease output torque by a factor of the gear ratio, not accounting for friction losses.

From Wikipedia:
A gear train can be analyzed using the principle of virtual work to show that its torque ratio, which is the ratio of its output torque to its input torque, is equal to the gear ratio, or speed ratio, of the gear train.

This means that the input torque T<SUB>A</SUB> applied to the input gear G<SUB>A</SUB> and the output torque T<SUB>B</SUB>" on the output gear G<SUB>B</SUB> are related by the ratio

<DL><DD>http://upload.wikimedia.org/math/d/4...74a6e6cc97.png</DD></DL>where R is the gear ratio of the gear train.

fredmeister 06-15-2013 04:41 PM

Quote:

Originally Posted by WinRice (Post 7498548)
Because gear ratio can increase or decrease output torque by a factor of the gear ratio, not accounting for friction losses.

From Wikipedia:
A gear train can be analyzed using the principle of virtual work to show that its torque ratio, which is the ratio of its output torque to its input torque, is equal to the gear ratio, or speed ratio, of the gear train.

This means that the input torque T<SUB>A</SUB> applied to the input gear G<SUB>A</SUB> and the output torque T<SUB>B</SUB>" on the output gear G<SUB>B</SUB> are related by the ratio

<DL><DD>http://upload.wikimedia.org/math/d/4...74a6e6cc97.png</DD></DL>where R is the gear ratio of the gear train.

Got it now i feel stupid for overlooking this. I was thinking wheel horsepower and mistakenly thought gear ratio has no effect but you are right sir. So I guess the dyno does a conversion based upon the gear ratio you enter and then converts to RWHP. The of course you can back calculate HP at the crank using a good suitable tranny loss number which most people accept 12-15% for the 911.

Thanks
Fred


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