PDA

View Full Version : Tech question


thilton59
03-02-2006, 03:18 PM
This really isn't specific to our marque or model, but nonetheless;
In reference to final drive ratios (3.25, 4.1, 3.73, etc...) my interpretation is skewed. It seems everyone says that a 3.25 will, all things being equal, accelerate slower than a 4.1, but how can this be? If the ratio is 3.25 rotations of the the drive shaft to one of the axle, then the wheel speed will be higher with the lower ratio, 1:1 being the fastest, albeit unpractical. And then there's the bit of more torque with the lower the ratio, which again make little sense to me, seeing that T= r x F, the higer the ratio the more wheel torque, I'd think. Any help un-conveluding this for me would be much appreciated.

Pinecone
03-03-2006, 08:44 AM
F=MA. or A=F/M Force is torque acting over the tire rolling radius. Gearing increases torque, so by a higher numerical ratio, you get more torque delivered to the rear wheels which mean smore acceleration.

It also means that you will need to shift at a slower raod speed. Which mans that you might end up wiht a slightly slower 0 - 60 time if yo uhave to add an extra shift. Most cars are designed to give 60 MPH in 2nd to keep the shifts to the minimun. But iun any given gear the acceleration will be greater with the numerically higher rear axle ratio.

thilton59
03-03-2006, 01:18 PM
F=MA. or A=F/M Force is torque acting over the tire rolling radius. Gearing increases torque, so by a higher numerical ratio, you get more torque delivered to the rear wheels which mean smore acceleration..

mmkay, but the higher the ratio (ie; 4.1:1) is torquier than a low ratio like 3.25:1?
This isn't what I've heard. I could be wrong, but if you could extrapolate that further, inasmuch that you define why "a higher numerical ratio gets more torque..." :dunno:

Pinecone
03-03-2006, 06:57 PM
You put 100 pounds feet of torque into the diff input shaft. A 3.25 diff will deliver 325 pounds feet of torque to the rear wheels. A 4.10 will deliver 410 pounds feet. More torque, more acceleration.

And I have never heard anyone claim that a 4.10 is less "torquier" than a 3.25.

You have to be careful, a lower diff, generally means a higher numerical ratio. So in common use a 4.1 is LOWER than a 3.25, even though the number is higher. Not sure why, but it has bbenn like this for YEARS. So that is why I specify numerical ratio.

thilton59
03-04-2006, 01:47 PM
Ok, the whole common use thing is where I'm having problems (intercooler/aftercooler...), but you've cleared up the mess a bit, thanks.