diff ratio and 4wd ability

[2002tiiguy]

Unless you have completely rebuilt 4.10 front and rear diffs sitting in front of you, just rebuild the 3.91.

[mrsleeve]

fuel mileage or top speed is what a lower ratio is good for, higher rates are for more grunt out of lower out put power plants or for adding torque

[ACVC325ix]

Thanks!

[nando]

sure - if you don't have a broad power band with a high redline, if your engine already has a lot of torque, or if you cruise a lot on the freeway - you don't want a high ratio for that.

[ACVC325ix]

So noob mechanical engineering question, but is a higher ratio diff always better then?
I have a manual but should I be looking to throw the 4.1 from an auto on there instead of rebuilding the 3.9?
Is there any scenario where a lower ratio diff is preferable?

[The Dark Side of Will]

The subject of this thread.

[2002tiiguy]

what am i doing with which car?

[The Dark Side of Will]

^^ this is what i was talking about, actual numbers, not percentages and personally i am as well, but i already have one screamer 74 2002tii 70mph @4k approx

Lol. What are you doing with this car?

40% is only relevant when the rear wheels have traction.

[2002tiiguy]

the VC doesn't really "transmit" torque. It essentially locks the front/rear output shaft speeds together so that one end can't spin faster than the other end. it's not possible for more torque to be sent to one end or the other than what the planetary gears are set to.

But yes, whatever the 40% split to the front ends up being, having a higher multiplication from the 4.10 will result in more being put down. Personally, I'm a fan of shorter gears and a higher redline..

^^ this is what i was talking about, actual numbers, not percentages and personally i am as well, but i already have one screamer 74 2002tii 70mph @4k approx

[The Dark Side of Will]

Absolutely it can transmit all of one end's torque to the other end. How do you think these cars can move with no front driveshaft?
(Or drive the front wheels with the rear on ice in my example above)

[nando]

yes, but it's in an indirect way. it can't for example transmit all of the torque that was sent to the rear wheels - the front wheels still only get 40%.

[The Dark Side of Will]

the VC doesn't really "transmit" torque. It essentially locks the front/rear output shaft speeds together so that one end can't spin faster than the other end. it's not possible for more torque to be sent to one end or the other than what the planetary gears are set to.

The VC absolutely transmits torque, but it does so on the basis of speed *difference*.

Example: If the rear tires were on ice and had zero traction while the front tires were on dry pavement... the rear tires will start to spin while the front tires won't. This will create a speed difference across the VC which causes it to transmit torque from the rear output of the T-case up to the front output.

[ndfspd67]

the VC doesn't really "transmit" torque. It essentially locks the front/rear output shaft speeds together so that one end can't spin faster than the other end. it's not possible for more torque to be sent to one end or the other than what the planetary gears are set to.

But yes, whatever the 40% split to the front ends up being, having a higher multiplication from the 4.10 will result in more being put down. Personally, I'm a fan of shorter gears and a higher redline..

Shorter gears and higher redline, my kind of man right there haha

[nando]

the VC doesn't really "transmit" torque. It essentially locks the front/rear output shaft speeds together so that one end can't spin faster than the other end. it's not possible for more torque to be sent to one end or the other than what the planetary gears are set to.

But yes, whatever the 40% split to the front ends up being, having a higher multiplication from the 4.10 will result in more being put down. Personally, I'm a fan of shorter gears and a higher redline..

[The Dark Side of Will]

but, lets say the viscous coupling can transmit 150 ft lbs of torque, thats 150 ft lbs to either the front or rear, multiplied by the differential ratio, therefore the 4.10 ratio differential would have more torque available correct?

If you're simply talking about ftlbs to the front axle, yes.
Are you going to enter a FWD racing class?