Diff gears

[dhs]

I'm just curious, how much it would cost to get my diff rebuilt with 3.9 gears? What kind of places do this work? It's sorta out of the question if I'm not mechanically minded isn't it?

My car has 2.78's stock, so would 3.9's give it much more whack off the line?

[LJ38L]

I take it that it's a VH V8 Commodore?
3.9's will give it more wack off the line but will rev much higher when cruising. If you dont mind reving the nuts of it go 3.9's, personally I would put 3.5's in it.

[dhs]

Yep.. Will 3.55's still make much of a difference?

[[ryan]]

Well it's a happy medium, it'll make a diffence just not as dramatic as the 3.9's i guess.

[dhs]

True, with a Trimatic+15" wheels what rpm will I be at on 100km/h? At the moment I sit on 2500rpm at 100km/h on the freeway..

[LJ38L]

Originally posted by dhs
True, with a Trimatic+15" wheels what rpm will I be at on 100km/h? At the moment I sit on 2500rpm at 100km/h on the freeway..

with 3.5's it will sit on close to 3200rpm 3.9's or 4.11's close to 4000rpm.

[FASTVP]

With 205/55/R15's, a 1:1 3rd ratio on your Traumatic, and your current 2.78 diff ratio, your 100kph engine speed should be about 2450rpm.
With 3.5's it'll be about 3050rpm.
With 4.11's it'll be 3600rpm.

[dhs]

Thanks for the help

[FASTVP]

Oh yeah, you'll need to account for a small amount of converter slip, too. Won't make that much of a difference though unless it's buggered.

[LJ38L]

I guessed pretty well then

[dhs]

Also if my VH is stock (well extractors, exhaust, shift kit) with 225's how much can the diff gears knock off my 0-100 times? About a second or so?

[FASTVP]

308 or 253?
Depends on what ratio you get.
3.5 is a fair jump from the crappy 2.78. 4.11 is an enormous jump.
I don't know what difference in 0-100 time it will make, but you will definately notice the difference.

[dhs]

253 for now. I'd probably get a 3.55 since they seem to be more common and it's a good compromise. Will it be dramatically quicker off the line?

If I just buy the diff center is it a big job to do it myself?

[FASTVP]

You will notice a fairly dramatic difference.
I changed my whole axle over when I did mine. Not sure how much work is in changing just the centre.
Someone else here should be able to help you out with that. Best to ask this in technical though...