Author Topic: replacing entire diff or diff centre with taller ratio  (Read 3808 times)

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Offline wombat

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Hey - I promise to make this my last post in ignorance (for a while)

R31 Pintaras, as we know have a 4.111 diff - giving 3100 rpm at 110 km/h. A 3.7 diff reduces this to 2800

I want to lower this by replacing the diff centre in the Borg Warner 78 diff with a BW78 from either a skyline or a VL Commodore. Probably to a 3.7 from an R31 Skyline (Manual - auto diff was 3.889) just to knock a few hundred revs off cruising speed.

I'm aware that i'll have to use a 3rd and 4th more around town. I'm aware and prepared for this.

What I want to know is how interchangeable the centres are, and also how possible is it to swap the whole rear axle from a skyline, just because i reckon that's easier than changing a diff centre.

Cost estimates are appreciated too!
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Offline wombat

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Re: replacing entire diff or diff centre with taller ratio
« Reply #1 on: Apr 4, 2005, 09:32PM »
have spoken to a diff rebuilder in town. It can be done... it's a question of money. About 1100 for a rebuild with a new centre gear. More than i can justify.
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Offline Budgie

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Re: replacing entire diff or diff centre with taller ratio
« Reply #2 on: Apr 4, 2005, 10:14PM »
You might be able to source a skyline wreck and make the entire swap wombat.

Sounds to expensive to consider any other way.

Do you do enough highway k/s to justify the lower cruise rpm the diff will provide?
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Offline wombat

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Re: replacing entire diff or diff centre with taller ratio
« Reply #3 on: Apr 4, 2005, 10:41PM »
The expense is too much. It'll stay whiney... just like all manner of girlfriends
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Offline KNG P1N

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Re: replacing entire diff or diff centre with taller ratio
« Reply #4 on: Apr 5, 2005, 08:35AM »
might as well get a second hand lsd
And there's not much I can say
But we miss you every day
And I'm bitter 'cause it's unfair
We'll be drinkin' in the sun
Playing music having fun
Cause one day I'll see you there
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Offline noss

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Re: replacing entire diff or diff centre with taller ratio
« Reply #5 on: Apr 5, 2005, 10:59AM »
the centres are interchangeable but i guess you know that now, i should have read this earlier. people with rb30's swap in pintara diff's to get faster acceleration, and i guess swapping in the rb30 diff would do the opposite for you so i guess you should ask yourself if you're prepared for your car to be slower?

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Offline wombat

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Re: replacing entire diff or diff centre with taller ratio
« Reply #6 on: Apr 5, 2005, 12:36PM »
I know the trade off with any gearing

taller diff/gear = acceleration is slower but lower revs for fixed speed

shorter = better acceleration, but you can run out of revs at terminal velocity

I asked, merely because I'm sick of sitting on 3kRPM at 100km, and wanted to knock it down a peg or 2. I thought that would be the easier option than trying to change the 5th gear ratio in the FS5W71C gearbox down to about .75 or .7 instead of .838.

It might be the easier option, but don't know about the cheaper option.

Speed is not the issue, it's comfortable cruising. Next fix might involve sound deadening foam just to shut the diff up for now.
I'm so far behind at work, I'm almost retro!