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9 inch diff transpalnt for au falcon auto

4K views 16 replies 9 participants last post by  useless 
#1 ·
Hi guys ...I have learnt in recent times that 3.7 gears on my au falcon will make the car hit rev limiter so ..i cannnot put it in with the auto.However I am contemplating a nine inch diff and was wondering how far we can take the ratios?
 
#6 ·
Plenty of different Diff ratios available for the Bog warner diff....
If you told us a bit about the car spec (6? 8? g.box? mods?) and use it'd be a bunch easier to help you pick a ratio.....

Ben
 
#10 ·
AUGZMK said:
Any gain you get from changing the diff gears will be lost by the weight of the 9" rear end. You'll do a full 360 degrees and end up where you started, with a lighter wallet and a lot of wasted time to boot.
Wrong. Yes, the 9" is heavier, but the gain made by a shorter ratio will easily outweigh the extra weight.
I've done it several times.

Rick.
 
#12 ·
But if you search the forums, you'll find heaps of guys running shorter Borgy diff centres... ranges from about a 3.55 (3.45?) LSD from skylines (and XR6's?) to 4.11 from a Pintara.
Gets you a shorter ratio, same weight as before (ie much lighter than a 9") and I don't think you'll break the borgy unless you turbo the six... The other thing of course is that while the 9" is reknowned for its strength, its also reknowned for being "lossy". ie it sucks alot of power.
Besides, at about $300ish swapping a centre in a borgy is alot cheaper than converting to a 9" ($400 + gears + conversion) and the car will handle better than with the 9"...

Ben
 
#13 ·
Soxx said:
Wrong. Yes, the 9" is heavier, but the gain made by a shorter ratio will easily outweigh the extra weight.
I've done it several times.

Rick.
On a standard 6cyl AU Falcon? With a 9" with 3.7 gears fitted?

Fitting a 9" with 3.7 gears onto a standard 6cyl AU Falcon IS a waste of time and money! Put the 3:45 gears that are already available, which can be done quite easily.
 
#14 ·
I agree for a daily driven ( not raced or dragged) 4spd Auto Falcon a 9" diff with 3.73 gears is not ideal.

I mucked about a lot with seeing what the std auto ecu could cope with and the best combination I found that works was just a simple change form 215/60R16 to 225/45ZR16 tyres with a grey speedo drive gear to correct the speedo.

This effectively changed the overall gearing of my stock 3.23 diff Auto by 8.35% or virtually changed the diff ratio from 3.23 to 3.52. A nice difference that allows you to leave it in drive and it revs out sensibly for a cammed non stock engine without going to the rev limiter.

I would recommend for a street car to either go 3.45 LSD diff and 235/45R17's or 3.23 diff with 225/45ZR16's. This is 10% and 8% lower than standard and remains fully streetable. 3.7 are 15% lower and this is great for acceleration but becomes much ore of a compromise.
 
#15 ·
AUGZMK said:
On a standard 6cyl AU Falcon? With a 9" with 3.7 gears fitted?

Fitting a 9" with 3.7 gears onto a standard 6cyl AU Falcon IS a waste of time and money! Put the 3:45 gears that are already available, which can be done quite easily.
I never said it is the right thing to do, I said it is easly done and sensible if you had one lying around, like I did.
I also meant I have doen several B/W to 9" conversions, not on an AU though, yet.

Rick.
 
#16 ·
i done it to my au using 3.7 2way LSD 9", however weapon of choice due to cupercharging and regular drag racing AND slicks ...

for street driven um ... useless

personally with and auto i would rather use the 3.45 out of the XR series for the BW, the 3.7 out of the VL or 3.9 out of the skyline or the 4.11 out of the pintara might cause the auto a big headache
 
#17 ·
You guys have been a great help...I now can see that anythig more than a 3.45 ratio is overkill.The increase in takeoff by bringing the revs on a bit quicker may very well the medicine this bus needs.to overcome takeoff launch bog.....once again thanks all
 
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