diff ratio change from 4.7 to 3.54

[tell]

Has anybody changed the diff ratio from 4.7 ratio to stock 3.54 ratio on 6x6 110 perentie?
would appreciate any comments
rear Salisbury diffs C8H  front land rover BYG70  gearbox LT95

[303Gunner]

Has anybody changed the diff ratio from 4.7 ratio to stock 3.54 ratio on 6x6 110 perentie?
would appreciate any comments
rear Salisbury diffs C8H  front land rover BYG70  gearbox LT95

No, I think you will be the first. Note that the stock ratio IS 4.7. Changing to 3.54 ratio on a 5.6tGVM vehicle with only a 4 speed gearbox will be too high. As suggested in the other thread you started on this topic, 4.11/4.12 would be better and even then you may have to lower the Hi range ratio in the transfer to compensate.

[Diana Alan]

Or boost the turbo charger pressure!  The current boost is quite low around 11cm, to provide longevity, you could increase the boost to about 15cm (without stressing the engine too greatly) and add an intercooler which will give you a little better performance to cope with the higher gearing.

There are people who have boosts at around 20cm on 4BD1-T.  Pop over the the Isuzu Land Rover section on  the AULRO.com site and do some searches on what can be done.

[303Gunner]

The Stock gearing for the 6x6 is:

LT95A
1st: 4.069
2nd: 2.448
3rd: 1.505
4th: 1.000

Hi: 0.996
Lo:3.321

Diffs: 4.7

Your current overall ratio in 1st gear Hi range is 19.04:1, while 2nd is 11.45:1

Changing to 3.54 diffs will make 1st gear 14.34:1 and 2nd will be 8.63:1.

To see what the effect of fitting the 3.54 diffs will be, load the vehicle up with a bit of weight or throw a trailer on and drive it around for a while using ONLY 2ND GEAR and above, because you WILL lose the reduction of 1st gear. Include a few hillstarts in the drive. You might find that while you can live with this at unladen weights without too much slipping of the clutch, at close to GVM you will inevitably be burning clutches at a rapid rate.

Unfortunately, this trial won't demonstrate to you the resulting increase in cruising speed, or even maximum speed. The 6x6 is currently advised with an 80km/h top speed, and your diff ratio change could theoretically increase that to around 105km/h for the same revs, but this won't be acheived at GVM weights due to avilable engine torque.

[fc101]

see what the effect of fitting the 3.54 diffs will be, load the vehicle up with a bit of weight or throw a trailer on and drive it around for a

This is an important point - these vehicles are designed to carry weight but in reality in civilian service most will only be lightly loaded.  So if it is going to be a working vehicle carrying a lot of weight you will need to leave the overall gearing as it is - but if a show pony like most of our vehicles and carry nothing like it is capable of the change the diff ratios.

My 101 has a 1 1/2 ton capacity and it normally gets around with only a couple of hundred kilos in it. 

[tell]

Thanks for all your comments greatly appreciated. I will be turning this vehicle into a motor home. The rear of this land rover has a very heavy roll cage, which I be removing completely. I hope by the time I finished with it, the weight will be much less then what is now. I want be loading 8 soldiers either. However Its probably better to build the motor home part first before I make any changes to the diffs. This will also give me the opportunity to do more research.
 
Thanks again, and if anyone of you comes across someone who has made any changes to diff ratios, I love to hear about it.

[Barefoot dave]

I can't add anything about the change in driving characteristics that 303 hasn't already said.
MR Auto has done 1 conversion already, they quote $7000 for the job. Other specialists say cost is dependant on parts price, but expect similar.
I am told it is not just a case of replacing the CW&Ps.
Am hoping to be put in touch with the new owner to get some feedback.

[Diana Alan]

On Maxidrive diff centres, there was a spacer ring that was needed to to be fitted for 4.7 CW to align with the pinion.

[303Gunner]

I am told it is not just a case of replacing the CW&Ps.

The differential carriers in a 4.7 diff (both Salisbury and Rover) are different to those in a 3.54 diff due to the different thickness of the Ring Gear, so factor in the price of 3 new diff centres as well. You can use spacers to fit gears designed for a 3.54 carrier (which include 4.11 R+P sets) to a 4.7 carrier, but not the other way around.

Most aftermarket carriers, including traction aids like Trutracs and Ashcroft lockers and LSDs are designed for the 3.54 dimensions (although some offer 4.7 alternatives), but with metric bearings and 24 spline axles. The 4x4 Perenties have 10 spline axles and inch bearings, but not sure about the 6x6. So possibly factor in the cost of new axles and bearing spacer sleeves too.

It may be that your cheapest option will be to find one and a half 4x4 owners looking to swap over to 4.7 diffs, but even then you would be looking at a bit in labour costs, especially with 2 Salisburys to set up. It was reported in another thread that someone was quoted $800 in labour to set up just ONE Salisbury diff with a locker.

[John H]

There is also the possability of adjusting the governer.After the mechanic I worked with tinkered under the bonnet we were able to travel at 110km+,this was needed to keep up with the convoy after unscheduled pub stops.

[fc101]

It was reported in another thread that someone was quoted $800 in labour to set up just ONE Salisbury diff with a locker.

No me, but I was also quoted about $850 to put a ARB Locker into the front Salisbury in my 101.

Garry

[Barefoot dave]

Mr Gunner, YOU are the God!

Nice explanation, thanks.
BDave.