How easy is it to change the crown wheel & pinion (p38)

[Chicken Drumstick]

I like my 4.6 p38, but with the auto and stock gearing it feels a little stretched. The engine does its best work at 3500rpm onwards. Stock gearing gives you a bit of a ratio gap between some gears where you end up fairly low in the rev range, but it is going to quick to want to kick down. I feel some shorter gearing would massively 'wake it up'. Albeit at the sacrifice of cruising rpm.

There doesn't seem to be any options I can find on the transfer box high ratio gearing. So I think the diffs are the only real practical option. Ashcroft Transmissions offers replacement diffs, but they work out about £1000 a diff with no extra's, just changing ratios. That is a little more spendy than I can really justify. So was wondering if there are other options? I see Ashcroft sell the crown wheel & pinion for £195 + VAT:

https://ashcroft-transmissions.co.uk/product/4-10-r-p-for-the-p38-type-diff/

Still quite a chuck of money, but less than a 1/4 of the price of going for fully rebuilt diffs.

Are these easy to swap in for a home mechanic with basic tools? Would I need to worry about setting backlash or anything or is it just a case of unbolt, swap in new bits and bolt back together?

[Bowie69]

Add new bearings, shim set for each, and hours of setting pinion depth, backlash and pattern - in short, it is really quite an involved process. 

All doable with dial indicator, hammers and lots of patience though. 

Conversely, swapping a diff centre out is relatively easy, you just need to set backlash and pattern, normally they are OK if you keep the r&p set. 

 

 

[Escape]

Getting the diffs out isn't too bad, though the front requires a bit more work than on the Classic.

As @Bowie69 says, setting everything up correctly is not that easy, and you ideally need 2 full sets of shims. You can cut corners of course, but not without risk and with the time and money involved, better to play it safe.

I am a bit surprised you feel your 4.6 P38 is over geared though. I rarely feel the need to go to 3500rpm, unless I'm towing a heavy trailer. My current one is a manual, but I've driven plenty of automatics and never noticed a problem unless the engine was past its best (worn timing chain and/or camshaft for example). In fact, I love the way the 4.6 cruises just fine at 30mph around 1500rpm, and can (gently) pick up without the need to shift down. I do have a high torque cam fitted and, more importantly, have a pretty relaxed driving style. Especially in the Range Rover. But it's not just me, I recently had a mate as a passenger, while towing his P38 on the trailer. He commented how smooth the car drove, without the need to go much above 2000rpm. Just my 2cents. 😉

[Bowie69]

What tyres are you on? Makes quite a difference. 

Have driven 4.6 P38s and always thought they had quite a bit of urgency if you pushed the loud pedal, never thought they were overgeared, the 3-4 shift is pretty short, but other than that, all good IMHO. 

As @elbekkosays, are you sure it's running right? 

[Chicken Drumstick]

It goes very well overall. Running near stockish tyres, 255/70R16 if I recall correctly, although they are an MT. Engine runs great, it is a Thor engine. Had it on a dyno a couple of years back and it was making all of its factory rating. But like all 4 speed auto's it has quite tall gearing. I think 1st gear is nearly 50mph at the red line and over motorway speeds in 2nd.

So there are times when it will end up fairly lowish in the revs, but too high to kick down. It is only a minor thing, but noticeable. Another 100hp would likely mask it mostly. But I suspect it is cheaper to look at the gearing.

The V8 does its best work at higher rpm, I think it makes Peak power at the red line. Not saying it lacks low end grunt, but in order to use its full potential when you go to wide open throttle, using the right part of the rev range makes a big difference.