dave4x4

Active Member
I have replaced the ZF 4HP22 gearbox on my diesel p38 with the stronger 4HP24 as used on the 4.6 V8. A few people have asked about the details involved so here are a few notes:

Background: I have got a chipped 2000 DSE. It has a Jeremy Fearne V2 chip and a larger Allisport intercooler / performance air filter. EGR valve has been removed. I know the J Fearne conversion produces 190 bhp with the boost turned up to 1.3 bar. I haven't upped the boost yet. Mine is still on 1.0 bar so I would guess the engine is putting out about 175 - 180bhp. More importantly torque is on a par with the 4.0 V8. I usually run it on a 50/50% mixture of veg oil/diesel. I tow a heavy caravan and I have had a couple of "gearbox fault" warnings, so I guess was on borrowed time.

I had the gearbox assembly done by Ashcroft Transmissions based in Luton and the actual fitting by Autoland, Dunstable.

As I posted before, apparently there are two versions of the HP24 gearbox used in the p38 - an early one and a later type known as an 065 box. The number will be found on the gearbox plate.

It should say something like 030 - 065. This is a direct swap for the HP22 after the diesel bellhousing and torque converter are fitted.

The earlier HP24 is 15mm longer and I am told by the main man that the diesel tailend housing can be fitted to bring it back to the same length overall. Other than that I don't know as I haven't tried it.

Another alternative Ashcrofts do is to fit HP24 internals into the HP22 box.

If anyone one else is contemplating a HP24 conversion, I would suggest the easiest route is to find a later 065 box. I am no expert in any of this however. If in doubt talk to Ashcrofts.

Also only the diesel torque converter will fit into the diesel bellhousing. The 4.0 and 4.6 torque converters are too big. I had Ashcrofts fit their own uprated diesel torque converter that is modified internally to have a lower stall speed. It quote "reduces the revs and takes up the drive earlier which results in much more relaxed driving". I don't know if anyone else does one.

I also had a new XY switch fitted and a boss welded on to the gearbox sump to take a thermocouple. For those who are interested it is tapped M16 and welded to the rear sloping part of the gearbox sump behind the drain plug. It is out of harms way yet still in the transmission fluid. I have modified my dash panel to fit a digital temp readout using one of the unused warning bulb spaces. Fits nicely behind the black panel.

The gearbox fitting was straightforward without any problems and was a direct swap. All the bolt holes matched up. The original diesel ECU was used.

I have done about 200 miles now so it is early days. First impressions are the new gearbox is very quiet compared to the old one and gear changes are very smooth. I didn't realise how noisy the old one had become, also when it is cold it takes up the drive immediately unlike the old one. Through the gears it takes up the drive better and there seems to be more bite or pull from the torque converter. There is definitely less slippage. It is nicer to drive making better use of the torque. It may not be suitable however for an unchipped diesel that has less torque, imho. The gear changes are earlier but I think it can still be further improved by altering the ECU. Dave Ashcroft thinks he has found someone who can alter the gearchange points. I am going to be the guinea pig so I will send him a spare ECU. The workshop guys also tested the car and also say it changes earlier. It is a nice drive was the general opinion.

Costs for the work that Ashcrofts did were £300 for labour and £150 for the uprated torque converter.
The situation with my gearbox was a bit complicated. I discovered the box I had was not an 065 but a HP24 for a disco 4.6, so required a different tail end to make it into an 065. I also px'd the new 4.6 torque converter, bell housing and XY switch I had. The costs were for assembling and testing the gearbox on their test rig. Included in the cost was a new p38 XY switch, diesel bellhousing, tail shaft and housing and welding the thermocouple boss. Two springs in the valve body were also changed. There is 12 months warranty. I could have got £200 off for my old box but as there is nothing actually wrong with it, I have kept it.

Fitting was £325 on top and was done at Autoland, one of Ashcrofts fitting stations, and included use of a courtesy car. Fitting was done in a day. All costs plus VAT. Not cheap but I intend to keep the car. I already had the gearbox so that is not included.

I have only got the engine to worry about now.

Dave
:)
 
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Hi Dave,
About the uprated converter do you really think it picks up drive (less slip) sooner or is it just a placebo effect?
Good luck with the box and thanks for being the guinea pig :)
 
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Yes. There is less slip. The change points for the intermediate gears are definitely lower. Not so sure yet about the lock up in top. At gentle throttle openings it locks up at about 45mph but at higher throttle openings it is about 50mph. When I floor it and use kickdown, it is as late as 60mph.
Should have taken some readings with the old converter.
3 of us have driven the car including the foreman and mechanics in the garage and everyone says it changes sooner. Not the placebo effect. I though am comparing it to before and after with my car. The mechs are comparing it to other p38's they have had in.

Dave
:)
 
Early days yet. Don't know about mpg figures, but yes in theory

less slip = less heat = greater efficiency = more mpg
 
We are talking about the HP24 gearbox here, as used in the 4.6 V8 and also the V12 BMW.
The puny 2.5 diesel won't generate anything like as much strain.

I have faith in Ashcroft Transmissions knowing what they are doing.
 
We are talking about the HP24 gearbox here, as used in the 4.6 V8 and also the V12 BMW.
The puny 2.5 diesel won't generate anything like as much strain.

I have faith in Ashcroft Transmissions knowing what they are doing.
they do but diesels are harder on boxes than petrols by a wide margin,petrols also produce there torque different not low down you will get less heat thats were torque goes when not past through to wheels ,overheating isnt a general issue ,you will also but more strain through box ,but not saying its a bad thing to do, but you dont get owt for nowt hp 24 improved clutch A ,C and freewheel are better ,if you look at earlier clutch A there is usually signs of slip even if still serviceable but later usually like new as other clutches
 
Overheating gearboxes is the biggest cause of failure according to Ashcrofts and slippage is the cause of heat. That is why I have fitted a thermocouple at their recommendation. Torque is still less than the 4.6 even with my chipped diesel.

James, ask me in 10 years how is my gearbox doing.:)
 
Overheating gearboxes is the biggest cause of failure according to Ashcrofts and slippage is the cause of heat. That is why I have fitted a thermocouple at their recommendation. Torque is still less than the 4.6 even with my chipped diesel.

James, ask me in 10 years how is my gearbox doing.:)
im not selling you anything ,and i didnt say youd done anything wrong ,but diesel boxes are by far the most likely to fail and freewheel isnt damaged by over heating 24 is the way to go
 
OK James. Thought you were saying I had made a bad move.

Ashcrofts also said they see more p38 diesel box failures.
 
James, maybe I should re phrase that.
Of the p38 gearboxes Ashcrofts see, the majority are diesel boxes.
 
James,
the main man said overheating is one of the main causes of warranty claims. That and wrong assembly when the pump is damaged by the torque converter.
They see the evidence when the gearbox is "cooked". 300 deg F and your gearbox is history.
 
their warrenty claims or lrs ,clutch burning produces heat its difficult to say for some which came first ,but over heating causes damage to all clutches not just one which you do see from time to time but usually not just 1 or 2 and the rest of box like new with no signs of heat ,std t/c can run around all day at less than lock up speed and not over heat as long as coolers good
 
You need to look up some torque figures, a chipped diesel is likely to produce more torque than a 4.6 and from considerably lower down the rev range but still within the limits of the 24.
 
I have got plans to increase power even more.
I want to maximise torque between 2000 and 3000 rpm

Dave
:D
 

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