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
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
Last edited:
