This LRO reader emailed LRO.firstname.lastname@example.org to ask Julian Gilling about replacing a 4.0-litre engine in his Range Rover P38 for a 4.6-litre block.
I bought a P38 4.0 HSE (W-reg, Thor engine) and have discovered I’ve probably got a blown head gasket, (according to tests that were carried out by independent mechanics). I’m now thinking of stripping the engine down to the block and replacing it. I’m tempted to buy a 4.6 block instead of getting the original 4.0 size – others who’ve had the 4.6 P38 engine say it would give better power and fuel economy. If I did this, can I do a straight short-block swap and bolt all my other
engine parts on to the 4.6? Or will there any issues that can arise from this mechanically? I’ll notify DVLA, of course.
Olie Hulse, Southampton, Hampshire
The 4.0 and 4.6 blocks are basically the same, the difference in capacity being due to a diering crankshaft, conrod and piston combination. If the change is simply to increase power and performance I would advise some caution afterwards, as the gearbox fitted to the 4.6 was of a higher torque rating to match the additional performance figures. If you decide to swap, you’ll need to ensure the bottom end is all 4.6 – to give the extra capacity – and that everything else is 4.0.
The fuelling and ECU from the 4.0 should cope with the change without issue. If you decide to go along the 4.6 route, make sure the block you use has had top-hat liners fitted, because the 4.6 has been prone to porous bores due to misaligned machining during manufacturing. Finally, if your original block has low wear and no serious problems, I would be inclined to cure the head gasket problems and leave it as standard.
This workshop advice appeared in the May 2016 issue of LRO. Back issues are available to download on digital devices here.