A Word On Engine Conversions
Engine conversions are very popular in the Land Rover world, most popular is the anything-to-200TDi conversion on the grounds that the 200TDi is a very good engine and has gained many fans over the years, and that everything else isn't a 200TDi and therefore is pants with sick on and must be exorcised.
There are, however, a lot of gaps in some people's thinking about engine conversions (as with most modifications really) and I'll list a few of them here:
Standard parts: Using an engine from a different manufacturer, or doing a conversion that requires special parts, means you can't get all your parts from one supplier and potentially can only get some parts from a single source. Not so bad for trundling round near home, but stranded in a foreign country with a busted clutch that can only be sourced from a specialist supplier with a 3-week lead time and a price tag to match is not fun. Even just getting service parts is a pain if you have to buy your oil filter from one dealership and your air filter from another. This is made even worse if you have to use non-standard parts to make everything fit, so before you know it you're using a Renault oil filter on an Isuzu engine in a Land Rover…
Standard fit: Manufacturers spend ages making sure everything fits, everything is more or less accessible for repairs & servicing, that nothing rubs against something else, or vibrates itself loose after 10,000 miles, or develops a funny rattle at 54mph on left-hand bends, or that the fan belt doesn't inexplicably shred itself every 5,000 miles, etc. etc. etc. Plopping a different lump in can lead to all sorts of issues, it may fit when you drop it in from the engine crane but when you find out you can't change the oil without removing the front propshaft or that the alternator wants to occupy the same space as your power steering box, things get complicated. When you start having to bodge around problems you then multiply the issues because now the non-original engine doesn't even have all its own bits where the manufacturer worked out they should be.
Standard performance: It can seem very tempting to look at some other vehicle and think it performs so much better than your old Land Rover, but there's usually a good reason for that. Land Rover have always tuned their engines on the relaxed side, designed to work hard in an unspectacular sort of way for many years and to discourage you from abusing them. By contrast, other manufacturers don't share that ethos - either because they don't want to or plain don't have to. No-one expects a Ford Mondeo to be towing a 2 ton trailer every day for 100,000 miles, so it's not built to do that - even though, on paper, it looks like it produces more power & torque than some of Land Rover's offerings, it would likely be outlasted several times over if used in the same way as a Land Rover.
When I was pondering the world of possibilities for the 109 I considered a Nissan RB26DETT lump, as the 109 was originally a 2.6 straight-six I thought it would be a good fit and an amusing swap, and those engines are legendary for being able to withstand 500hp+ with minimal fettling. On chatting to some of the Nissan guys though, it quickly became clear that the RB only really got going once the turbos spooled up and everything came on song over 2000rpm, below that the dyno chart dropped like stone and the boring old Rover V8 could walk all over it. Having to hit 2000rpm before anything happens does not make for relaxed driving or easily controlled off-roading, and that puppy would be throwing out heat like a volcano, not to mention the plumbing nightmare of twin-turbos, intercooler and all that. The 4.6 V8 slugs away from idle in a very relaxed way with all the torque in the world available any time you ask. Dyno plots don't usually start from idle but from what I can tell, a stock 3.9 makes 100-150lb/ft just off tickover and a stock 4.6 maybe half as much again.
Overkill: Sort of follows on from the above - some engines can provide genuinely
better higher performance, usually ones which are significantly bigger than the original. However, the rest of the vehicle has not been designed with that in mind. Everything down-stream from the engine - the clutch, gearbox, transfer box, propshafts, diffs and axles were all designed for the power & torque the vehicle left the factory with. Some engines (notably the horrible GMC 6.2 V8 diesel) are also insanely heavy and are likely to not only kill your vehicle's drivetrain but also give the suspension a hell of a time to boot. While it's nice to have sufficient power on-tap, with great power comes great responsibility - a moment's red-mist or inattention, especially off-road in low-range, can grenade a diff in the blink of an eye. It can also happen over a much longer period, a bigger engine can stress everything that bit more, I have a few friends who have or had big(ger) diesel conversions and these seem to really get through driveline components even when mainly used on-road.
R&D Work: The manufacturer has spent tonnes of time, piles of money and thousands of hours testing the setup that was fitted to your car and it should all work together nicely. Changing one part for something completely different can totally cock this up - gear ratios that fit the characteristic of the standard engine beautifully can be completely awful for the characteristic of the donor engine, to give just one example.
Age of donor: I'll come back to the the beloved 200TDi here as people seem not to have realised yet that no-one's made a new one for decades, so no matter how brilliant it was, there are no donors out there that haven't been round the block a few times. Engine conversions haven't really moved with the times as no-one's made a purely mechanical engine for a long time and electronics are Land Rover owners' kryptonite. To add insult to injury, it's now harder to make a modern diesel engine run outside of its home vehicle than it is to make a petrol one run. This is especially
amusing bad news as the diesel conversion fan club's main argument was “it don't need wires to make it run”. Doing a conversion with an ancient shagged-out old nail seems like a pointless proposition, but new (or properly reconditioned) engines are proper money.
Cost Vs Saving: Even done on the cheap, an engine conversion is a pile of time & money. If you don't do many miles, any fuel saving may never be recouped. You might be better off investing the money having your standard engine reconditioned or at least given a damn good servicing.
Legality: In the UK we can get away with a lot. People like to bitch & moan about rules & regs but we've got it very easy (and very loosely enforced to boot). You do need to tell the DVLA and your insurance company about an engine conversion or you're on dodgy ground, but that's about it. However, if you are going travelling, you could invite a world of pain or be flat out refused entry to a country if your vehicle is non-standard or engine numbers don't match. The USA is very jumpy about Defenders being imported, and also non-approved engines because of strict emissions laws.
Chris Scott agrees with me on most of this, especially the difficulties of finding a good candidate for a modern diesel conversion.