I have a 1998 Discovery I and am looking to convert it to a diesel. I have heard several suggestions such as 4bt cummins, Mercedes, and Isuzu engines, and am looking for any guidance either by someone who has already done something similar, or any advice at all.

 

This will be a labor of love, and since you obviously love your Disco, how will you stand it not being able to drive her while the transformation drags on? I own a Mercedes and can confirm their engines are very high quality, best choice for long life are the older ones with iron heads and block. Isuzu makes a lot of fork lift engines, and my pal's forklift shop works on ones with over 10,000 hours on them (mostly idling). The Cummings shop I take some office trucks and generators to has a client with Dodge pick up with 800,000+ miles on his (must have a girl friend in Key West, and another in Bangor, and another in Vegas).

Of course, there will be another outcome from this project:

 

It depends on your mad skills as a fabricator and your access to tools and a garage/shop. Some people have spent more than 6 months doing the conversion from scratch themselves. Others have spent a few days. The ones that did it from scratch, spent usually around $2k. The ones that spent a few days spend upwards of $7-10k. Only you can weigh your 'opportunity costs'. The main problem is the adapter to bolt the diesel to the LR tranny. There are some adapters available online, but they will cost you. The cummins is the least done conversion. The is most popular outside the NAS market. The Mercedes is fairly popular in the US because of availability of Mercedes inline 5 cylinder engines still working and cheap(ish). IIRC, the cummins produces the most power, then the Isuzu, then the Mercedes. All of them will put you in the low 20s for mileage. If you are doing the conversion for mileage, you are wasting your time(unless you can do everything yourself and have a mill to make the adapter on.). If you are doing it fun, then have a it.
Now, the adapter could be made with 1/2" plate steel, cut out with a cutting torch or plasma cutter and drilled with a drill press for next to dirt cheap, but will take a lot of time. And you will be unable to drive the LR during the process as you need the bolt patterns off of the LR tranny and Donor engine. You also have to consider your tranny...is it the auto, or manual? The manual is said to handle the diesel engines. The auto is said to have had issues with the vibration of some engines(just hearsay).

 

On stock land rover diesels, the autos are valved slightly differently to get different shift points. You might over-rev the diesel with a stock v8 land rover transmission.

 

what about replacing the transmission as well? right now its an auto tranny, but I would have no problem with a manual (although from what I hear that conversion is even more complicated) but another auto would work as well. Basically would it be easier to change both the engine and tranny with a new set or just the engine?

 

How much money are you willing to spend, and why do you want to do the conversion?

 

I know a guy with a 01 D2 that threw a cummings in her, supposedly he gets 28 mpg's, 315 HP, and 600+ torque!

 

You need a diesel engine and a manual trans or a diesel automatic.
You are looking at least $5,000 just in used engine and trans and ECU's.
Plus fab/parts to make it fit/work.
Changing the fuel system, etc.
Do you have emissions testing? It wont pass.

 

a cummins 4BT would be a fun one...

 

I doubt he's getting that kind of power out of the 4bt. The 5.9 can put out that much, but would need major improvements in the suspension to handle the huge weight. And even with that kind of power, you would have to change your tranny to a diesel tranny and upgrade the rest of your drivetrain(axles, t-case, prop shafts). Lots more money.