View Full Version : ENGINE PROJECT (swapping 3.9 with a 4.6 vs. rebuilding 3.9)

11-26-2007, 07:36 PM
I have a 1990 rover with a 3.9 which most likely blowing oil past the rings. i have the heads of it now and there is a real lot of carbon on the heads in the cylinders. Now I am wondering if anyone can give any insights on the viability of switching out this motor with a 4.6 liter. I know that everything bolts right up to the 4.6 and that when done right they give more power with the same or better gas miliage, and they are just better motors. I am not sure if it would be worth getting my current moter back in shape vs. finding a 4.6 long block that is in good shape and just using that. Anyone who has any ideas let me know. i am excited about my project and like getting other people in on it!
Thanks Dan

11-26-2007, 09:12 PM
let me see if can get this right without getting my head chewed for tipyng the wrong info. the main difference between the 3.9 and 4.6 is the stroke, that is if you took the 4.6 crank and rods into the 3.9 with the 3.9 pistons you will have a (stroker) high revving high torque 3.9 with less possibility of loosing a rod thru the block. the 4.6 has a big bore piston. this is just off my mind, I have all the info on the RAVE cd at work to completely veryfy this info.

11-26-2007, 10:37 PM
You mean that I could in fact clean up my heads and block and by placing the crank and conecting rods from a 4.6 I would be able to increase torqe and horse power? Where would that put me though with the ECU? Would I have to rechip it or would it be withen the computers spectrum. I am interested, please tell me more.

11-27-2007, 08:47 PM
some people say yes you need to upgrade of chip your current ecu others say no, I cannot be sure on that, here are the bore and stroke specs....

engine cyl. bore rod stroke
4.6 94.00 mm. 81.92 mm
4.2 94.00 mm. 71.12 mm
4.0 94.00 mm. 71.04 mm
3.9 94.00 mm. 71.12 mm

according to specs all motors share the same size journals including 3.5, 3.9,4.0. and 4.6 have cross bolted main caps which makes them a strong lower end ideal for hi compression upgrades, all cylinder heads are compatible with all motors, even the old style with the four bolts near the exhaust manifold will interchange with the newer block which are not drilled for them. simply use the newer head gasket and do not use the botom row of bolts because you cannot.
I dont have the 3.5 bore size but is the only small bore out of all, it shares the 4.2 stroke.

11-29-2007, 10:00 AM
When I rebuild my other 4.2, I think I'll put in a buick 340 crankshaft and a higher lift cam. That will punch the displacement to 5.6 L with bigger 95.2 mm Buick pistons. The only mods to the EFI will be to put in Mustang 24lb injectors. All else can stay the same.

11-29-2007, 08:23 PM
do your research because there is a limit on the cam size before the ecu begins to have problems and giving malfunction codes for running too lean or too rich and unability to keep idle.
I have a crower cam on my 3.5, I cannot remember the size but i do have the specs in the car with the owners manual.

11-30-2007, 04:26 PM
You mean that I could in fact clean up my heads and block and by placing the crank and conecting rods from a 4.6 I would be able to increase torqe and horse power? Where would that put me though with the ECU? Would I have to rechip it or would it be withen the computers spectrum. I am interested, please tell me more.

You are correct on the 4.6 - this is basically what was done with mine. I had a 4.6 that had the 2001 new style heads and the crank/pistons/connecting rods from the newer engine put into a 4.0 block. It bolts right into your existing engine location, with a different timing cover and cam to adapt to having a distributor.

I've had mine for a little bit - it's got about 40K on it, of which I have put about 20K of it on - and it runs great with the 4.2 ECU and a Crane cam and Edelbrock timing stuff. I am, however, in CO, so with altitude it might make the 4.2 chip work OK where lower it would be leaner (more air).

I've mostly been happy with it, but have decided to switch to a Tdi instead. I prefer the gas mileage and the ability to run green fuels (WVO, biodiesel) that you can get with the TDI.

That being said, I do have a 4.6 in good shape, complete, that I am looking to sell! You can email me at sdejong at airbits dot com if you want more info.

12-08-2007, 11:07 AM
Are you saying that you are going to put a Tdi into a RR? If so, what year?

I was under the impression this was a tough conversion for a RR. I have a '90 County and am interested in this but have been steered away from it by those more mechanically inclined than I.

My brother-in-law is upgrading his '95 Disco (manual transmission) to a 300 Tdi which I will be involved with so I guess I will get some first-hand experience soon, but this is a fairly easy conversion.

I am recently back on this forum having not been a member in several years, I'll search around for info on this as well.

12-10-2007, 08:33 PM
I have been a mechanic for about 25 yrs, i have done various fuel conversions, from gas to diesel, diesel to gas on brands other than rovers. during the last 8 years i have dedicated most of my career time to rovers, that having been stablished i must convey my experiences. Ihave driven and serviced about 7 different td rover conversions, 1 english conversion range rover auto, a couple of american discoveries auto and stick a couple of d 90's by ECR and an english built d 110 stick.
I must say that I found those little 4 cyl turbo diesel engines quite torqey but lacking enough umph to get the truck moving and keep a steady highway speed, constantly having to down shift and the most scary of all keeping those rpms close to red line, i have never been too avid of keeping an engine at or near red line.
on the other hand, I met a guy named STEVE at MAR one year who had done a shadetree (backyard) intallation of a 5 cyl benz into his 94 disco 1
5 speed, he claimed he did not have a problem merging into the higway or mantaining speed because of the size of his motor, idid not drive his truck so i cannot attest, how ever he did mention that his truck did have an edge over his buddie's factory td.
I have heard of some guys looking into the beetle or jetta tdi as a venue and I know of a guy in arlington va with a melted engine in his 98 disco 1
looking into the 4 cyl cummins found in the cheetos vans.
my personal choice the v6 olds cutlass of the mid 80's to early 90's (olds sierra front wheel drive).
just my experience and opinion.

12-11-2007, 09:34 AM
[quote=FairfaxCounty]Are you saying that you are going to put a Tdi into a RR? If so, what year?

Yes, I'm putting a 300Tdi into my 89. I've got the old engine out and am waiting on the new engine/tranny/tc to arrive - hopefully today, but it depends on the weather. I'm really excited that the big day is hopefully at hand!

My hope is that most of the work can be done in the next couple days, so it is running again for the weekend. Some of the big things you need to do are: replace the engine mounts with new ones welded into the correct location, update the fuel tank and remove the fuel pump, replace the transfer case with a correct ratio LT230(1.41:1), especially important for those running larger tires, like my 33's, and sort out the wiring. It is also important to get the correct transmission if you are keeping your autobox - it has different shift points, a different bellhousing, and the TC locks up earlier. You can do this with mods to an existing transmission, or by getting a whole new transmission.

So far, I'd say that it has been lots of legwork and lots of research. Having someone experienced in doing it is a gigantic help, and it makes everything much easier, which on a task this size is pretty crucial.

I did research some other options - the 200Tdi and an Isuzu from Robert Davis, but I liked the idea of keeping the Rover all English and more or less factory condition, and I liked that the Rover engines would fit in with minimal adaptation.

From what I understand, it should be fairly straightforward to do the swap. I am having help from a reliable, experienced mechanic who has done these swaps before, which is a really big!

Anyway, my plan is to try to document things each step of the way now that the engine is out. Here in Colorado, I've talked to several (4+) who have done the Tdi swap into RRC's, Disco's and Defenders and so far have heard nothing but rave reviews. I'm very excited to be able to join this list and hope I have similar results...