1998 Land Rover Discovery LSE D1 with a Bosch D2 4.6, Hedman headers, Cyberdyne gauges, Magnecor wires, viscous fan delete, dual side exit exhaust, subwoofers, and some other stuff.

On a very cold January morning my trusty Disco dropped a liner that slipped far enough to hit the crank and the pieces spun around and pulverized piston 1 and 2, fortunately this happened at idle in my driveway. The 4.0 was well maintained and only had 108K on the clock. When I took apart the old engine it showed very little wear and was quite clean. The only sign that this was going to happen was a slight tick that popped up about a week before it let go. I thought it was a worn lifter, but the slight loss of coolant that popped up at the same time should’ve told me to not be so optimistic. The damage:

So after the initial frustration and thoughts about parting the thing out passed I set about finding a replacement engine. I found a new Bosch 4.6, at a good price and the project began.

Removing the old 4.0:

The best gas mileage my Disco has ever gotten:

The Swap

Where a Bosch to Bosch swap and a GEMS to GEMS swap is pretty much a straight take
out drop in swap, putting a Bosch 4.6 into a GEMS 4.0 equipped truck takes some
patience and modification.

Before starting this project I read a lot of conflicting information and heard
varying opinions on what could and couldn’t be done. 
I retained the entire Bosch 4.6 engine including the Bosch engine front
cover and the Bosch engine oil pan.

Here are the tricky things that
need to be considered when doing this modification.

Intake and ECU
I just reused the 4.0 intake and 4.0 ECU. The OBD system threw no codes even while I
was running without rear O2 sensors.

Crankshaft sensor
A Bosch 4.6 engine has a different style crankshaft sensor and bracket that holds the sensor in place. This is on the lower driver’s side of the engine. Take an angle grinder and cut the spot welds holding the bracket in place and tap it out. Replace the bracket with the bracket from a 4.0 GEMS vehicle. You can either weld it back in or just use some J B Weld. Reuse the 4.0 crank sensor and flywheel.

Practice cutting the crank sensor bracket on the old 4.0:

4.6 crank sensor bracket removed:

Camshaft sprocket and sensor
You must use the camshaft sprocket and camshaft sensor from a GEMS vehicle.

GEMS 4.0 sprocket on left and Bosch 4.6 sprocket on right:

Secondary Air Injection
The Bosch 4.6 engines have 4 holes in the block near the exhaust ports for the S.A.I. system. When installing in a GEMS truck you don’t need this system, but you need to plug the ports. Getting a set of the exhaust port pipes that screw into the holes and welding shut the ends will fix this. You also can just weld shut the holes.

Welded shut port plugs: 
Also pictured part numbers for 4.0 crank shaft sensor bracket and flywheel cover rubber plug for 4.6 oil pan:

Oil pan
A 4.6 oil pan includes the flywheel bottom cover opposed to the 4.0 oil pan which has a separate bottom cover for the flywheel. You can use the 4.6 pan, but not all the holes leading to the bell housing will line up. Enough of them will so you can use it though. You will need some bushings to press into the 4.6 oil pan or some shoulder bolts as it has larger holes than the holes that are in the bell housing. You may need the rubber plug that goes in the access hole for the flywheel bolts too, see above picture for part number.

Front cover
A common misconception is that you must reuse the GEMS 4.0 front cover on the Bosch
4.6. Doing this makes the swap easier, but is not required. I used the 4.6 cover.
Wrenching on the 4.6 front cover:

Dropping in the 4.6
Despite what the service manual says you don't have to take the hood off to do this even with the oil pan attached. 
Just prop it up, but watch the windshield.

One main difference is the 4.6 cover isn’t as wide front to back. Because of this you will need to put a spacer on the camshaft sensor to prevent it from hitting the sprocket, about a 1/4" inch is all that is needed so an extra nut or thick washers will suffice. Also the water pump pulley will be too far back to line up with the other pulleys for the serpentine belt. Adding 1.75” of space between the pulley and the water pump and some 2” 20 t.p.i. bolts will get the pulley lined back up. I found as a temporary solution a pair of exhaust manifold bolt spacers were the perfect amount needed. You will lose your viscous fan in this process unless you fabricate some sort of shaft extender, but that would put a lot for extra stress on your water pump so better to switch to electric fans. No viscous fan frees up a slight bit of power from the engine and electric fans can be shut off when crossing water or turned on manually when sitting in traffic.

Bolt and spacers for waterpump pulley:

Pulley installed with spacers on Bosch front cover :

Electric fans
I used Flex-a-lite 210 dual fan setup. It has a thermostat and relay and adjustable on/off temperature range. The 210s mount nicely into the stock radiator rails. The control box I mounted right next to the radiator, you can see it in the photo below. To wire you need positive and ground and then a switched 12 volt source if you don't want the fans to run when the key is off. 

There is an empty 12 volt lead in the under dash fuse box you can hook a spade clip to. Thick red wire on left:

The 4.6 front cover has the oil filter coming off at a different angle so your oil lines going to the cooler in the radiator are a bit of a tight fit, but they do work. One other thing to note about the front cover is the GEMS 4.0 oil pressure sensor will not screw into the 4.6 front cover, however the 4.6 oil pressure sensor will plug into a GEMS harness even though the plugs are different. Plug it in and tape the connection and it will work fine.

Additional gauges
I added a three gauge pod with some Cyberdyne gauges to provide me with some extra protection. I added oil pressure and water and transmission temperature sensors. The gauges have a numeric led display. I used the 12V positive going to the cigar lighter for an easy place to tap switched power and fed all the wires up through the vent on the dash.  The pod is secured to the vent with bolts coming up through and a washer and nut on the top.

The Bosch 4.6 front cover has an extra port with a screw in plug right above the oil
pressure sensor. I took this plug and tapped it so I could thread in some NPT fittings for a real oil pressure gauge. I also swapped this plug to the bottom hole and the stock sensor I put in the top hole so I would have more room for the new oil pressure sensor.

For a real water temperature gauge I used the drain plug hole in front of the starter for the thermostat. It is standard 1/8" NPT. Its location causes it to read hotter than one mounted in the stock location on top of the engine. Using a real temperature gauge shows you how the stock temperature needle will sit at halfway for a wide range of temperatures and won't let you know there is a problem until it is too late.

Gauges all wired:

The Cyberdyne gauges have adjustable low and hi flashing and scrolling warnings to catch your attention.

Exhaust Headers
I figured as long as the engine was out I would add a pair of headers. I used Headman Hedders part #39800 and then coated them myself. These will not work out of the box. In order to clear the steering column you will have to do a bit of heating and bending or hammering. I also switched the driver's side knock sensor to a 90 degree knock sensor found on D2s. They plug in, but you will need to file the connector down a bit. This gives you more space to maneuver the headers. You will have to crush down the heat shield over the starter and the one covering the driver's side knock sensor.

D2 angle knock sensors, the one on the right is filed down to fit the D1 harness:

Headers being stripped:

Headers being coated:

Collectors with O2 sensor bung on top:

Headers installed, tight fit:

Dual Exhaust
After a few days of running open headers I put the exhaust on. I had been running the stock system minus the resonator. The rear muffler was in bad shape so I figured I'd just replace the whole setup. I went with a dual side exit setup. The headers run into 2.5" collectors with holes for the front O2 sensors and then to pipes that curl around and down each side of the frame rails. I debated about putting cats back on, but decided to be smart and put in a pair of free flow cats with a spot for the rear O2 sensors. It passes NYS emissions. The mufflers are from Xlerator, part # X-102, and are loud. It is not too much louder inside than without the resonator and on the highway it doesn't get annoying to me, but might for some. I'm not sure what the intended application is for these mufflers, but they are the smallest performance muffler that they make, not much else would fit nicely. The system has no cross over so the sound at idle is a bit like a dragster as you are getting 4 cylinders per side. A full pull with the windows down sounds great. The exits are right in front of the rear wheel. Nothing hangs down lower than the stock system with it tucked under the frame rails and there is no longer a stock barrel muffler to rip off.

Sound clip of exhaust:

Sound clip of Frankendisco from behind doing a pull to third:

Video of dual exhaust setup:

In the end the performance gains are amazing from the stock 4.0 Disco. It feels stronger on the road than a D2 4.6 as well.
Having turned every bolt in this project myself I can say it was not the easiest engine swap I've done. Just dropping in a replacement GEMS 4.0 would have been much simpler, but this was fun and it is great to find people on the road who appreciate it as being different.


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