Monday, March 15, 2010

Jaguar XJS to 6.5L Diesel Engine Swap

My initial intention for the Jag was to put a Small-Block-Chevy (SBC) in it. Having had the opportunity to drive a number of different Jags, I must say that I have always enjoyed the Jaguar ride: it is an awesome vehicle to drive. My wife liked to call her ‘93 Jag Sex on Wheels. I don’t know that I would go quite that far in trying to communicate the thrill of driving one – but they are a great ride.
The electronics and power plants, however, have always left much to be desired. The V12 is notorious for dropped valves (which I think had been the case in our motor) and the Lucas wiring is way beyond quirky… leaving no doubt why a normal conversion path involves using a GM motor as the power plant. This SBC arrangement mates the awesome ride of a Jag with the reliability of a GM. That there is a lot of support available for such a project – it has been done often enough before – is a bonus.
Never one to go the normal route, I began looking at various engine options. One of my early considerations had been a GM diesel of some type. I initially shelved the idea due to weight, size and power issues. I have ultimately come back to the GM diesel for a number of reasons.

Here’s Why…

First, there is what I would call the cool factor. There really isn’t a cool diesel car to burn biodiesel in. Most of the television spots you see on biodiesel will show some guy collecting grease at a local Chinese buffet and pouring it in a beater Mercedes or Volkswagen covered in green Astroturf. Who wants that? Second, a supercharged diesel would help with power, lowering the top profile of the engine and also help with dimensional issues. Third – and here is the question I ask whenever I get inquiries on oddball and gas-to-diesel projects – is there a SBC kit available for the car? If there is, it makes the swap far easier. The SBC, 6.2 and 6.5 diesels all share the same form factor, so a kit with motor mounts, adapter plates etc. for the SBC would work for the earlier GM diesels as well.

Getting to Work

Once the bonnet is off and the motor is on the way out – you get a feel for just how big the OEM V12 really is. I am thinking we might even get away without having to do a spring kit or suspension work to the car. I might be wrong but I believe the stock motor and the GM diesel weight are about the same – we’ll see. The GM should weigh in at about 900 pounds fully dressed. The Jaguar transmission was originally made by GM – the plan is to upgrade to a GM four-speed. Installing the GM transmission will also require a few modifications, but we’ll cover that later.
Pulling Out the Jaguar V12: Once the bonnet is off and the Jag V12 motor is on its way out.

How we built this engine…

Now for the fun part. I know I will get some flak for this but the block I chose to start with is the GM 6.2 liter 660 casting. This is statistically the strongest of the GM diesel blocks. I know the new Navistar and XL 6500 castings are all the rage right now, but there is so little data on the fleet that it is impossible to say how they will really hold up. On the other hand, having disassembled thousands of these engines, I can say the 660 block shows web cracks in about one per 200 examples. The particular block we chose was first 
  • align-honed,
  • washed,
  • baked,
  • needle scaled
  • and machined to the standard 6.5 bore.
Some might worry about this last step, but I believe that there is plenty of meat in the cylinder walls of the block for this operation; it will fail no more often than a normal 6.5 engine.
Next, we had the rotating assembly balanced. The crank, rods, piston assemblies, balancer, flex plate and anything else that spins were sent off to a specialty speed shop and balanced. I would not normally do this to a diesel, but if it turns 4,500 RPM – ever – it will be good to have it as true as possible. As you can see, the crank has some welding where weight was added and a couple of the rod caps have material taken off – not the norm for a diesel but it should be smoother at high RPM.
As the engine assembly was being put together, we were able to use standard main and rod bearings. Though I do not mind it when necessary, it is not often that we have to use oversized bearings. This motor also got a set of new Elgin roller lifters, a set of timing gears from DSG Services and a crank stud girdle kit for added stability in the center three main webs. This stud kit will add even further strength to the crank when it is under higher than normal stresses. Once the heads are finished being ported and polished we will use ARP head studs to firmly clamp the heads on.
Timing Gears: The motor is timed just like any other 6.2/6.5/SBC with the exception of the geared timing set from DSG Services. The benefits include constant timing for the life of the engine and a snappier throttle response with virtually no lag between the petal and the fuel delivery.

Fly Cutting the Pistons

As you can see while it is being tapped together, the pistons appear to be silver with a dark coating on just the valve reliefs. I chose to fly cut the tops of the pistons by 0.020 which should effectively lower the compression by two points. The main reason for lowering compression in this application is in anticipation of pushing up the boost. If you did this to a mostly stock truck, the engine would tend to last longer but might be a pain to start on a warm days; at least, this has been my experience. There are a couple of other ways to lower compression: thicker head gaskets, marine pistons and so on. I felt this fly cutting procedure would produce very reliable results and be far less expensive. Having experienced about 50,000 hard miles on a similar motor, I am not too concerned about potential heat issues.
Tapping the Pistons: We fly cut the tops of the pistons by 0.020 to lower the compression in the supercharged engine.

Stay Tuned...

For more on the motor, head work, fuel system and the super charger…
In the conclusion of the project, we will give you a peek inside as we finish the super secret head work and a description of how I am building the fuel system specifically to match this car. We will also have more on the supercharger installation and specifics about how it works. We will cover the motor as a complete drop-in unit, the actual installation of the motor, transmission and some of the quirky issues you might run into while doing a project on your own. Finally, we’ll get to see how it runs... I can’t wait!
Stone Cold Sexy Motor


  1. ...and then enough people told you that diesel in a classic jag was as boring as it gets and you gave up? too bad, I was interested. You're just one more quitter I guess.

  2. Looked much more interesting than he usual LS1 clump too. I guess we may now never know... Was thinking of a Nissan turbo 4 for my Jag with that annoying Kapooooosh!!! sound... But then again there is always just sending it to the wreckers option.