SuperCharger Requirements

Just swapping out the engine for the SuperCharged version of the AJ16 4.0 Litre engine has a lot of consequences. These then have knock on consequences that can make the swap very difficult. I decided to use the AJ16 engine and Automatic gearbox and the minimum amount of the XJR electronics needed. The alternative is running a standalone engine management system although I may have to go this route if I cant get away from the dreaded “limp home mode” with the original electronics. Very little else from the XJR is needed, I hope 🙂 Below is a list of the bigger issues and I will mention my thought process for each one and the proposed solutions.

• The XJR6 engine requires different engine mounts
  • I tried with the standard XJ6 mount and the engine is too far forward so the oil pan hits the subframe
    • I purchased XJS facelift mounts (they have the AJ16 non supercharged engine) and hooray the engine will fit into the space. It it very close to the passenger inner wing because of all the intercooler add-on parts. It is also “swapped over” compared to the NA engine for inlet and exhaust. On the NA engine the inlet is the drivers side and the exhaust is on the passenger side in the engine bay. Now it is reversed.
    • Swapping the exhaust over should be OK as there is an offset Y shaped part than can just be rotated. Not sure on the whole exhaust setup yet but thats a minor issue to resolve anyway. I will use the AJ16 SC down pipes and Catalytic Convertors for sure.
  • I will use the XJR radiator, Fan Assembly and the majority of the XJR ancillaries as needed
  • Power steering pipe will need modification to fit the old steering rack. Need to research the pressures involved too.
  • I will use a mixture of old and new electrics as needed. For example, the horn, lights, indicators will be completely using the old XJ6 Electrics whereas because there are now electric fans, these will be driven via the newer XJR electrics. The Reverse lamps will use the same wiring as old but be driven by a new relay that is engaged via the reverse switch on the J-Gate lever assembly so a mixture of old and new.
  • By far the biggest challenge is the Engine electronics. I have researched this quite a bit even before I decided to purchase the XJR. I aim to use the following ECUs and wire them up as if the engine was still in the original car. I will mount the Control units in the passenger footwell area
    • Engine Control unit – obviously
    • Gearbox control unit – obviously
    • Security and Control Unit – to get over the inhibitor issues (will use the original key and reader but see more on that in later post)
    • Body Control unit – needed along with the others to run things like the starter and gearbox interlock process
    • For the gauges and warning lights I will use the circuit board that drives the XJR display but merge the wiring into the old display to keep it looking like a classic Series 2 XJ6
    • Getting the wiring into/out of the engine bay is challenging.
      • I will use the hole for the LHD steering option that currently has two vacuum pipes for the AC system running through a circular plate and make another hole for a single XJR bulkhead connector. Not all the connections are needed but enough to make it worth wile using one of the original connector pairs.
      • Need to add some fuse boxes into the engine bay. The XJR had 2, I think I can get away with 1 as I will not require all the extra items such as the lighting, fog lights etc that were in the engine bay area because those are already catered for on the XJ6 wiring loom.
  • Not yet decided what to do about the fuel vapor evaporation control system. The system on the XJR is quite complex with a carbon canister, a pump and a valve. This is combined with pressure sensors in the fuel tank as well. As I am using two tanks now and neither has the pressure sensor I will have to do something different and maybe send some fake signals to the ECU to stop any limp home issues.
• The XJR gearbox requires different mounting points and the change to a J-Gate gear leaver assembly
  • Need to modify the mounting plate to fit body
  • Need to modify the central console and the underlying metal work to fit the J-Gate set up
  • Need to upholster/cover with veneer the modified console to hide the changes
  • Need to modify/merge the old/new wiring to manage the gearbox interlock and reversing lights
• Drive train – I am initially going to use the old XJ6 rear end as it stands. If the gearing appears poor in use and I want to change it I can revert to an outboard braking system and the full cage from a facelift XJS. Maybe the 4.0L or even the V12 setup.
  • The propshaft will need modification. The XJ6 is a long wheel base and a cursory look shows that there is a bearing assembly half way along the propshaft with a UJ joint. This means that I could potentially use the front half of the propshaft from the XJR and the rear half from the XJ6. Later investigations proved that although this looked like a simple change, the sizes of the UJ’s are different. The XJ6 is imperial and the XJR is metric. Still it was worth a check 🙂 I will get the propshaft sent off to a specialist to modify it to fit. I need to get the engine and gearbox fitted in their final resting places so I can measure up properly.
• Knock on effects
  • Fuel pumps – The XJR has two fuel pumps inside the tank with the second pump to boost the pressue over 3,500 RPM. I did look at the potential use of the XJR tank and did remove it from the car but I will stick with the original tanks and set up a new system using a high pressure external pump and the change over valve system.
  • Speedo and Mileometer –  These are mechanically driven by the old gearbox on the XJ6. On the XJR these are electronic with the speed signal pulse being generated by the ABS module and its sensors. I am not going down the ABS route so that will not work out for me. There is a output speed based square wave signal from the XJR automatic gearbox however it is 15 time faster than the signal provided by the ECU unit.
    • My plan is to use the small electronic gauges from the XJR inside the original gauges for speed and set the mileometer just under the dash somewhere discreetly to show the milage. I will use a mixture of the old and new sensors/gauges as required. The goal is to for the gauges to look the same as they do on a Series 2 XJ6 but working. I will wait to finalise this until the engine is in and working.
      • I did test fit the electronics to the original speedo and it looks like this will work out OK. The photos below show the initial attempt. It still requires some finessing so the plastic is not under stress, but it looks like I can get it to fit inside the original gauge.

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      • One issue is that there is a gap for the trip and mileometer in the gauge. I resolved this by taking a photo of the original gauge with the trip and mileometer in place. I then printed out the photo and stuck it behind the faceplate aligned with the holes. It looks quite effective even it it will not rotate. Note this test configuration used poor quality photo paper so it seems to have gone a little reddish whilst sitting in the garage for about a year. I will do another one and maybe laminate or spray clear coat over it to keep out the elements.

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      • I will need to divide the input signal down to reduce the rate. It is directly 15 times slower however I have yet to do the calculations regarding the rear end differential ratios and tyre diameters so it will need adjustment. To logically divide a signal by 16 is pretty straight forward so I will start there and adjust the analogue feed to the gauge to adjust to get a representative speed.
      • The circuit board that drives the XJR gauges is planned to be installed to the left hand side of the drivers foot well under the console

Wow thats a lot to contend with but thats the plan with no doubt loads more to decide upon and try to resolve as I go along.

I decided to replace the original normally aspirated 4.2L engine with a Supercharged 4.0L engine from an X300 XJR.

Why would you want to do that you might ask?

Well a number of reasons as per below:

• The car is of little financial value and with the amount of restoration already taken place and still required it cannot be considered original so why not?
• Considering the state of the cylinder heads, a complete engine rebuild is probably the best way forward but will cost a few hundred pounds at least
• It will be something a little different from even the very few Series 2 XJ6s I occasionally come across
• I have seen an E-type with the same engine so I am not necessarily the only person considering this
• It will be more of a challenge to sort out all the electronics and associated issues
• The X300 XJRs do rust a fair bit and so a complete car can be purchased for a silly amount of money!

So a few ground rules!

• Limit the amount of modifications to the car so it can be undone reasonably easily and retain any parts swapped out – keep the original engine & gearbox for example
• Limit the amount of holes drilled in the chassis where possible
• Limit the outwardly visible changes to a minimum
  • discrete addition of some kind of XJR badge and maybe 4.0 instead of 4.2
  • use same wheels
• It may be nice to add all of the features of the newer car but there is a point that you need to stop!

I therefore went and purchased a 1996 XJR from that well known online auction site for just over £750 and with new enthusiasm the challenge really begins!