There are too many differences internally in the engine and with the car systems to try to supercharge a regular engine. The electronics, sensors, fuel system, intercooler mount, pumps, hoses, etc are all different with some needing modification I found the entire car is needed and although I completely (almost) stripped my XJR I wish I had kept it until I had finished the project.
I knew I needed some inline 6 cylinder engine mounts and found they use the V12 rubber (metalastic) isolators. This places the XJS engine slightly differently from the x300 with the TH400 auto gearbox being longer than the BW. I have not measured a V12 TH400 yet although the mount will not work. The overall difference between the 5.3 and 4.0 sc TH is about 4.5 inches longer which means the driveshaft will need to be shortened. I have not been down to the driveshaft shop to find out which version to use, BW, TH300 HE, x300. All the mounting flanges are the same between trans, driveshaft and read end.
For the trans mount a possibility is to make some spacers to move the x300 trans mount down from the XJS body and allow me to use some of the bolt holes the XJS trans mount and tunnel stiffener plate(?) use as these items are removed.
The ’94 XJS shifter is for the TH400 behind the V12 6.0 and will work in place of the J-gate. The trans lever is the same length meaning the shifter detents are correct.
The auto trans cooler is in the x300 radiator as is the power steering cooler. I will be using the x300 radiator, AC condenser, s/c intercooler and electric fans. I cut the front sheet metal off the x300 where the bonnet and radiator attach as I might need these later to weld onto the XJS. I also kept the lower x300 radiator support. The future problem I see is getting all the coolant hoses and electric water pumps in place. The XJS does not have the open space as the x300 does and this is a LHD car with the brake setup on the s/c intake side along with the 100 or so hoses. Well maby not that many but the heater has 10 and the intercooler has three very long ones. Coolant tank, air mass meter, air filter, two coolant pumps, harness and some other items must be jamed in the left side on a LHD car. Power steering resivoir will be moved to the other side.
I have not figured out the throttle yet as the two cars are very different. I will not be using the traction control. I am looking to use the x300 pedal box to have a throttle connection. The biggest difference is the 10 degree difference in slope of the firewall mounting. The x300 is slightly flatter requiring a wedge shim around the edge. I will use only 4 bolts similar to the XJS pedal box perhaps I can get five by using one behind the fender. This allows better master cylinder reservoir and booster along with the much better switches and easier throttle cable connection.
I was able to leave the ground strap and formed fiber heat shield in place. I removed the metal heat shields going back to the trans mount but left the rack bellows heat shields in place. I will need to combine the ’76 power steering hose with the x300 hoses by having them made using one end from each.
The AC and heater fittings are reachable with the AJ16 s/c engine in place. However I may change to the x300 climate control unit. I cut this part of the body firewall out of the x300. This next spring I will pull the interior to install the wiring and will look further into the climate control.
The fuel system uses the push lock fittings. Nice leak proof connection so I kept the fuel pipes off the x300 from the engine to about the rear seat. Not enough time to drop the rear end and disconnect properly. I can weld another piece to these to go to the fuel tank. After working on the fuel lines I wish I would have saved them.
The 6.0 V12 XJS and x300 use twin fuel pumps to get enough fuel to the engine under load or high RPM. I have not pulled a XJS V12 6.0 to see the tank/body configuration. The x300 tank has a lower center portion and fuel pipes out the bottom. My ’76 does not allow for this and I will need to modify this area after dropping the rear cage. I need to put the drive shaft in anyway. After further measuring, forcing and thinking about the fuel tank with twin pumps I have found the sedan tanks with the sump will not fit in the early XJS due to this sump not width or height. I will try the '94 V12 tank then. Fuel hose connection to the '94 V12 XJS tank will require either XJS or XJ40 V12 hose\pipe connectors which I have.
Because the engine management system harness makes an easy install of the ECU it still leaves much out. This is because the systems have almost all the electronics and computers in one harness, almost!! The engine side (red I think) is 5 volt for the sensors and the 12 volt side (black I think) is for the connection to trans, body ECU, fuel pump control, cooling fan control, traction control, compressor, etc. I have not looked precisely into the cross connections perhaps the security system and heater AC are tied in also. The US person I emailed stated it was very important to think about the scope of the electrical items that must be removed from the donor car. I think he is missing some important bits. I have decided to use all the x300 harness and remove all the XJS harness. After I get it running I will remove the un-needed circuits. Like the adjustable rear seats, seat heaters, rear door lift and locks, etc. Rather than try to figure out
various controls i.e. how do you up the idle when you switch on the A\C? The x300 body controller runs the various electric water pumps, radiator fans, dash and interfaces with the security system. The x300 climate system turns on the cooling fans with A\C. The trans conputer interfaces with the engine management and security. Lots of the harnesses tie together.
I plan to use the x300 wheels. And the x300 ventilated outboard rear brakes. The lower hub mounting is the same as the XJS and the axle flange at the diff is the same bolt circle with various shims to allow camber adjustment. I have already done x300 to XJS rear ventilated brake swap on a ’94 XJS. I have saved the steering column (upper and lower) thinking to use the switches and wood steering wheel. Somehow I will use the instruments and saved the x300 gauge cluster.
I have a flat hood/bonnet on the '76 meaning I will have to cut it and place a bump over the top front of the AJ16 engine. If you use a later one with the raised center it will fit without issues.
That’s about it for now. The car will soon go into storage for the winter and I will not get back to it until late spring. It would be great to have the various players update this thread as we go to let others know how involved this is.
Geoff Green
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