About the Car

The stock Jaguar XJR is a fairly fast car. I have read test reports that suggest it should be able to complete the quarter mile in anything from 13.6 to 13.9 seconds. It has a 4.0 liter V8 engine with an Eaton M112 supercharger and power output was originally rated at either 358 or 370 horsepower at 6200 rpm (depending on who you believe). The changes that I have made give it more power and make it much more consistent.

Unfortunately, although the situation has improved in the past couple of years, there are still not many performance parts available in the USA for modern Jaguars. Those that are available are usually expensive and often they are not emissions legal. On top of this, the manufacturers claims about how much performance improvement to expect, usually don’t give you a good indication about what you will really get. Performance is anyway surprisingly difficult to measure in a consistent way. Especially with these cars, because they are very temperature sensitive.  Check out my Temperature Effects page for more information about this. Also, I have an E.T. Calculator if you want to see how much horsepower you need, to give you the performance that you want.

Hopefully the information below will be helpful to anyone hoping to do some of the same things that I did.

Supercharger Pulleypulleys

The supercharger is driven by a drive belt from a pulley that is connected to the harmonic balancer on the crankshaft. The easiest way to increase engine performance is to connect a larger pulley to the harmonic balancer to drive the supercharger faster.

The stock pulley is 5.875 inches in diameter and produces a boost pressure of 11 or 12 psi. My largest pulley is a bit over 7 inches and produces 17.5 psi.

This is, by a long way, the most effective change that you can make but, depending on the quality of fuel that you use, if you raise the boost this much without reprogramming the ECU you risk destroying your engine.

The only other problem that I’ve found is that the drive belt tends to slip and it wears out fairly quickly. Goodyear Gatorback belts seem to be better.

I can’t honestly say how much additional power you’ll get from making this change on it’s own because I did it in stages and changed other things at the same time but I guess it would be around 50 horsepower.

Ported Supercharger

I bought a spare supercharger on ebay and sent it to Stiegemeier to be ported. The difference isn’t very great as you can see in these pictures but the inlet is very slightlylarger (about 1/8″) and the triangular outlet is about 1/4″ wider. The surface just inside the inlet has also been smoothed out.

Supercharger OutletI changed the supercharger before I changed to the 7 inch pulley and when I first tried it out, it didn’t seem to be making a significant difference. The peak boost was about the same as before and my quarter mile times were at best a few hundredths of a second quicker.  A friend of mine who is a bit familiar with ported superchargers on Ford Mustang Cobra’s told me that this was fairly typical. The Eaton M112 is a roots type supercharger which means that it delivers a fixed volume of air with each revolution. Porting can’t make it deliver more air but it can increase it’s efficiency. This means that it will require slightly less power to drive the supercharger and the temperature of the air leaving the supercharger should be reduced. The main advantage of this improved efficiency is that you can now drive the supercharger faster without excessive power consumption or outlet temperatures. In other words, it’s only worthwhile doing this if you’re also going to change pulleys to drive the supercharger faster.

Supercharger InletDo I believe all this ? Probably yes because I have a temperature sensor in the intercooler coolant circuit and the new supercharger is clearly generating less heat at full boost than the old one. Of course the only way for me to really know if my ported supercharger plus high boost pulleys is better than just high boost pulleys on their own, would be for me to put the old supercharger back on with the new pulleys and see how it compares. Since it took me about 10 hours to swap the superchargers I’m going to leave this experiment to someone else.

At first it seemed as though the new supercharger was a lot noisier under boost than the old one but after a while the noise gradually went away. Maybe it just needed to be run in. Also, it’s noisier when I have a high boost pulley installed.

I have the Borla cat-back system. It’s only minimally noisier but I also think it only makes the car minimally faster. At most it makes a 0.1 second difference over a quarter mile. Maybe if I start removing some of the mufflers (there are 5 of them) it will make some difference but it’s probably not worth the effort. It would certainly make more noise.

It’s also worth noting that this system was a real pain to fit. It required so much altering and welding that in the end it would probably have been cheaper to buy one of the Arden systems.


I have Brembo rotors (disks if you’re English) and ceramic pads all round and Brembo calipers at the front. The front rotors seem to be about 13.5 inches. These work extremely well and are also a little lighter than the stock rotors.

Wheels and Tires
I experimented with Michelin Pilot Sport Cup tires which will fit the standard wheels. These work well when they are hot and are probably ideal if you’re into autocross or some other type of racing that requires you to be able to corner. Unfortunately, unlike drag racing tires, you can’t heat them up effectively by doing a burnout.

I’ve tried Nitto, BF Goodrich and Mickey Thompson drag radials. The BF Goodrich tires are nice because they have sizes that will fit the stock rims but I use the Mickey Thompson’s because they give you by far the most grip. I have some 9″ wide, 17″ diameter BBS wheels mounted with 275/40-17 tires. I put them on the rear when I get to the race track and put the stock wheels back on before driving home.


The drag radials that I’m using only work well if you do a burnout before each race. To do this, you need to get both rear wheels spinning. Unfortunately, even though the XJR apparently does have a limited slip differential, the clutch pack in the differential was not tight enough so more often than not, only one wheel would spin. Then, when I went to launch, the other wheel would spin on the line. I eventually found that I could get a custom built racing differential from an XJR-S that would have a tighter clutch pack and could also be made with different ratios. I went with a ratio of 3.55:1 vs the stock ratio of 3.06:1. It didn’t make it significantly faster (only about 0.1 seconds over 1/4 mile) but finally my traction problems were solved.

About a year after installing this differential I noticed a strange new noise coming from the rear of the car at certain speeds and during a scheduled service, the dealer noticed oil was leaking out of one of the axle seals. It turned out that there are a couple of brackets which are spot welded onto the rear sub frame to hold the differential and they had broken loose. Also, oil was leaking from both axle seals and from the drive pinion seal and the drive shaft flex disk was cracked. These problems are apparently not uncommon but I’m sure that I’ve been abusing it more than most people. Anyway, a few days and a couple of thousand dollars later it was all re-welded and fixed and the noise is gone. I’m told that it will be stronger now than it was before. I certainly hope so.


The intercooler is basically a radiator that sits in front of the regular engine radiator. Coolant is pumped around between the intercooler and the charge coolers. By installing a larger intercooler you can hope to cool down the air more after it comes out of the supercharger. In practice, it makes the car more consistent rather than significantly faster.

Intercooler water pump
After installing the larger intercooler, I noticed that the intercooler was often much cooler than the charge coolers. My solution was to install a larger electric water pump to circulate the water faster. I’m using a CSR #923 dragster water pump. I tested it, and through a short 3/4 inch pipe it flows at least 3 or 4 times as much fluid as the stock pump.

Mixture/Timing Piggyback chips

After a disastrous experiment with another system (blown head gasket due to excessive timing advance) I eventually found a device that could make the mixture richer or leaner at full throttle by adjusting the signal from the MAF sensor. I also have a system that uses a wide band oxygen sensor to measure the air/fuel ratio. I found that at full throttle, the afr was already around 12.5:1 so when I’m using high octane fuel I leave it. When I’m using ordinary pump gas and the engine is running high boost I make it rich (around 11.5:1) because I think that this will help to avoid detonation and preserve my engine.

I’m no longer doing anything to alter the standard ignition timing.


The stock air box is a bit restrictive but it does a fairly good job of making sure that, as much as possible, the engine breathes cold air from outside the car instead of hot air from around the engine and exhaust.

I have experimented with every possible configuration that I can think of including various cone filters, removing the fog light and replacing it with a secondary air intake and a Aluminum/fiberglass heat shield sealed around a cone filter to keep hot air out. I even purchased a small radiator fan and fitted it next to a cone filter to try to make sure that it sucks cold air from the fender well instead of hot air from around the exhausts. Nothing works well.

Here’s my theory about what’s happening: On a hot day, especially when the fans are running, air is sucked through the radiator into the engine compartment and the only way for it to get out is downward past the exhausts. This means that you have very hot air not just under the hood, but also coming out underneath the car. When you’re moving or facing into the wind this isn’t much of a problem but if you’re stationary then the hot air is going to come up the front and sides of the car making it almost impossible to get any kind of cold air intake to work. Even the stock air box setup sometimes gets very hot when I’m warming up the engine before making a run. All the different cone filter configurations that I’ve tried seem to make it worse. Once you start moving it cools down very quickly but the first 60 feet are very important in drag racing and hot air is making me sometimes slow and more importantly unpredictable.

For the moment, I’ve gone back to using the stock air box with a K&N flat filter element. I’ve also slightly moved the fender, the bumper and the headlights to increase the size of the gaps around the fender. I might be giving up a few horsepower compared with a large cone filter but I think I’m gaining a little consistency. I’ve also made a large hole in the bottom of the air box and attached a fairly small fan with a 3″ flexible duct that goes behind the headlights and emerges in front of the car next to the intercooler. When I’m racing, I keep the fan running all the time. It delivers just slightly more air than the the engine uses at idle so when I’m warming up the car or when it’s cooling down after a run, the fan circulates air from the front of the car through the duct into the air box and then out into the fender well. This seems to help keep the inside of the air box cooler. Finally, I wrapped everything in multiple layers of aluminum and fiberglass heat insulation. It still sometimes gets warm but it’s better than it was.

The only thing that I think might help much more would be to louver the hood so that the hot air can escape up instead of down. I notice that some of the new Jaguar XK’s have louvered hoods. Maybe I’ll try it one day but it’s an expensive project and I’m not very confident that it will work. Does anyone out there have a can opener that I can use ?

Fan Override
I managed to wire a manual override switch into the cooling fans circuit. This somewhat helps me control the temperature of the coolant in the intercooler circuit and so hopefully gives me more consistent performance. I usually run the fans on low during a run and switch them to high as I’m coming back along the return road. So long as it’s not a very hot day, by the time I get back to the start line, the coolant will have cooled down enough that I can make another pass and get almost exactly the same performance as on the first one.

Low Temperature Thermostat

I managed to find a thermostat that, so long as there is air flowing through the radiator, keeps the engine coolant temperature at around 185°F. The standard thermostat lets it rise to about 210°F. This enables me to get it up to temperature quicker and keeps the under hood temperatures a little lower. There’s a picture of the box that the thermostat came in with part numbers etc. here.

Intake air temperature override

There is an intake air temperature sensor in the same place as the mass airflow sensor. There is also a second temperature sensor at the back of one of the charge coolers. The reading from the second sensor is more meaningful as it measures the air temperature after the supercharger and charge coolers, and so takes into account fluctuations in supercharger and intercooler efficiency. I haven’t messed with the second sensor but the first one was causing a problem. On hot days, as I warmed up the engine and approached the start line, the sensor would sometimes read 30 or 40 degrees hotter than the ambient temperature. This was giving me very sluggish and inconsistent performance at the start line. I guess this was because the ECU was retarding the timing when the temperatures were high, to prevent detonation.

I wired a switch to bypass the temperature sensor and replace it with a fixed resistor so that when I’m racing, I can make the ECU think that the intake air temperature is always 55 degrees. This seems to have improved the situation. It’s still fairly slow in the heat but it’s more predictable. I don’t worry too much about detonation because there is still the second temperature sensor and knock sensors, and the changes that I have made to the intercooler circuit will keep it cooler than it would be. Also, I use high octane fuel when I’m racing. I can’t say that I’m 100% sure this isn’t causing me any problems but how much damage can you do in a 13 second race ? Maybe more than I think but I’ll just have to take that chance.

Torque Converter

It’s noticeable when accelerating that the torque doesn’t peak until the engine gets over 3000 rpm. I had my torque converter modified to give it a higher stall speed so it will get into the power band more quickly. The result is about a 0.1 seconds improvement in 60′ and 1/4 mile times. I guess that although this change makes the car quicker off the line, it’s less efficient at other times otherwise a 0.1 second improvement in 60′ times would translate into a much bigger improvement in the full quarter mile.

High Octane Fuel

On days when I’m due to race, I mix some 100 octane unleaded racing fuel in with the regular pump gas. This makes a big difference. I’m not sure what is the best ratio, but my record time so far was set using almost 100% high octane, and I would guess that it was at least 0.2 seconds faster than I could have done with regular gas. (Premium pump gas is 91 octane here in California.)I think that the ECU must be somehow sensing the higher octane and advancing the timing a little. Maybe 91 octane causes a little detonation, the ECU detects it and reacts by retarding the timing. With higher octane this doesn’t happen. That’s the best theory that I have. I don’t really know what’s going on but so long as it works I’ll keep doing it.

Other Changes

Other minor changes are:

  • Temperature sensor in intercooler circuit.
  • Replaced coolant with about 80% water, 20% anti-freeze and a bottle of Redline water wetter.
  • Slightly smaller tires (245/40 -18) all around during normal driving. I only put the 265/40-18 drag radials on the rear when I get to the race track.
  • I remove the weather strips around the hood (bonnet) when I’m racing to let some of the hot air escape. I don’t honestly know if it helps at all.