Experience in a Book
Weight Reduction / Relocation

The XJ-S is a heavy beast. Let's face it, it's a luxury car, not a performer. The best way to aggravate the Corvettes would be to pull about 800 pounds out of this car. If you can do without the electric windows, electric door locks, stereo system, 5 mph bumpers (see below), air conditioning, back seat, spare tire, carpeting, sound insulation, etc., etc., then go for it.

Another popular way to reduce weight is to replace components with lighter versions. Chad Bolles has replaced the radiator with an aluminum model, the bonnet with a fiberglass one, the A/C compressor with a smaller model, and other mods. Effective but expensive.

When making weight reduction modifications, bear in mind that such mods will affect acceleration in the non-wheelspin realm by the proportion of total weight of the car; removal of 200 pounds will improve acceleration by 5% in a 4000-lb car. In conditions where the wheels spin, weight reduction in the front of the car will help slightly, weight reduction in the rear will hurt slightly. Weight reduction has almost no effect on top speed (other than how long it takes to get there). Weight reduction has a similar effect on maneuverability as installing bigger tires; it may be negligible if the present tires are adequate. Although it might feel different (and that alone may justify the modification), the actual timed performance may be insignificantly changed.

Also bear in mind that the springs, shocks and other components are designed for a 4000-lb car. If the weight reduction is significant, the car will effectively be sprung more stiffly and will ride higher. If the stock springs are cut shorter to bring the ride back down, it will increase the stiffness even more. All of this may be considered desirable in a car as soft as the XJ-S.

Weight reduction from points high on the car is preferable, since it will lower the center of gravity and make the car lean less in corners. It is even beneficial to relocate components lower on the car.

Weight reduction at the extreme ends of the car is also preferable, because it reduces the moment of inertia of the car. In other words, it reduces the car's tendency to resist beginning to turn when going straight, and its tendency to keep turning when beginning to straighten out. It is beneficial to relocate components from the ends of the car to more centralized locations.

If weight is removed entirely from one end, it can change the balance of the car. Since the XJ-S is obviously nose-heavy, it would be helpful to either remove weight from the nose or relocate components to the rear of the car.

Weight reduction on rotating parts, such as engine/drivetrain moving parts, wheels and tires, brake rotors, etc., can have a much greater effect on performance than general weight reduction throughout the car.

On to Body Modifications