(Click photo for SOUND!)

How many of you “older” gearheads remember the sound of a 389 Pontiac four barrel with your foot on the mat? How about a 1950 Olds “Rocket 88” with the air cleaner in the trunk and your foot on the floor? It's a sound that has always made the hair on the back of my neck stand out. 

Well I have captured that sound again and of all places to hear it, in the comfort and luxury of my ‘91 XJS convertible. Not only the sound but the fury to go with it. Following is my trip through the transformation from whisper to ROAR !!

This project is not my idea by any means but my version of DIY technology in my own garage with the stereo loud on Classic Rock Radio. Somewhere, you must have heard about larger throttle bodies and air intakes. I decided to make my own since Radford Engineering commands a $1350.00 premium for this upgrade. 

Armed with a spare set of throttle bodies from a trashed 5.3 at Foreign Accents, a friend with a machine shop in town and a white paper on the specifications of the system from Radford Engineering, I set out to do it myself.

The only new parts necessary were two throttle discs from a ‘88 through ‘94 4.0 liter engine. Part no. EAC 4820 and a few gaskets. Having those in hand from Bob Dunn Jaguar, I made my way to Mike Troxler’s shop, Advanced Machine Concepts, with my request for some milling work. Figuring the correct diameter for the throat was not the easiest task. The bevel on the edge of the disc had to be taken into account to provide a stop position at just the right point in the diameter to prevent it passing through the throat. Mike had the expertise to make things fit and look great for a very reasonable fee. My turn now to put the spit and polish on the project that I always feel is necessary. 

Designing the air ducts for optimum cool air delivery from outside the engine compartment was next. Three inch flexible metal duct was fitted from the front of the air boxes to the panel bulkhead behind the headlight enclosures. The headlights were then removed to cut open the rear of the enclosures to allow fresh air entry directly to the air boxes. New K&N low restriction filters now live here. My primary approach to the additional air, before the throttle body modification, was to add two 2” inlets to the bottom of the filter air boxes and run ducting to just behind the lower spoiler. I retained these inlets  and now have a considerable amount of outside cool air induction being fed to the engine from four separate inlets. With the four inlets, two 3” ID and two 2” ID, the engine really has a tremendous amount of fresh air to ingest. The larger throttle body discs provide a full 32% more air volume per throttle as the stock configuration.

To provide the other end of the larger pump theory, I removed the catalytic converter internals. (all of them) This provides more air in, more air out, making a more efficient pump and more horsepower. The result is an estimated 25 horsepower increase. This is calculated by Radford Engineering dyno testing in their white paper report. The results I experience is that feeling I remember when I planted my right foot on the floor of that old Rocket 88 and the ROAR that resulted. I remember people on the street snapping their neck in my direction to see what in the world that noise was. 

The results in the XJS is a clear and present increase in performance along with that wonderful sound. The acceleration is increased and the midrange pull actually plants your hips into the seat. This is by far the best increased performance I have been able to realize in my quest for more. The best part is the large smile that is present when my foot hits the floor and the ROAR fills my ears and my senses. The second best part of the project is the less than $200.00 and several hours of recreational labor for a $1350.00 plus labor update.  YES !!

Editors Note: JCNA recommends you check with your local DOT before making any modifications involving your pollution control systems!