Two other changes were made to the engine, both sourced from AJ6 Engineering in
the UK. The first is a modified inlet manifold, designed to provide ram-forced
induction. Most modern engines make use of this, whereby having sufficient length
in the inlet ram tubes allows pressure pulsations as the inlet valves actuate to
assist with drawing in the inlet charge over a limited rev range. Many modern engines
have variable length intakes to spread the effective range. It can be considered
the inlet equivalent of exhaust extractors
The 2nd modification was a 304 grade stainless steel exhaust, with 2 key differences from a standard system. Firstly,
no mufflers are used prior to the axle overpipes, and secondly the engine pipes are
considerably extended before joining. Pipe diameter is standard 1.75”
AJ6 engineering claim that the use of this exhaust system and inlet manifold
considerably enhances mid-range torque. And they should know – Roger Bywater
worked with Trevor Crisp at Jaguar in the mid to late 70’s developing the EFI
version of the XK engine, as well as doing development work on the V12 and AJ6
engines. AJ6’s website is definitely worth a visit.
It is all very well to research and go to this trouble, however was it worth
As the biased owner, I am probably the last person who should comment, however, for what its worth, my assessment
is as follows:-
throttle body hole of extended ram tubes.
Ease of use – The engine is definitely much smoother, with none of the idiosyncrasies
when cold that the SUs had. Starting, cold hill-starts, throttle response and cold
exhaust smell are all on par with a modern car.
Go – Peak power levels tell only a small part of the story, however I am
advised that an everyday carb’d 4.2 will produce 80-90 rear wheel hp on a rolling road,
and a triple SU, manual car might deliver 110-120hp. I have seen a good V12 HE XJS
plot showing 168 hp. As such the 140 we achieved is pretty good. When you look at
the plot however, it can be seen torque drops away from 3700 rev/min, and the power
plot is pretty well flat from 4000 rev/min. Possibly the cold-air intake is causing
a higher speed restriction that is robbing maybe 20hp. On the road the car feels
responsive with loads of mid-range torque. Higher speed performance is impressive
– 2500 rev/min in 5th equates to 115 km/hr, and acceleration at these speeds in 5th
feels stronger than the 350 Chev, which is doing more like 2800 rev/min.
Fuel consumption – Each of the tanks hold ~ 42 litres when filled with the gauge
showing empty. The car will do ~ 375km of freeway driving from a tank, and I’ve
not seen < 300km for local running around.
Mechanical work and modifications were only part of the story with restoring this
car. Other modifications included re-bushing the front suspension and steering rack
with polyurethane, and fitting Koni shock absorbers all round. Fairly substantial
changes were made to the air-conditioning system, by scrapping all the original
electronic controls and servo mechanism, and replacing them with pneumatic valves
and actuators, a microcontroller, and a little LCD screen that sits just below
the centre air-outlet.
Chassis Dyno Report
At the 2007 JDCA concours (Greg Jones
ex multi-award winning coupe in the background)