Greetings! I am loking to start building an XK 4.2 for a future project. Either XJC or MKII, can't quite decide although I'm leaning towards the MKII. I've heard of these motors being built up in the 4.2 E-Types,and getting some great power numbers. I'd love to see 250-300HP at the wheels. From what I've read, a Series III XJ6 is the best place to start, since it's valves are aleady the largest. I haven't been able to find too much (if any) info on performance builds for these motors, although I know it is done. Does anyone have any good advice on where to start? Best block and heads to start with, good companies to get performance parts, Compression Ratios, stroker crank and rods, largest bore to go over, etc. This isn't my first motor build, but it's been a while (8 years).

 

Reliably, about the best you will be able to extract from a 4.2 for a road car is around 230hp on pump gas. The hemi head likes a lot of octane to prevent knock, that will always be a limit to power. If you're looking for performance parts, Rob Beere racing in the UK is the best place. Not cheap though. Rob Beere Racing: Performance Jaguar Parts Also look on the XK engine list at jag-lovers.org, there is lot of advice there. Make sure you go through the archives. It's a different sort of interface, as it started as (and still is ) an email list, not a forum like this one.

A slotted block would be good, Series 3 XJ head, any of the rods, and about a post 1983 XJ crank, as those were Tuftrided. I believe that Rob Beere has stretched the XK out to 4.7 litres, but even 4.2 was already a stretch. There is very little room left, 4.2 is about the max.

Unless you are running high octane racing fuel and high compression pistons, you'll never see 300 hp at the rear wheels. A friend of mine is a racer and has 12:1 pistons, a race cam etc, and he had to struggle to get 270hp at the crank on the dyno. It needs 115 octane fuel to run as well.

You should read the history of the XK engine by Roger Bywater, you'll learn about the weakness and strengths of the XK: The Jaguar XK Engine / AJ6 Engineering and here: http://www.jagweb.com/aj6eng/xk_performance.php also look at his site, AJ6 Engineering

 

Could be a rediculous idea, but I wonder how the AJ6 would be once stripped of all its EFI and emmisions bits and fitted with either ITB or Webbers. Sort of a classic take on a modern engine. I'd like the motor to seem at home when I open the bonet, nothing beats a clean XK, they are beautiful to behold, and a plastic covered AJ6 jsut isn't my style. I like to see motor, not plastic to cover an ugly motor. So, if I simply take an AJ6 long block and bolt on (with some fabricating) some vintage speed goodies, it might not be as pretty as an XK, but could still look nice and perform well. I like the idea of 4 valves and aluminum block. I'll kick it around in my head for a couple weeks and see how many holes I can poke in my theory.