Performance Mods

I’ve seached through the archives and couldn’t find anything that was general or definitive to not write this post.
I have a XK6 engine sitting in the garage for a couple years now, that came out of a xj6 S III. I have several xj6’s and S-III and S-II. The series II rattle like a diesel at low rpm and I’m suspect the head is warped. So the S-III engine sitting in the garage is destined for the Series II as a stop gap until I get around to rebuilding the number matching S-II 4.2 xk6

First off, in all of the specs I’ve read the euro version of the XK6 has better performance numbers than the north American cousin. How come?

Here’s the questions which are centered around performance “mods” rumor or fact?

  1. removing the throttle plates (plugging the shaft holes (of course) inside the water heater in or over the intake ports. Performance gain True / or False
  2. gasket matching and polishing the intake and exhaust ports Performance gain worth the effort? True or False
  3. Cam timing mods? The thing that I love about the XK6 is the ability to time each cam individually. That is so awesome!
  4. Three carbs instead of two. I don’t have the manifold, but I do have aluminum, a TIG welder, and Vertical Mill and more time than sense. is the performance gain worth the effort True or False.
  5. rotating mass balancing? true or false

A proper manual (still available) for the XK6 engine? sources?


2-sorta true

3-true, but dependent on camming, timing, and flowing the entire induction system


5–tune your suspension, and gain 100 hp…:wink:

buy “How to power tune XK engines” by Des Hammill

example, it is advertised at various prices

(my copy is misplaced atm)

Each question you asked is covered in great detail

One point he does make is nearly all mods will affect more power/torque, but mainly/all at higher RPM.

He makes the point that 3 carbs vs 2 is only advantageous at higher RPM

No comments on the other mods, all I’ve done but not sure where gain came on some as they were done together.

Big fan of triples, with one caveat, especially if it’s a sedan.

Unless that top speed is really what your after…ditch the two inches in favor of a smaller carb.

Smaller carbs, say 1 3/4 will yield quicker intake velocity or more response at low end… This also will be limiting at the top end likely speed.

The cam adjustment is sweet with early era sprockets, they discontinued them for a while on later sedans.

Sorry to be picky but this topic seems to be specifically relating to XK engines rather than XK cars. Might I politely suggest that you move it to the XK Engine forum rather than continue here.

Shropshire, UK

Well this is going into a Xj6 Series II and hopefully a Series I
No more cam adjustment? when did that stop? I haven’t taken a XK6 apart yet, but occasionally I take my car to a specialist that deals with only British cars, and only pre-electronic and I saw a XK-E series II up on the lift with the head off the fabulous 6. I noticed the notching in the cam sprockets; and I thought how cool is that!!! you can independently adjust the cam timing.

Sorry Eric, I thought I was in the right place. I bumped up to the discussion category and am I’m looking at the forum discussion topic as I write this thread. It reads XK-Engines; the sub heading reads, "For discussing the Jaguar XK engine, found in many different Jaguar models… "

Is there another XK Engine discussion group that I’m not aware of? that this topic should be directed to?

Thanx for the heads up


This is the correct list for XK engine matters. Eric must be confused.


Paul M. Novak

1990 Series III V12 Vanden Plas

1990 XJ-S Classic Collection convertible

1987 XJ6 Vanden Plas

1984 XJ6 Vanden Plas

1969 E-Type FHC

1957 MK VIII Saloon

Ramona, CA USA

The US imposed far stricter emissions laws than most European countries. Jaguar were forced to run lower compression ratios in XK engines destined for North America - I think to manage NOx emissions. They also added catalytic convertors which were mandatory in the US long before Europe followed suit. It all added up to a reduction in power.

Yes, sorry, it is me that’s confused. You are in the right place and it’s me that’s wrong. Somehow, I have got subscribed to the XK Engines forum. I didn’t do it, honest, but I am going to unsubscribe myself now.


1 Like

lower compression… Different pistons? or different head configuration? I’m going to assume different pistons. That would be an easier solution than a different cc chamber on the head. Assuming it’s pistons, does anybody know if the higher compression pistons are available in north America?

Thank you gentlemen for the information. The one thing, that I’m truly interested in is a manual gearbox. Being a North America kinda guy; we didn’t see any manual gearboxes; so I’m not sure what was in the car in Europe. Manufacturer? Model No? 3 speed? 4 or 5 speed? Any conversions available? yes I do have a welder and a Mill, but it’s a baby one. I was thinking I’d end up making 3 manifolds 1 for each carb.

IN the XK, compression changes are accomplished by different dome configurations of the pistons.

Thete are a few: one of the better is a place in Florida, calked Medatronics, BUT…apparently that may be coming to an end.

There is a thread somewhere on JL, talking about a group buy from Broadspeed in Britain.

Don’t know about in the USA, but 9:1 pistons are still available in the UK +0.010"
Are getting more scarce but standard, and +0.020" are still readily available over here.

yes you can alter the cam timing with the serrated plates, (some engines around '72 had inferior serrator plates that had “cutouts”, and are considered no good)

despite an extensive search Hammills book is still missing, :slightly_frowning_face: …but from memory

some things he notes; (makes big distinction between street and track mods)

  • S3 head is the one to use, (big intake valves)
  • Stock exhaust manifolds is best value on a street car
  • smooth out rough cast & mismatch, but no point polish or enlarge (on street car)
  • discusses vary camshaft in great detail (and carb mods)

The engine in my 420G atm is a built S2 XJ, with .030 overbore 9:1 pistons
It presently has dual HS8 carbs, with manual choke conversion,
but in due course, I will install the 420G manifold, and 3 X HS8 manual choke carbs.

A 420G triple will fit under an XJ bonnet, and the MKX 3 x single manifolds probably even better

I just purchased Des Hamills book from Book depository < $40nz free delivery from the UK to NZ, what I really want is David Vizards book on modifying SU Carbs, hens teeth scarce. :frowning:

HammilL has one on modifying SU carbs, as well as the XK engine book,very comprehensive

True and some must be printed with gold leaf :slight_smile:

Des reworked my cylinder head for me when he was in town, I think his SU mods were a little drastic for road use, boring the bridge out for example, but creating a radius on the piston each side of the bridge makes sense so I did it without any unfavourable results…