Hi All!
Recently as ive been diving into getting more power/ a better overall feel from the XK engine ive discovered that the US models were rather significantly handicapped compared to EU engines, it began to make a bit more sense why many posts i see dont share the same opinions about the lack of power i often felt behind an engine i knew was capable of much more.
So where should i start in this journey?
ive read a transmission swap is the best way to go as of right now and that the trans from an XJ40 is the best option, any opinions there?
im a beginner in most things past surface level issues so ive been looking into taking on bigger projects with this car to gain some more experience!

 

how handicapped is the 4.2 engine compared to the European engine?
is it so handicapped for the addition of an air pump system which everyone removes only to find out it makes little or no difference? what else can be removed to gain a lot more power?

My opinion:
the XK engine is old, it was great during the 1950's and 1960's and the 1970's, but then it became old shoe.

you might race-prep it but then you end up with a short life. And even then you will be beat by modern cars with modern engines. Is it worth it? NO. Might as well buy a plastic-body Corvette if you want to smoke the tires at red lights.

the XJ-6 is a very stable high-speed cruiser, not a red light racer. It is too heavy compared to a Honda. 3 thousand,+ 800 pounds.

and last but not least: I have a 1965 S type with a 3.8 liter XK engine that is faster than the XJ-6 with the 4.2 liter engine. doesn't make any sense to me, but it is what it is. The 3.8 has more power, roars louder, revs higher and faster, something happened along the way with these engines.

could it be carburetters vs. fuel injection? I don't know.

 

Most go with with the GM TH700R4 or 4L60 transmissions. Good trans, there are readily available adapter kits.

With a modern trans, especially the 700/4L60, you're making better use of the power available. And it helps mitigate the handicap of the very lazy 2.88 final drive ratio...which was standard issue after 1981 or so.

AJ6 Engineering used to offer some power upgrades for the 4.2 engine. Not sure if they still do. Not inexpensive. Worth a look.

Low restriction intake and exhaust systems might give a small increase. Advancing the camshaft timing helps a bit --- not really more total power but the engine is more responsive.

The main difference between USA and UK/Euro spec is the compression ratio. Increasing compression to UK spec is a big project; tantamount to an engine overhaul.....and you're only gaining 25 horsepower or so.

The easiest choice is to find and buy a V12 Series III...which has almost 100 more horsepower. But, they're not always the easiest to own, especially for a beginner. Or, a V8 engine swap....but that's a very big undertaking as well.

Cheers
DD

 

Who said anything about wanting to smoke the tires? Not everyone wanting more power it interesting in racing or burnouts


It's very simple; total vehicle weight and axle ratio are the main factors in your S type accelerating faster. It isn't so much the engine itself.

Cheers
DD

 

 

My 4.2 was no slouch, not sure what else had been done to it by previous owner, but I do know head and intake were port matched, and then I installed a European front half of the exhaust with no cats, It was still not a race car by any means.

 

I agree with Doug......get a V12.
I had an XJ63, and it was very slow. I swapped in a 3.31 diff, and couldn't tell the diff-erence (sorry).
I don't think you'll ever make an XJ6 quick. A T700 and 3.31 will help, but those are big costly jobs.
Rob

 

My 4.2L 1975 XJ6C has the 3.31 gears and I find it has plenty of pep. However at 70mph and above it feels and is really winding out. I'm always wishing for a 5th/OD gear. And yes I know I can cure that with the GM TH700R4 transmission, but at what expense. I use my Mercedes when I want to go on long trips. Not a bad ride (but not my Jaguar either).