What is the compresstion on an XJ6 motor?

(Pat Leask) #1

I am very new in to Jaguars, and I am restoring an XKE and I am having to use an XJ6 block (XKE one was cracked, this one has those relief cuts between the cylinders - ya hoo!).

I need to get a distributor (was missing) so I need to find out what the compression rate is on the XJ6 motor in order to get the vacuum advance correct.

Is it 8:1 or 9:1

If needed, the S/N of the block is 8L106462-8
Thanks all !

Thanks so much!

(tony) #2

The indicated compression ratio is 8:1, indicated by the last digit (as is usual)
(provided it has standard pistons).

Mine has 9:1 pistons installed, and has 160 psi on all cylinders, measured with a quality screw-in gauge (170 with a cheap press-down gauge!)

you may “search” under compression psi, or others may answer,

from memory, my 8:1 motors were in the 130-140psi

(Mark Lee (Pay Pal Patron)) #3

A fresh US spec XK6 is 150psi well kept under 100k 145psi. Clapped out XJ’s (like the ones I have) 130psi

(Mark Lee (Pay Pal Patron)) #4

I have a points distributor; (series II XJ) but like the car, it’s pretty sloppy; in the console was a feeler gauge, couple sets of points and condensers, a screw driver, jump start button & flash light. . I can’t count how many times I ended up on the side of the road with cooked points.

I gave up on the distributor and installed a Pertronics Electronic distributor and coil. it looks OE until you pop the cap. Extremely pleased!

(Frank Andersen) #5

The main factor in compression testing is evenness between the cylinders, Pat - confirming that engine’s insides in in working order. And possibly verify piston types…

The nominal compression pressure of the 8:1, your block number, is 135 psi, but as Tony says; a PO may have refitted pistons for higher compression. Apart from the wear factors mentioned; test conditions variances may influence actual pressure readings - crank WOT for 4 seconds…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

(Pat Leask) #6

Thanks Frank (and others). I perhaps was in error when I mentioned that my
motor is having a 100% rebuild from a non running state. All new OEM pistons
and all the other goodies that go in to a new build.

So there for I am afraid that I have no idea what the PSI will be of each

From what Frank says, stock my XJ6 motor (XKE head) will be an 8:1 correct?
So I need to find a distributer that is designed for an 8:1 compression -

Thanks for the tip on the Pertronix, I’ve been 50-50 on it.

Thanks, you are all so very helpful and I appreciate it.

(Carl Hutchins, Jr. ) #7


I don’t think the vacuum advance is that critical as to a point or two in differing compression ratios.


All the signs of a failed capicitor, aka condenser . But, the Pertronix electronic method of “make and break” is light years ahead.

I messed with a lot of point and condnesor ignition over the years. My ain issue was the right gap with the feeler guage
Pure luck got it right and the engine would start and run properly,. Sure don’t miss 'em !!!

Carl. .

(Paul Wigton) #8

I could/can set them by eye: that said, I am with you…good riddance!!

Next to the EDIS, going with the Mallory Unilite made a big difference in how Tweety ran. My Rover has an Allison/Crane, and will soon have an EDIS.

(Jochen Glöckner) #9

Hi Pat,

though I can’t help about compression figures, I can recommend the 1-2-3 ignition system, NAYY. The PO installed it in my car well over 10 years ago and it hasn’t given any trouble since. It is not only electronic, but includes spark balancing, resulting in near virtual ignition pictures on the oscilloscope.

Most recent versions include a USB socket for individual programming, so you can find the best settings with a laptop computer on the co-pilot’s seat while you drive.

Yet, while it is nice to have, it won’t turn a good engine into a racing engine. If you’re going to modify the engine in the course of a rebuilt it might be the easiest thing to adapt the ignition to hotter cams and the like.

Good luck


75 XJ6L 4.2 auto (UK spec.)

(Mark Lee (Pay Pal Patron)) #10

I too am no stranger to points. my 70 Camaro still has points; heck I have a dwell meter. Under circumstances, you’re right about the cooked points, What I had going on was a completely warn out distributor. the shaft wobbled around in the bush which made it nearly impossible to keep timing as well as the gap would change depending on the RPM Because the Dizzy shaft would wobble in a circle instead of just spin.

I looked into rebuilding it, but couldn’t find parts, I did find a classic Brit car specialist locally, but he wanted twice the price of the Pertronics unit and the coil combined. So that’s the direction I went.

With respect to compression and distributor, I sincerely don’t think the two are really related other than timing. Depending on what is available for petrol, will more or less determine what the timing will be. Normally it’s 8 BTC at 750rpm; but that may change due to fuel & the higher compression. Essentially do what ever needs to be done to stay off detonation.

(Frank Andersen) #11

But you can mess with on the road, Carl - getting the engine going with a nail file. When electronics fail; you are stuck…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

(Paul Wigton) #12

Yea, WHEN: they fair far less often.

Seeya, points and condensers: wish I’d never knew ya!


(Doug Dwyer) #13

I work on oldie cars all the time…which is generally lots of fun. And I have full appreciation for that which is quaint and charming or I wouldn’t be a Series III enthusiast…the Series III being ‘the most modern of all antique automobiles’. :slight_smile:

But, I find no great pleasure in fussing with ignition points…working in some awkward position while trying to adjust the gap, tweaking the assembly so the faces of the points are parallel, dropping the tiny screws into the distributor, re-setting the timing whenever the points are changed which typically is every 12,000 miles.

Kits to convert points-style systems to solid state are popular for a reason :slight_smile:


(Robert Wilkinson) #14

Sure was easier in the old days. :slight_smile:

Those screws now seem tinier than they used to be. But back in the day I remember working on family cars (probably GM; was often my dad’s choice) whose points I could adjust with a dwell meter and a flexible hex wrench–dizzy intact. That was a pleasure…wonder why it didn’t catch on? What/when were those cars…I’ll bet DD knows.

(Pat Leask) #15

I agree, I’ve had electronic points and it was like a new car. But just last
month I stopped to help a guy on the side of the road with a failed
electronic points set up… which makes me, as I’ve said, think hard about
it. But that is/was one guy out of how many? (he should have bought a
lottery ticket at the same time)

But as you say, how often do they really fail? And if they do, carry the
points etc in your repair kit sitting in the trunk, or, simply make a call
to BCAA (I live in BC) for a tow and get something back on the membership
dues you pay :slight_smile:


(Paul Wigton) #16

Moi aussi…!

That, and a buck-and-a-quarter will get me a cuppa!!!

(Doug Dwyer) #17

That was the GM cars, yes. I think the externally adjustable points came out in '57. Very clever, probably patented.

'Long about 1970-ish GM introduced “UniPoints”…points and condenser in one assembly.


(Paul Wigton) #18

That was a definite improvement, and the ability to set dwell whilst the engine was running was a good feature of the GM ‘window’ dizzys.

There was also the trick of being able to take off the cap off a Mazda rotor motor, swap a few wires around, then START the engine and set the points while running!

On that system, getting dwell dead-nuts was actually important.

(Mark Lee (Pay Pal Patron)) #19

I still use those tools on the '70 Camaro. yes it was a snap, and far less painful than on a Series III. Could Jaguar made the distributor any harder to get to? I know, it’s evolution. the Series I and II isn’t hard to get to all.

This brings up a point, that I’ve always wondered. Coil placement on the Series III. I would think there’s a better place to put it than right on the top where the heat settles. I went through 2 Lucas coils in a year on the Series III until I stepped up and purchased a High Energy “racing coil” if there is such a thing. The coil carries a significant amount of gravity over the Lucas, which explains it’s girth. I copied the stock bracket out of aluminum bar stock in order to mount the larger coil in the same location as it’s English brethren.

Much to my surprise there was a noticeable gain in performance. Two years later, no coil related issues. The downside is the coil is red.

Side tracked again. sorry, back on track and my question
My question is this, has anybody relocated their Series III XJ6 coil?


(Robert Wilkinson) #20

Not exactly, but I have a Series 3 dizzy in my Series 1, with the coil in the S1 location (on inner wing, as far forward as it will go). The lead from dizzy to coil is longer than most. I also put the AB14 on the inner wing. This required extending the twisted pair sensor cable–which you’re not technically supposed to do.