Haynes Ignition Timing

[Tintin]

If anyone is using the Haynes "Restoration Manual Jaguar XJ 6 Series 1,2&3" beware of the ignition timing data. Page 135 gives the dynamic timing of the 9:1 compression ratio 4.2 litre engine as 10 degrees BTDC at 1700 rpm. The Jaguar handbook gives 20 degrees - quite a difference. I have run my engine at both settings and 20 degrees gives the better result.

Robbie

[oldtimer]

How do you know yours is 9:1 compression ratio ?

[mike020150]

Ignition timing information in general is fraught with careless info. Writers have been known to list only the mechanical advance degrees required, omitting the (usually) additional static setting degrees. Equally, people get confused between crankshaft degrees and distributer degrees (half the first number because it turns at half crankshaft speed). So, all in all, it pays to establish the detail of any ignition setting before assuming it is the total ignition advance expressed in crankshaft degrees. Mike.

[Tintin]

How do you know yours is 9:1 compression ratio ?

The engine number has an "S" suffix, which indicates standard compression of 9:1. An "L" suffix indicates a low compression engine of, I think, 8:1. Looking at the data again, the 8:1 CR engine also has a dynamic advance of 20 degrees BTDC at 1700 rpm - the same.

However... the N American specification 4.2 9:1 engine is listed with a dynamic timing of "TDC at 650 rpm". Why it is set at a different engine speed I have no idea - possibly something to do with engine emission control systems?

I read recently that a rule of thumb for approximate ignition timing was as follows. At idle turn the distributor until maximum revolutions is achieved and then retard by 5 degrees. I don't know whether that is reliable but certainly, the idle rpm increased as I re-timed from 10 to 20 degrees.

Ignition timing information in general is fraught with careless info

Yes Mike, I have found that it is a bit of a minefield. Interestingly, the original Haynes manual for the car (1976, re-printed 1980) lists the dynamic timing as 20 degrees BTDC at 1700 rpm for the 4.2 - no idea why the later publication (1997) introduced a different figure.

Robbie

[oldtimer]

Hi Robbie
Certainly I recall S and L meaning Standard and Low compressions. I seemed to recall that Low was 7:1 and Standard was 8:1. In some counties that these cars were exported too, had questionable fuel quality !

I've just spent some time trying to find conformation of my thoughts on the subject. All I can find is in my 1956 Mk 7 workshop Manual , it refers to compression figures for 7:1 and 8:1 . As there is no reference to 9:1 I'm guessing 8:1 is standard.

My 1966 S type manual says 8:1 compression pressure 155 psi
9:1 compression pressure 180 psi
Interesting to have a read of these old Jaguar Workshop Manuals

Robbie, Make of it what you will , ref above

Paste my bedtime now

[Tintin]

'Morning Old Timer.

The XJ6 4.2 litre CR values are tucked away in the Workshop Service Manual after the Index and before the section on Engine Data - not the most obvious place IMO!

Robbie

[PPV]

Hi, for the Mk2 the Jaguar manual says that 2.4 engines are prefixed BG and have CR's of 7:1 and 8:1, 3.4 engines are prefixed KG and have CR's of 7:1,8:1 & 9:1 and the 3.8 is prefixed with LA and have same cr's as 3.4. The Cr's are then identified by a 7.8 or 9 following the engine number. Once the engine number has reached 9999/ the second prefix letter is changed and the numbering is supposed to recommence. My 1961 3.8 engine number is LB4894-8, hence identifying the second letter has changed to an A and it is the 8:1 CR.
Hope this may help.
Regards,

[oldtimer]

Hi Tintin and PPV,
After so many years of involvement with Jaguar cars I find all these Letters and Numbers so very confusing. There is an opportunity for the right nerd to compose a book listing these figures into a sensible reference book , sadly I'm not the nerd to do it. I shall have to be satisfied with just being a WAX OIL nerd and as Mike K says keep bleating on about it !

[Tintin]

You are far too modest OT. I reckon you should write a book about your experiences - they deserve to be recorded for posterity!

Robbie

[XJR308]

In my experience with carburetted cars it is best to set the timing initially with a vacuum gauge. Disconnect the vacuum advance pipe from the carb and connect the gauge. With the slowest tickover and balanced carbs, slacken the distributor and turn until the highest vacuum reading is achieved. If the tickover rises significantly the slow it down and recheck the vacuum. Then road test for pinking (detonation); if pinking is present due to poor fuel, wrong heat range spark plugs or carbonated valves/chambers, then retard a degree at a time, using a timing light, until it stops.
Bruce