123 ignition for XJ6

Got quoted AUD 250 TO refurbish my original dizzy. Landed a 123 (eight switchable curves) for AUD 525. Wife’s Christmas present - it is her car. Curve number 8 runs very smoothly. No moving advance parts - mechanical or vacuum. It’s my second one in a car - nice, quality kit made in The Netherlands. Paul


1 Like

Hi All,

I too purchased a 123 Distributor for my XJ6 Series 1 and I have been very happy with it.

Mine is the Tune+ Bluetooth which allows me to map the Mechanical and Vacuum Advance. The software can sometimes be a tad flakey but the updates over the years have helped.

I especially like being able to store multiple curves and load as required. Further, it allows me to lock the distributor immobilising the vehicle which is handy on the odd occasion.

I would be very interested to hear from others who have used this and what your ignition curve looks like. I run 7 degrees initial timing with the following curves (currently 31 degrees all in) - the plan is to increase all in advance by a couple of degrees and then reduce the all in RPM. Currently, the engine is very happy through out the rev range - no pinking!

The real question is, Edward - what does the engine want…:slight_smile:

Any engine has a specific advance timing for each combination of rpms and loads for best performance - giving the ideal advance curve.

The closer you get to this the better - but this ideal curve is sort of theoretical; the mechanical distributors simply isn’t precise enough. In practice we will be lucky to find the advance curve given for a specific distributor fitted - which of course Jaguar picked after due considerations…

The 123 is not xk specific, and the best bet is to find the curve used by Jaguar - and pick the one closest to it. Bearing in mind that the initial setting moves the curve in toto, but does not change it - and the initial setting also relates to petrol quality…

The xk will likely run on any reasonable curve, but for best results from scratch would require a dyno for proper assessment and selection - seat of the pants selection is a crude approximation. Crudely; the closer the engine is running to the ‘pinking’ point - the more power…

The crux is that the curve must fit the engine - the engine’s requirements are fixed…

xj6 85 Sov Europe (UK/NZ)

I quite agree Frank.

Although I must note it is nice being able to easily pull some timing out of the engine should you be forced to run a lower octane fuel. I use my car as a daily driver and this occasionally happens.

Whilst the curve is loosely inline with the Jaguar engineers, the engine is some what different to the original, most notably much higher compression, and thus the original curve is not a perfect fit. Eventually, I will organise some dyno time but at the moment simply enjoying the process of an educated ‘seat of the pants’ approach. :slight_smile:




my twin SU XJ was treated to a 123 ignition by the PO. I do have 16 different curves, but, frankly, access to the switch is so hard that even the PO probably never cared to try them all out. I for sure didn’t as the car runs well. Sadly, I don’t even know which curve is selected :frowning:

That being said I appreciate the more recent and advanced models permitting to adjust “while you go”. One of the nice consequences is that you don’t have to ask others about their experiences ;-), because you can simply go out and have a try. As Frank wrote: everything is good that makes the car go better without pinging.

A “normal” reaction is that more advance leads to more top range power (most times useless) to the detriment of low range torque (most times required). Without taking up expensive dyno time you can achieve similar results with a stopwatch and a stretch of open road, preferably uphill. You can easily find out how easy the engine revs up and - beyond shift points - how hard it pulls, finally, whether it starts pinging under load.

I suppose your 123 ignition came with the same suggestions for timing curves as the simple systems:

As you wrote, with a considerably higher compression than standard “S” your mileage may vary though …

Good luck and keep us posted on the results


75 XJ6L 4.2 auto (UK spec)

This is of course a very valid point, Jochen…

One would expect Jaguar to have taken this into account when selecting advance curves to suit a driver that drives a Jaguar…:slight_smile:

But it also means that experimenting with timing is legit - bearing also in mind that ‘our’ engines are no longer factory new. And that petrol qualities are not necessarily as Jaguar intended for the xk…

xj6 85 Sov Europe (UK/NZ)

What about getting the cheapest fuel and then fine tuning it on a hot day… make notes rpm/load. Back off when you hear it pinging. Then advance everything a bit for better fuel maybe, and colder days. Finally, take timed runs and see what works best.
Starting point a known useable curve and go from there… I’d really like that adjustable on-the-fly 123 too!
It would probably pay off to get it to a dyno if available.

The E-type list has some curves if the engines are comparable at all. I guess a little.


The only issue with this might be that there is some pinging - at high rpm - that you might not be able to hear. Some of the 123 curves are quite aggressive at the beyond 3000 rpm. Paul

1 Like

Hi Ed,

This link has some info which might be of interest for DIY optimisation of ignition lead:


This provides insight into the process from a tuner, but you will need to overlook the EEC specific stuff…


What is missing in the very good discourse, Simon, is the precision of the system throughout the rev and load range…

Concentrating on a specific rpm/load for tuning the ign timing is fraught with a distributor. What is dead right at one point may be horribly wrong at other stages. While compromises are mentioned in one context, like gain/degree, omits the compromises that must be made in other contexts…

With modern ign systems, computers give a capacity for handling a lot of inputs. Theoretically giving optimum ignition timing in all conceivable combinations of rpm/loads - backed up by anti-knock sensor to avoid the destructive effects of knocking.

Knocking is spontaneous detonation of remaining petrol due to mixture temp reaching petrol ignition point before normal combustion is completed. Basically due to ignition occurring too early - due to high air ingress at suddent


…WOT which also increase fuelling. Compression in itself generates heat; the increased air ingress combines with burning petrol to rise temp - and the engine knocks…

But your intent is worth while pursuing - so there…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

My late e36 always knocked shortly ad sudden full throttle. Never hurt it. The full throttle and high rpm conditions are most important for the damage they can do and everything else can be approached later on, hence my suggestion with the lower grade fuel…?
I have a part throttle pinging at 60-70 mph and not solved it satisfactory yet, and still the plugs and cylinders show no signs of damage (the engine is not worth anything with 5 broken head studs and a smashed tappet guide). It’s been more than thousand miles by now. Were it on full throttle I‘d have it fixed right away.
I wish there was a self learning 123 with a knock sensor.

I would back of the ignition a tad, David - whatever the reason. Apart from loss of power while it pings; damage will invariably occur over time. It’s petrol detonating - certainly increasing stress on the studs…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

It started out when driving back from the UK last year, I backed it off a little but seemingly not enough. Now for the driving I do in winter I‘m okay and just stay below or accelerate past that range and therefore it can wait until April.
But yes, surprisingly, the studs still do very well. :slightly_smiling_face: Happy you remember.

My point was, when we drove home we just stayed below audible knocking and I was sure that at that speed we just couldn’t hear it. Consumption was low and I was surprised that there was no apparent damage when I inspected the plugs during and after that drive, and even now there is still nothing; they look perfect. The engines can take a little, for adjustment purposes that is.


1 Like

David - are you using the highest octane fuel available in your market?

Tuning literature that I’ve seen suggests that MBT timing is commonly 4 degrees or more less that the point of audible knock, and detonation can’t be taken lightly, so I agree with Frank on pulling timing. If this is really wrong for lower load operation, you might see coolant temps lift as combustion energy sees more cylinder wall exposure. But if there’s a problem with timing indication, maybe you will get a general performance improvement.

I suffered this problem in a car that I later discovered had about .125" of material built-up on the piston crown - too much CR for any octane available to me.

First thing we did was 98 or even 102 octane. Not much help, strangely.
Second thing was retarding as much as I could with the ‚vernier‘, that is the little slotted hole in the dist. Bracket. Helped a little! I could/should have tried disconnecting the vacuum advance but that is a can of worms.
Of course it did run a little hot after hundreds of miles of road on a warm day, but that couldn’t have been the cause since it still does it.
I think the distributor is no longer matching the engine. Someone before me went in and put in colder plugs, way overadvanced the timing (it would kick back most of the time!) and the cams had a lot of overlap.
I overadvanced the intake cam and now it idles smooth, lost some top end and probably lead to the distributor being a little too early on light load situations. The proper solution might be as easy as just slightly retard the intake cam and leaving the distributor as set.

Anyways, my point was that it won’t be killed by slight detonation, but you also couldn’t tell from plugs and pistons. Of course I will correct it as soon as I have time to spare, but in the meantime when I‘m driving I‘m rarely in that band and it is cold outside which helps. Thank you both for your empathy!

What does MBT mean?

Apologies - Maximum Brake Torque - this seems to be the preferred expression of peak torque vs timing in the tuning world. There’s much on the interweb for more…

Oh I see, makes sense. Thank you!
When I come around to it, I‘ll report, if I don’t get it in a 123 might pop up on ebay… for €320, I‘d probably take that after cc some fuss.
MBT timing sounds and feels alright if not too late: part throttle is too early. If it was knocking uphill I‘d rather stop the car and walk.


Even ‘slight detonation’ means loss of power, David - but there will be no visible negative signs on plugs and pistons…

In principle, there should be no knocking with steady pedal. Opening the throttle increase manifold pressure, forcing more air into the cylinders - which raises compression temps, and residual fuel detonates.

You may have increased compression for reasons mentioned by Colonnial - carbon build-up in the cylinders, which renders original settings useless. And indeed some fault with the distributor as you imply; have you checked that centrifugal advance moves as it should - and vacuum advance ditto…?

Have you also checked vacuum output to the dist in idle; ‘US’ usually use ‘ported’ vacuum - basically 0 with throttle closed. ‘Europeans’ use manifold vacuum to the dist. In all cases; dists has a maximum combined advance before end-stops. And what is your actual advance setting in idle…?

Like others; I think your attitude to knocking is somewhat cavalier. While brief knocking during heavy loads indicates that the ign setting is ‘au point’ - knocking in itself is bad.

And backing off as much as necessary is the proper remedy. If using the vernier, with its limited range, is insufficient - the distributor should be repositioned. In principle, distributor position is to be set to the desired advance with the vernier midrange - then the vernier to be used for finer tuning…

As an aside; having the vernier is a great asset…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Frank; you‘ve seen my pistons when the head was off, they still look about like that.
I also couldn’t rule out a weaker injector but it idles well. Centrifugal and vacuum are moving as they should. Manifold vacuum. 10° (I think; to spec for the set up) static before I dialed it back a little. It is just a small window at part throttle. It runs well in all other situations hence my somewhat cavalier attitude. The car is at home, 100 miles away so it will take me some time to solve this. Until then I‘m a lame duck.
Thank you