Total advance on XK engine

I recently had the frustrating experience of a 2 week old condenser going weak, causing really confusing run problems until I figured it out. So, I’m getting ready to convert my points distributor on my new Mk10 4.2 engine to Pertronix I, which I’ve been running on my Mk2 and an e-type for 25 years with zero issues.

I do find myself confused about timing on this engine. On reviewing previous posts, I think there are considerable differences in opinion on the best way to set timing on an XK. I may have to time it dynamically after the Pertronix is in.

This engine, stock 9:1 compression, is currently static timed at 10 degrees BTDC, runs ok, no pinging. Acceleration is decent on premium no alcohol fuel, currently running stock UM needles, UB were too rich. Idle at 650-700 is just a little irregular.

My dynamic timing light shows around 15 Degrees BTDC (vacuum line off), and total advance (hard to read) is around 34-35. The manual gives advance figures that suggest total advance of 10+9+10.5=29.5 in a best case scenario and about 25 with numbers at the low end. I do have significantly more than that.

I’m guessing my distributor springs might be weak. Seems like it might be better to use total timing. Or replace my distributor springs. If I set my timing dynamically, I’ll be putting my initial time closer to 5-6 when starting the car.

Any thoughts about why my dynamic timing is so different from static?

What total advance are the experts using on stock xk engines?

Is there a downside to 35 degrees total timing?

Depends: what is your altitude, and what is the final compression ratio of your engine?

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I dont have source to quote, but it was my understanding that 35 degrees “all in” was good for an XK engine.

I suggest you check the archives, as this has been discussed before

I have not looked at any of my manuals, but cant recall under 30 degrees as being correct

sorry to be light on detail, but from memory, isnt it static + twice what is stamped on the dizzy arms (around 12 ?) + relevant vacuum

Fuel octane will play a large part of the amount of advance you can get away with. High altitude, hot ambient temperatures and high compression all require higher octane to keep from becoming a diesel. If you are just driving it on the street, tune it just below the part-throttle detonation threshold.

Nope… it’s the opposite. The higher the elevation, the less tendency towards detonation, ceteris paribus.

That’s how I always set up streeters: there’s no real upside to squeezing out the last few degrees of advance.

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I am not quite sure where the 9 comes from, (maximum vacuum ?), but my interpretation of the data in the MKX manual (now that I am near my source) is 10* for static advance for a 9:1, plus 10.5 all in dizzy advance, but you have to double that for the crankshaft where you are reading it from, so 10 + (10.5 x2) = 31 + whatever vacuum advance is applicable, if you dont have it disconnected and blocked off for this test ?

Interestingly, Hamills book " How to power tune XK engines" mentions that advance somewhat higher than stated is ideal, pages 80-84

Excellent book, ideal for any Jag-Lover, do you have it ?

Thanks for the input.
There are lots of varying opinions in the archives, just revisiting, wondering whether I should have my distributor rebuilt to restore its advance curve, leaning towards “yes”, but waiting until after the driving season.

The numbers are from the Mk10 manual
10 degrees static
7-9 vacuum advance
8.5-10.5 mechanical at 2300rpm

As I stated in my original post, it’s a 9:1 stock engine, running on on non-ethanol premium pump gas. I’m at sea level - no plans for mountain driving.

Really, my question boils down to how much advance to expect at idle in a stock distributor in perfect condition. In other words, what should I expect the dynamic timing to be after setting it at 10 statically. The manual also states that the is no advanced timing before 300rpm, implying that there is certainly advance at 650. Is 5 degrees more than expected? I think so.

Looks like you’re misunderstanding the numbers. The Static number is measured in crankshaft degrees. The Mechanical number is measured in Distributor degrees, so to convert it into crankshaft degrees you need to double it. I wouldn’t worry about the Vacuum number for now, as you should block it off for setting up and testing the mechanical advance. So, the Mk10 manual settings result in 10 + 2*(8.5 to 10.5) total advance - “all in”. i.e. 29-31 crankshaft degrees.

No measurable economy gain?

Here’s the chart from the manual. It does not state crankshaft vs distributor (I was assuming it’s what I would read on the crank - that may not be right), but I understand what you are saying. Using your numbers, when you add the vacuum component, that’s going to add up to a lot of total advance, which doesn’t make sense to me. I do know that when you set up an engine based on total advance, most builders keep it under 32 degrees total advance at the crank.
Mk10 distributor chart.pdf (295.8 KB)

I agree that it could be clearer, but it does tell you to mount the distributor in a “centrifugal advance test rig” so it is clearly not mounted in the engine.

Vacuum advance only comes in when the engine is not under much load. When the engine is loaded, the vacuum advance capsule will not produce any additional advance. When we talk about maximum or “all in” advance we never include vacuum advance. Though the maximum advance that Jaguar specified for the early 4.2s was pretty conservative (as you say, 32 degrees or less), they later increased it for the Series 2 E-Types to the high 30s, or even 40 degrees. Those engines did not have vacuum advance, so the centrifugal maximum (plus static) is all you would see.

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Damn. You mean I’ve avoided Colorado for 40 years for no good reason? :man_shrugging::joy:

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Not to mention you have missed the golden oasis that is my personality…:wink:

Margaret, 10:1, tuns quite happily on 91 R/N octane.

Thankyouveddymuch, Mile High State!

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Minimal, if you don’t push the last few degrees of advance.

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Interesting. Thanks for explaining that. I do recall that the series 2 e-type that I had years ago was all mechanical. Since, like most home mechanics, I do not have a distributor testing machine, the chart is largely not helpful.
While this information is good, it does not answer some of my basic questions. I have an old distributor in a new engine which is advancing a fair amount at idle (without vacuum attached). I am trying to understand whether this amount of advance is normal for this distributor or whether it has weak springs.

I’ll take Tony’s suggestion and dig out my copy of Hammil’s. What I would really like to see is the data for dynamic crankshaft timing over a range of rpms, i.e. a curve. We do have tools with which we can look at the real world timing numbers, nothing very exotic about an induction timing light. Without data, it’s very hard to draw conclusions.

Tony,
thanks very much for your suggestion. The Hammill book is quite helpful, tells me what I need to know.
First off, while he suggest initially setting static timing to get the engine running, “Strobe light ignition is the only way to check and set the ignition timing of any high performance engine, simply because all variables are taken into account in the reading.” That goes along with my thinking.

He suggests that even with a high performance engine at 1000-1200 rpm idle that the advance should be 12-14 degrees, no more than 16. I have 15 degrees at 700 rpms, so may be the cause of my slightly rough idle. His performance engines have minimum 38 and maximum 44 degrees of total advance, so no need to be concerned about the amount of advance I’m seeing on this stocker.

I’m concluding that I have too much initial advance at idle from a weak spring, and I intend to change it. If it makes a difference, I’ll report back. Better not to convert to the optical trigger until I have this worked out.

Ron give this a read.

It will explain how the distributor works and what influences advance with increasing revs and total advance.

Pay particular attention to the design of the cam mechanism that defines total mechanical advance.

To check your total mechanical advance remove your distributor and mount in in a vice by the drive dog. Using a protractor you can measure the number of degrees the rotor button can turn. If you remove the top plate you can see the cam and the degree stamping on it.

Here are several! These are advance curves for different E-Type distributors used throughout the lifetime of the 6-cylinder models. There are a couple of additional non-standard curves from folks on the E-Type list. As you’ll see, the 41060 curve (for the S1 4.2 with triple SUs) is very conservative - maxing out at 29 degrees, while the later 41207 curve (for the S2 4.2 with twin Strombergs) is more aggressive - maxing out at 39 degrees.

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Now it gets interesting. To be sure that I understand, the 0 rpm number is the static component? If I read this correctly, for the 4.2 SU, it gives only a degree or so idle advance, which goes along with what I learned from Hammill. At the suggestion of a friend (thanks Doug), I’ll pull the distributor and see if I can tighten the spring at the ends until I can figure out where to get replacements. Distributor Doc has been suggested, but that’s England. Any domestic sources?
Many thanks.

Everything a guy could want to know. I’m printing a copy to keep on my bench.
thank you