Seeking feedback on 123 ignition (Bluetooth) curves I set

Hi all. After going through all the archives I’ve come up with some curves for my 4.2 with triple SU’s. For those that are familiar please take a look at these and see if you spot any concerns.

(I used David’s idea having the “dip” to help manage engine idle.)

One thought which occurred to me is that electronic distributor are dealing with set data points versus the infinitely variable weights and springs in a conventional distributor. So that as rpm fluctuations occur the conventional distributor is making incremental changes to the advance. For this reason I decided to fill in the gaps between the different rpm settings. So the timing doesn’t jump to far from one data point to the next, but moves along a series.

(I should note I have my distributor set at TDC so there is no static advance. I’m also close to sea level)

Looks ok , why not take it out for a road test and see how it performs ? IIRC someone did a dyno test and found no advantage in increasing the max advance beyond early 30 s , so that setting looks ok .
Maybe the vac settings are less important , more related to fuel economy .

I had my freshly rebuilt 4.2L engine on my 67 2+2 dyno tested years ago. Indeed, when we added advance, nothing good happened.
My thoughts:
The advance the engine wants/needs to obtain best horsepower/torque is a function of the hemispheric combustion chamber and the fuel quality/composition. Unfortunately, if you are running pump gas (versus racefuel you buy in 55 gallon drums) you really never know what your are dealing with day to day.
The human ear cannot hear the knock/detonation that damages engine internal components, as the frequency of the noise is very high. Which is why knock sensors were developed that feedback to an engine control unit that can pull back the advance if need be, on the fly.
I read somewhere, maybe Roger Bywater, that when then did tune by ear, there was a plink around 3000 rpm, which they lived with and did not seem to damage the engine.
Most curves I have seen are “all in” by 3000 rpm.
So in conclusion, your curve looks pretty good but don’t get greedy. I personally would back off to 30 degrees all in. Gives you a cushion over varying fuel quality.


A very good point, Harvey, that I have harped on in the past.

The detination that destroys an engine isn’t the stuff you can hear, but the stuff you cannot hear at high piston speeds. On a street car, it’s a zero sum game to try and absolutely maximize your advance for power. On a Jag six, 95% of the time, about 30° total advance is plenty for what anyone will need.

Are we talking 30deg advance with static set at 0deg or at 10deg…Steve

30° total crank degrees as verified by a timing light.

What he said. A dial back timing light makes it very easy to check.

It can be the sum of static advance plus mechanical distributor advance, or in the case of the 123, the distributor does all of the advance, at least the way Bob has set his up.

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This comment is confusing me. The 123 extrapolates between the data points you enter and delivers the advance figure represented by the red line for all engine speeds. It doesn’t hop between the discrete data points as you suggest. So, for example, for you curve, the advance at 2250 rpm will be half way between the data points either side at 2000 and 2500 - ie 29.5 degrees.

Oh! That’s interesting. I didn’t realize this. Thanks for pointing this out.

I read on another forum , someone who fitted a 123 with vac , reported a max advance of 48 * , but only at higher speeds when the throttle was closed , so maximum vac in the manifold . I presume this won`t have any adverse effects ?

Mark, generally correct. Not discussed in Bobs original post is that vacuum advance can be added onto mechanical advance at “light throttle” as a means to improve fuel economy. I don’t recall exact numbers on the Etype but your value is in the ballpark. That advance is pulled back when the throttle is substantially opened under hard acceleration, to avoid detonation/knocking. The fact that Bob has a graph for MAP (manifold air pressure) indicates he is also thinking about this aspect. Light throttle equates to high MAP. Full throttle equates to low MAP.

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What is the reason that your vacuum only starts at 30 and nothing at 0 or 20 ?
Where did you get those figures from and why 10 degrees advance. Page 338 and 363 of Bentley book suggests far more advance, if you are to double the figures as they are distributor speeds not crank speeds. Or is it a fact that on those pages you do double the ignition curve but NOT the vacuum amounts. There has been a huge amount of discussion about the programming of Bluetooth 123’s, with so many opinions. I have corresponded direct with 123 in Holland and not really had an answer that I can understand or makes sense to me. One of the major differences between 123 and ordinary Lucas distributors is that the 123 vacuum works from absolute vacuum.

Hi John the distributor figures in the service manual for mechanical advance are distributor degrees and revs…the vacume figures are shown as the amount of distributor advance degrees for the given vacume…so degrees and revs shown need to be doubled to represent crank…Yes for the 123 you need to calculate absolute vacume and not just enter figures from the Jag service manual…Steve

A time(ly) article…:grinning:

Sometimes I read these articles and see things that look wrong.

“As the throttle opens and load on the engine increases (it works harder), then pressure in the inlet manifold drops, which in turn advances the ignition timing at the distributor.”

I keep re reading, wondering what I am missing. I haven’t found it yet.
I would say when the pressure in the inlet manifold (vacuum) decreases the timing is retarded.

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I don’t think you missed anything. The statement in the article is nonsense. No wonder folk get confused, when articles that profess to educate actually contain complete falsehoods…


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Indeed… I saw that after I posted that article. I’m going to write to Hagerty…

I already added a comment to the article, so hopefully they will correct it.

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I just tossed off a letter, too: let’s see if the misinformation gets corrected.

Over 15 years of racing an E Type I spent many hours with both stock and modified XK engines on a engine dyno. On a stock engine there is at least a 15 plus hp gain on going to 45 degrees at higher rpm (3500 plus), and almost as much again with richer needles being used. Most of us seldom go there, but I autocross with my car and do, so I’m not recommending this for everybody. If you research recommended timing for stock hemi engines you will see 45 degrees commonly recommended. The timing curve for the Ser II distributor states max advance is 40 degrees as I recall, so Jaguar liked it.
I have a knock sensor that I’ve modified to permit me to install and remove it. I run a modified stock dist on a stock engine in my coupe, with 40 degrees max advance mechanically. There is no knocking detected, on 94 octane fuel.

One of the problems with the crankcase ventilation system on an XK engine is that crankcase fumes have the effect of increasing the risk of detonation. If you run a smoker you should be careful. When I autocross I have the system disconnected from the intake. Additionally the UM needle being quite lean can also contribute to detonation.

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