Ignition timing

[Chris_R]

In another thread related to engine temperatures (https://www.joc.org.uk/forum/viewtopic.php?f=20&t=26143" onclick="window.open(this.href);return false;) Kevin posted

It sounds like timing, if it was ok until it was tuned. I set my timing as per the book, and got it spot on, 5 degrees advance at tickover, with 38 degrees maximum advance above 2500 revs, with both vacuum and mechanical advance working as they should. The car lost power, and the temperature just kept going up and up. Increased the advance by a couple of degrees at a time and the power came back in and the cooling went back to normal.

This got me wondering what governs the ignition timing and why a particular setting is chosen by the engine manufacturer.
Interceptor Mk1 with "B" series engine: 12½ degrees, with "C" series engine also 12½ degrees, with "D" series engine now 5 degrees. Interceptor Mk2 with the "E" series, also 5 degrees. What is the difference between these engine series particularly between the C and D that would change the ignition timing value? Very little that I can see.
All have the same compression ratio of 10:1. The "B", "C" and "D" have the same camshaft part number, the "E" has a different one. All use the same inlet valves.
Each engine series has a different carburetor. The "B" series has the Carter AFB 4131S, the "C" series the AFB 4299S and the "D" series introduces the AVS with the 4401S and the "E" has the AVS 4682S

So, among all this, what influences the ignition timing settings?

[Kevin Birch]

Interesting post Chris, I haven't got a workshop manual for my engine, and I went off a Mopar site to get settings, and nearly all said 5 degrees and 38 degrees full advance. No wonder I had problems if it should be 12 degrees, which is about where it is at the moment. Bear in mind that most engines these days have been re-bored and the heads probably skimmed so the compression ratio will be higher, so timing should really be adjusted to suit.
I've always been lead to believe, the advance is down to camshaft profile and compression ratio, then a compromise is made to give good all round performance. Obviously these days its all down to the ecu and knock sensors etc, but back in the 70's emissions were coming in so that was probably a factor, and power output wasn't a priority. Available fuel octane would also be a consideration, in Italy I put some 100RON fuel in, it made a noticeable difference to the performance.
I've thought of fitting an Aldon Automotive Amethyst mapping kit, so it can be setup more accurately to improve running in all circumstances.

[Grant]

Kev and Chris
All these figures quoted in workshop manuals are non-applicable anymore due to the pump fuel being vastly different from years ago,
The best solution now is to fit a mappable ignition system as Kev suggested, or if you don't want to do that.. simply advance your timing and drive the car locally up hills & on the flat pressing your foot to the floor, if you hear it ping retard it a bit, try again and keep retarding it until it doesn't ping anymore, sadly the more you retard it you loose power, if you put a mapped ignition system on you can map the pinging out with a laptop and still keep the power. Of course I have suffered all these problems with my 10:1 comp Jensens and 3.0ltr Capris driving them up and down the lanes lifting the bonnet with an 1/2 socket adjusting the dizzy until happy .. hence the rolling road that I have now got in pieces!!! ... we live in a throw away world now and also a mapped ignition one!!

[Chris_R]

Grant, the point of the question was more the theory rather than what might apply today. Like Kev, I thought it was related to compression and camshaft profiles but here we have the "B" and "C" engines with same compression ratio, same camshaft part number (so I assume therefore identical camshaft) yet originally quite different timing. I know of course the principle of advancing the timing but why a particular setting would be chosen i.e. 12½ degrees and then why another engine with apparently the same characteristics would be changed to 5 degrees is something I can't discover. I can't find a good explanation of why a particular setting is chosen. I can't believe it was originally selected by trial and error!
I assume that when new, all the "B" series engines worked fine at 12½ degrees, as did all the "C" series at 5 degrees. So what changed and why is what I want to know.

[Grant]

Oh I see your point Chris ... yes that is an interesting question of which I wish I knew the answer but sadly I don't, Oh yes It wouldn't have been trial and error like I do to get Max performance No, but maybe the difference between these B, C, D engines could be something to do with the heads, they seem to change the heads slightly throughout the years, what the actual differences were I don't know, and maybe distributor advance springs that were also altered on various models,... I guess Chrysler must of bench tested or road tested these engines and set the figures at the engines best performance though ... Now I want to know the answer too!! Could be time to raise a thread on the Mopar forum

[Steve Payne]

My guess would be it is to do with the advance springs and maybe the vacuum advance cylinder specification.

You can only put so much total advance in an engine and maybe they found with softer or harder springs the engine performed better mid range.

I know when I mapped my ignition on my car I was suprised what worked best and how much advance I could get in at some points yet a few 100 rpm later it needed to be 5 or more degrees less or it was pinking. Obvioulsy with a conventional system this is not possible so taking some of the initial out but having softer springs could make the car feel more lively and responsive.

Steve

[Joe Schiavone]

Just a thought. Have you verified that top dead center is correct to your motor indicators. I know my gas guage low light is way off. It is way easier to adjust top dead center for reference. I know this does not answer you question in theory but you have a lot of variables that can occur between years of manufacture and displacement size and coincidentally same numbers. RacerJoe

[Christian]

What is the difference between these engine series particularly between the C and D that would change the ignition timing value? Very little that I can see.

From 1967 (C-Series) to 1968 (D-Series) there was the change from 516 heads to the 906 heads which means from closed chamber to open chamber geometry. This should have a huge effect on combustion process. And the change from AFB to AVS adds a considerable difference that could effect combustion and therefore ignition timing.

Christian

[jglarkin]

I had an issue with timing being nonsensical, it turned out that I had fitted the wrong damper so the timing marks were in the wrong place, caused the engine to run hot. Why they have to be slightly different is beyond me.

Jon.