This was a techtalk post but I have decided it is more appropriate as a blog entry:

Every day is a school day. This is for a 1987 car with Carbs, no ECU with Lucas 43D Dizzy. 

I thought ignition advance was set with a timing light (or statically with a bulb) at x degrees before TDC and thereafter modified by vacuum advance (vacuum being taken from the inlet manifold - not ports). 

Not quite. Massive absence of understanding by me. 

I noticed what I thought were vacuum advance weights in my non vacuum advance distributor (Lucas 43D (45D has vacuum advance is the only difference)) and so I started looking. 

Mechanical ignition advance is as old as the hills and new to me. It has bits inside the dizzy that move as centrifugal force increases, and in this way mechanically advances the ignition (how much before TDC the spark ignites combustion so it has time to burn the fuel before exhausting it. So loads of exhausted fuel means high (usually too high) HC hydrocarbons in the emissions test.

O2/Lambda sensors test the level of O2 but only on one exhaust primary (ignoring wide band for now, though even that tests only the average) so the ECU (if fitted) can only do its best - and we do not have one.

Vacuum advance is for when the engine is under no, or low, load (i.e. under say 3,000 rpm) whereas mechanical advance is R.P.M. based and advances according to engine speed, not how much vacuum is in the inlet manifold (I am ignoring if 4 separate inlets). 

So. Do I want vacuum advance? It gets better m.p.g. when not maxed out (which is seen as 3,000 or 4,000 rpm plus - I need to firm up on this.)

I primarily want to optimise my stationary (tick-over at 950, no cat) emissions (merely as a personal challenge). Up to now with tuning activity I have reduced the HC from several thousand to several hundred and the CO2 and CO with that (she is street legal in any case). Lambda is at about 0.98 which is very good for an old lady and borderline start of good for a more modern car (with an E.C.U.).

Now then. Before I discovered all this I was going to fit vacuum advance (distributordoctor.com) for the first reason given and also to see if I can improve my 17 m.p.g. fuel consumption. That's expensive even if I am enjoying every drop.

HOWEVER, given that the existing mechanical advance will have already built in to it the advance for tick-over at 950 (anyone have specification for this?) it's not like I need vacuum advance for that. AND over 34 degrees (I think, maybe it's 38) of advance no further advance is applied and this is built in to the mechanical advance from I think about 3,000 rpm (i.e. where the Dellorto progression holes no longer have any influence). In which case any vacuum advance will be in action between 1,000 rpm and 3,000 rpm which is a range of only 2,000 rpm.  (Does it go to 4,000? Not sure.)

IF I had a 3.54 diff and five speed box and my use of the car was/is "round town" and part throttle cruising in fifth, when. that 2,000 r.p.m. range would be in frequent if not constant use THEN methinks vacuum advance would make a sensible reasonable effective choice. 

HOWEVER, I don't. I have a 4:1 open diff married to a Caterham Sixspeed gearbox and I never go round town or cruising (in any gear). The rev counter is rarely lower than 4,000, r.p.m. No it never goes on tracks or races. 

In other words given the revs are nearly always above the useful range of vacuum advance it would be a waste of technology. Also given that mechanical and vacuum advances are added together the mechanical advance profile would have to be modified as well, introducing more complexity and room for error. Just doesn't seem worth it. Better to replace all the 37 year old worn out rubber seals and tune what we have. AND of course obsessive tuning will improve the m.p.g.

If anyone got this far, bravo. 

Anthony

here's what chatgpt 4.o thinks (in short):

Application to Your Car: Given your driving style and high RPM usage, vacuum advance may not offer significant benefits.

and mine: chatgpt is always polite, it will never say "don't waste your time".