Technical Distributor trouble

[ppalma]

Hi all,
I own a 1987 (88?) X1/9 and have had it in my possession for the last 10 years.
Very early on I switched the Marelli S178MK(?) distributor (just looked it up in the Marelli catalog...) for a SE100EX(?) taken from a Uno 60.
Now some ten years after I'm having second thoughts about the ignition advance curve. The issue is that the original distributor doesn't have vacuum advance, so the centrifugal mechanism is able to turn some 25-30º on it's own.
Now the SE100EX (breakerless) originally had vacuum advance which I blocked using the arm from the S178, but from what I've been able to find out, it's only able to turn 16º on the centrifugal advance.
The bottom line is that I've been running (and lots of other people who run the same distributor) a seriously weird ignition advance curve. What's even worse is that the car runs pretty well, so I don't want to "fix it" without having a very clear idea of what I'm doing.
Any thoughts / ideas?

 

[ppalma]

I'm adding to my own messy post a few comments and questions that I hope someone can answer:

1. I wrote that the original distributor of my X1/9 is a S178MK, at least according to Marelli's catalog. After checking once again my Haynes manual I realize that the distributor shown there doesn't look like the one I had, in particular because mine had a plastic part screwed in place of the vacuum advance, which part had an arm fixing the whole vacuum mechanism. As far as I can tell, the S178MK doesn't have a vacuum mechanism at all. Correct?

2. After some careful consideration I realized that I understood the vacuum advance the wrong way. The centrifugal mechanism advances the ignition, the vacuum retards it. At WOT there is very little vacuum in the manifold, so we have almost only the centrifugal curve at work. At partial loads we have vacuum in the manifold and the ignition may be closer to TDC because the mixture is less likely to pre-ignite.

3. Because I'm running 95 octane fuel, the ignition timing is more than likely closer to 10º than to the original 5º. With the SE100 I should have a maximum advance of 25ºBTDC, which doesn't look so bad compared with the 28º maximum advance specified in the Haynes manual.

4. This set up runs OK on a daily basis because I drive moderately. Because the ignition is a bit retarded compared to the original, it actually feels as if the engine has a bit more go. But the risk of detonation at high loads is real, although I've never noticed it in 10 years