It was 2008 I think (twelve years ago, gulp!) and I can’t recall where I got the info.123 is a Dutch product and I may have emailed ib Flemish - which Philip would read as Afrikaans!) or spoken to one of their people at a show.
I still find it mind boggling but from what I recall it was based on minutely sensitive time measurement, whereby the time between spark signals was used as a surrogate for power generated by the timing on the previous firing. I don’t recall if it was the previous cylinder in the firing order or the previous firing of that same cylinder (I guess the former). The net result was that the timing for each cylinder was micro-adjusted, presumably within the parameters of the core map, and it was technology derived from F1 racing that did seem to work on that Weber D replica. I’m sorely tempted to get a positive earth one for the current D but it goes against my originality quest for this car.
Since my knowledge of electronics stopped around the time of schoolboy crystal receivers and two-transistor radios, I may have been too gullible when this was all explained to me, but they are a serious company…
Phillip, I believe it is both ways. It “knows” misfires by this time difference.
thelews:
From the article…
“Ported Vacuum” was strictly an early pre-converter crude emissions strategy and nothing more. Don’t believe anyone who tells you that ported vacuum is a good thing for performance and drivability – it’s not
I agree with that article 100%.
[/quote]
I am not going to argue the “big” advantage you and others see in vacuum advance at idle, but I do firmly believe it was NOT some emission solution. This is from a 1947 Packard manual:
I do not believe the engineers back then were stupid. They knew the difference and they decided on ported vacuum for a reason. I agree with Kilbert that it may be steady idle. Or maybe better response, or simply that ported allowed the advance to come in slow, get large, then back off slow. But what ever there reason, it was deliberate. That does not mean I think YOU are wrong for desiring vacuum advance at idle. Everything is a compromise. Apparently you desire the compromise that includes vacuum advance at idle. Apparently some pre-emission engineers desired ported vacuum advance. Being non-imaginative, I desire the stock setup.
Tom
I don’t see how the article does that? Regardless, one clear aspect of ported vacuum advance is the fact that it makes for a quicker warm-up of the engine by rendering the engine less efficient at idle, thus producing more heat, faster. The down side is that this inefficient idle now also exists when the engine is warm, every time it idles, all the time it idles.
And then I read how many owners suffer from overheating cars… Sad.
Tom’s assumption is that no OEM was trying to control emissions in 1947, so if they opted for ported vacuum over manifold vacuum, then it was done for reasons other than emissions control. This seems like a reasonable conclusion in my mind.
I have read numerous article that tout the benefits of manifold vacuum at idle, so I have experimented with it quite a bit across several cars. I would say that about 90% of the effects seem positive, but I have also never been able to achieve a stable idle. This thread is actually the first time I’ve ever seen that symptom mentioned, so I’m glad to see I’m not alone.
For me, manifold vacuum at idle is one of those things that makes a lot of sense in my mind, and is something that I WANT to work, but every time I experiment with it, I remember why all my cars are still plumbed to ported vacuum.
What bothers me about this “unstable idle” thing is this: Are we talking about engines where ALL OTHER POSSIBLE CAUSES of unstable idle has been eliminated? Or are we resorting to Ported Vacuum to HIDE the causes of unstable idle…??
As I have stated before, I do wish there was some documented reasons from OEM’s that discuss why ported vacuum advance was chosen pre emissions. In this attached article from Delco Remy in the 1960’s, they also show ported vacuum, but no specific reason. They seem to imply they want the vacuum advance to come in slowly. And on the page numbered 13, they show a graph of mechanical and vacuum. It seems to indicate they do not want the vacuum advance to come in all the sudden as one speeds up, but they want it to come in slowly as the throttle is open. And yes, that is an assumption on my part, so others clearly have room to disagree. And as I have stated before, I can see that if the vacuum advance had the timing advanced at idle, then, as one floored it, I could imagine a split second of ping until the vacuum advance retarded.
edit sorry forgot the attachment: timing (3).pdf (556.9 KB)
Tom
I’m talking about two, Holley-equipped V8’s that both idle rock-steady with anywhere between 10-14 degrees of initial timing. I hook vacuum advance up to manifold vacuum, and they’re now pulling in more timing at idle, so idle speed rises.
I now have to back the idle adjustment screw WAY off to get down to a reasonable idle. Cool. Take it for a drive, and now it’s idling too low. Adjust the idle screw back up a bit. Cool. Take it for a drive, and now it’s idling too high. Rinse and repeat as many times as you like, until you’re fed up, and connect the vacuum advance back to ported vacuum so you can get on with your life.
When we say idle instability, I don’t think anyone means an engine that sits, and surges up and down…that certainly hasn’t been my experience.
Another complaint I’ve heard against manifold vacuum is that as you crack the throttle to pull away from a standstill, manifold vacuum drops, timing then retards, and you can get a pronounced off-idle stumble.
I have personally never noticed this during my experimentation with manifold vacuum, but an Ignition specialist that I have a lot of respect for often cites this as his biggest gripe against using manifold vacuum.
Using ported vacuum, you only gain timing by way of the vacuum source as you pull away.
I took this one step further with my 123 map as illustrated recently in this, or another related, thread. I set up a negative gradient advance curve section around my desired idle speed of 700rpm. Below 700rpm I INCREASE advance, which causes rpm to increase back towards 700rpm. Above 700rpm I DECREASE advance, which causes rpm to descrease back towards 700rpm. This negative gradient curve just covers the 600-800rpm range. Below 600rpm I drop advance to zero below 500rpm. This makes advance zero at cranking speed which eases load on the starter. After a short constant advance section above 800rpm I program a curve similar to the Lucas distributor centrifugal curve. I have set up the Vacuum Advance curve (Advance Increment vs Vaccum Level) to only come into effect above 1000rpm. This keeps it from interacting with the idle control that I so carefully set up above! The advantage of a programmable device such as the 123 is it gives you a lot more flexibility to give you more ideal control of advance. The disadvantage is that it gives you a lot more flexibility to mess things up…
One thing to bear in mind is that the amount of advance is a function of the advance capsule fitted and the desired advance capsule may be a different capsule for a ported vacuum source vis-a-vis a manifold vacuum source.
The other thing to remember is that ported vacuum is manifold vacuum but with the vacuum thrown away when the throttle is closed and much diminished when the throttle is only just open. It is no different from manifold vacuum once the throttle is significantly open, so all you are talking about is how much vacuum is used at tiny throttle openings and how it cuts in immediately the throttle opens.
I posted a table derived from a datalog from a drive showing the difference between the two on either this forum or the UK forum a while ago as my car has multiple sensors all round the place.
Ported vacuum is a rubbish idea for this application as it means the 123 user cannot meaningfully control the advance when throttle is closed or only just open, whereas a manifold vacuum source still gives him that choice.
kind regards
Marek
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
75
A clever engineer could have designed a vacuum advance system that would avoid the possibility of an unstable idle. Basically, if you get 18" of vacuum at idle, design a vac capsule that runs up against a stop at 15" of vacuum. So, anywhere from 15" to 20", the timing doesn’t change. That’d ensure a stable idle, but it also might provide less than optimum vacuum advance at cruise, I dunno. In any case, it’s evident that most automotive engineers felt that using ported vacuum was the better solution.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
76
I recently bought a used Lucas 41060A distributor to play around with on my '68. I like the way it is running now with the stock non-vacuum advance distributor so I wanted a second one to be able to easily take it back to original if I do not like the results. Given the debate here of ported vs manifold vacuum I think at first I will just try teeing into the little copper pipe that runs between the manifold and front carb with a rubber elbow to see how I like the result. This way I would not have to drill and tap at the carb for the vacuum source.
123 suggest a 1 ohm minimum and their position is as long as the coil is operating well, the “brand” hasn’t shown to make any difference. I’m happy with my new 3 ohm and the car is running super well now.
thelews:
