Eh, I got turned around (180 degrees). Thanks for straightening me out. Must have been too soon after I got up.
Mistakenly, yes. It does not have vacuum retard.
Ben, I believe there is a pretty significant downside IF the vacuum advance is not properly calibrated. Too much advance plus a lean mixture can cross the heat threshold into detonation & preignition.
The pro’s of vacuum advance are what, 3-4 mpg?
If one emulates the vacuum in./advance degrees matrix in the factory manual you’re likely ok, right? But run more “mechanical” and more vacuum adv. than stock…all bets are off. Tough to hear light pinging over 3000 in an OTS at least.
My 2cents.
Fair point, I guess…but even an old vacuum advance diaphragm can be ‘tuned’ for the application. On the 123 being discussed on this thread, it can be tuned to absolute perfection.
But, more importantly, you’re most likely to encounter pinging under load, and so in an engine under load (particularly are higher rpm), manifold vacuum will have fallen to the point where vacuum advance is well and truly dropping out of the equation.
Even a little vacuum advance can really enhance part-throttle driveability, so I would argue that erring on the conservative side with it is well worth the almost negligible “risk”.
I agree with you about timing & response. I’ve thrown a lot more mechanical advance from idle on up. Maxes out at 36. I have no vacuum.
My old S2 diz was all in at 45 all in w/no vac. so when I went to 10 initial…how does 50 under load sound? So there is some leeway for sure.
I spent time in the powersport industry, there were a number of engine designs that had failures at “cruise” conditions. Just sucking a lot of air, light loads, and a normal amount of advance. Admittedly most aircooled. But my point is, there’s a line there. The beauty of the 123 is also the danger. So many variables.
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John,
One thing you may want to watch with using the vacuum unit on the 123 TUNE+ (Bluetooth version) is that the pressure transducer appears to measure absolute pressure, not relative pressure, unlike the traditional vacuum capsule used on the Lucas distributor. My 123TUNE+ equipped E-Type lives at close to 6000’ feet altitude, where air pressure is a little more than 80 kPa versus 100 kPa at sea level. When I programmed the vacuum advance curve from the Jaguar Service Manual into my 123, I found myself getting 10+ additional degrees of advance under almost all circumstances, even when under load, because the baseline pressure was so low. Though I didn’t hear any detonation, that can be hard to hear in an OTS at 3000+rpm, so I ended up configuring a much more conservative vacuum advance curve. This is probably why I have seen little benefit in fuel consumption. A little extra advance (say 0.5-1 degree per 1000 ft) at altitude is generally a good idea, but I was getting more than 10 degrees just from the vacuum curve. Because the old vacuum capsule generates advance from the position of a diaphragm with atmospheric pressure on one side, it automatically compensates for altitude…
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Does anyone know what’s inside one of those 123 distributors? I’m guessing it’s not a mechanical advance with a vacuum capsule. I looked over their web site but didn’t find any description of the innards. Is there even a rotor under the cap?
Yes, zere iss a Bosch rotor und kapp but nothing mechanical.
What I found most mind-boggling was the spark equalisation, where the 123 measures the effect of each spark in each chamber and adjusts their timing to equalize the acceleration from each firing. After I bought my first Weber-equipped Jag I spent a bit of time with six Colortunes to adjust mixtures for each cylinder. It made a difference but when I then switched to the early 123 it was an even bigger improvement.
Pity it was from a faulty batch and seized after five miles… but that’s another story.
Please tell me it didn’t take out the brass gears…
6000’??? No wonder the little rental car took a long time to get to your place! That’s 30% higher than Ben Nevis. Nice views though 
I’m at 9100’. You might require a bit longer to here! 
So wouldn’t this pose an issue for say driving from sea level to say Lake Tahoe? Would it be safe to say that for even those of us who live at high altitude, we should simply get the one without a vacuum advance and plug the hole in the carb?
You SHOULD be glad for the “mistake” you made.
I can not think of a single reason NOT to use vacuum. Even a racing car can do very well with vacuum!!
Another “specialist” who has no clue what he’s talking about.
Phixed.
Phixed.
“Cruise” IS a “Part throttle” condition.
Phixed
MAYBE. Should you wish to simulate ported vacuum advance, it can be PROGRAMMED that way (with the BT version)
Hear! Hear!! Give this man a Be!!s!!
Your engine certainly does have vacuum at part throttle (and idle is also a part throttle condition). If your 123 does not have vacuum, I’m very sorry for you. If it does and you are not using it, I’m even sorrier for you and even more sorry for your car.
Correct. I tried to convince 123Ignitions to also install a barometric sensor and allow barometric timing correction. Sadly, “none is as deaf as those who do not WANT to hear”. This concept appears to be one of the hardest concepts to explain(for me). Even the Megasquirt code writers could not see my point.
NOT SAFE TO SAY!!! Any automotive petrol engine will benefit from vacuum advance.
IF you have a high compression enigine, tuned at altitude, on a dyno, for optimal performance and driving down to lower altitudes, you would have to either retard the timing map, or simply rotate the dissy to retard timing. A low compression engine is generally much more tolerant of too advanced timing and MAY not require timing retarding, but it would still be safe to do so.
IF you have a low or high compression engine, tuned at sea level and driving up to high altitudes, you would not HAVE TO change the timing, but you WOULD find the engine losing more and more power as you climb. You would be able to claim back meaningful amounts of power by ADVANCING the timing as you climb (either by rotating dissy or raising the CENTRIFIGAL map - NOT the vacuum map) but never as much as it did at sea level - unless you have a turbo (a different story…)
Vacuum advance is VERY, VERY, VERY, good for any automotive petrol engine. Period.
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You must be up the slope at Angel Fire! I’ll hafta take a drive over Palo Flechado pass, and come visit ya!
That I am! And you are most welcome to visit anytime. But simpler to drive down I 25 and come through Cimarron past Philmont and down through Eagles Nest.
But Palo Flechado is a hoot to drive!
… in winter, sometimes not so much…
But, that sounds like a good trip: I’ll give ya fair warning!
I am getting the distinct impression, I should use the vacumn feature…
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Very true. We have to keep a close eye on the weather, but in the summer it’s da bomb!
A pic from the serenity of the hot tub watching the night skiers
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No kidding, but most people talk in terms of vacuum advance only having a benefit on the freeway.
My point was that it benefits around town too, and in a vast majority of driving conditions.
Most people are wrong. You are correct.
Vacuum advance has major benefits in town too (even for idle)
When I was a kid, we had some family friends who lived part of the year in Angel Fire. For some reason, I always pictured it as a big town.
Then my wife and I drove through it a few years ago, and I realized I was very wrong! 