Vacuum at idle - and vacuum in general

[esv]

Hello everybody,

I recently got my Fiat Panda running with the Megajolt. Great! For the time being I use the default "safe" map, it is working reasonably well already

I have seen various discussions about vacuum takeoff positions - on some cars, the vacuum is zero at idle, since the throttle valve covers the takeoff position.

My simple question / statement: The Megajolt should replicate the distributor behaviour. So, if the distributor is running with no vacuum advance at idle, that is what the engine wants, hence the Megajolt should be set to do the same thing (?)

Furthermore, if the vacuum takeoff is in a "funny" place (in or close to the venturi), or is ported, that should not matter either. The vacuum advance data is related only to the vacuum value. If the engineers at the factory have decided that the vacum values fed to the distributor should vary in a complex matter, it is probably what the engine wants anyway to behave properly. So we don't have to worry about this at all. In fact, it would be wrong to take the vacuum from somewhere else.

In my car, the vacuum advance is simply stated (Haynes manual) as 12+/-2 degrees, i.e. no curve is given. So what I plan to do is to just see what the maximum vacuum can become (driving the car on idle and under part load at various engine speeds), and set 12 degrees at that load value, with a linear decrease to zero at full load (no vacuum). If the car idles at zero vacuum, that would be perfectly ok.

Or am I missing something here?

[alexander]

some thoughts on the above:
moving weights, the vacuum tube and ported vacuum are just the best mechanical means available to achieve certain desired advance levels at various combinations of speed and load. they create an ignition map, just like the mjlj does. generally, from the point of maximising power and fuel economy, ignition is advanced with more engine speed, and less load (ie high vacuum). idle is a special case, where advance is retarded from ideal, in the interest of having a stable idle and nothing more.

the only way to cause the vac advance to move from nil at idle, to maxiumum just off idle, was ported vacuum. with the mjlj, you an achieve that completely with the ignition map. from the point of view of constructing your map, it makes more logical sense to sense vacuum from some other spot, ie in the manifold, where vacuum varies in a (relatively) linear fashion without the odd disconintuity on/off idle. if you run mjlj from ported vacuum, the active cell on the load map is going to jump from the high vac end of the map, to the low vac end as the throttle closes, making your map a bit awkward at that point. can be done, but i think it makes more sense to use manifold vacuum and avoid that 'jump' caused by the ported vacuum.

re the 12 +/-2 reading in the Haynes manual, i would guess that is a maxiumum number, as i think you have concluded too.

anyhow, those are my thoughts on the matter, so hope is of use and i realise it doesnt address all your questions.

regards
alexander.

[esv]

Thanks a lot for your useful input.

If I take "linear vacuum" from the manifoil, I would still want zero vacuum advance at idle, right?
How do you guys normally solve that when constructing the ignition map? Do you just set the vacuum advance to be zero in the low rpm bins?

[brentp]

esv, the original complicated design created by the car mfg (using ported vacuum, weights, vacuum advance, etc) is neatly solved by by the ignition map of the MJLJ: At X vacuum and Y RPM, set Z ignition advance in degrees BTDC.

Connect the MAP sensor to a source of vacuum (or boost, if forced induction) where the reading can be measured reliably under all conditions. A common point is on the intake manifold, often where the brake booster line is fitted, if outfitted with power brakes.

So, if you want to set your idle advance, you would measure what your engine pulls at idle- typically fairly high vacuum. set the ignition map at that load and RPM to what your idle advance would normally be at idle, and you can continue across the rest of the RPM and load range. Further tweak that area of the map (low RPM, high vacuum) to get the desired idle performance you want.

Take a look at other ignition maps for similar engines- I'm sure one would make a good starting point for you.

[esv]

I understand and agree. However, in order to know what the advance would normally be at idle, I have to first investigate how the ported vacuum (or whatever) behaves - hence what it used to tell the distributor.

What I _should_ have done, of course, was to measure total ignition advance at idle by using a strobe light with the original ignition system. But it would partly be difficult to get good readings, and besides the old system has been removed.

Anyway: this is going to be great fun! Thanks a lot.