10 degrees or vacuum for timing?

[Nikkum]

I know this is an old nutshell but my dilemma is that the engine runs pretty well when set at 10 degrees BTDC. it runs much better and smoother when set at just before max vacuum (21 inches at idle). With best vacuum at 850rpm idle the strobe indicates about 25 degrees BTDC. Should I use the vacuum gauge or set at 10. Even with engine wear I can't believe 25 Is acceptable at idle. Any advice?

[focodave]

Pretty hard to help, effectively, not knowing the specifics of your engine.
I will assume it is stock, with no modifications to the distributor, injection, or engine internals?

If so, you should be setting your advance at idle, with the vacuum advance disconnected, and the vacuum hose plugged.
10 degrees BTDC is right for the 2L, fuel injected engine which I am assuming you have based on your user profile showing this as a 1985 car.
If you are at a significant elevation above sea level, you may get away with a little more advance ( a few degrees) but you don't want the engine to "ping" under load, so you have to adjust using a little bit of trial and error if you are going to add any more than the standard 10 degrees.

[Nikkum]

Yes it's a stock 1985 2litre except for the Allison electronic ignition. Sea level. Shouldn't the vacuum be best at the engineer designed 10 degrees?
Tx for advice.

[focodave]

I don't have any experience with Mark's ignition system.
You are probably best to call him and get the specifics.

I know on the stock system, total advance is supposed to be around 28 degrees, all in, on the 2L engines.

[RRoller123]

+1, he is available on the telephone most of the time and is very helpful. On the original Compu-Tronics, set at 10 degrees, although I found that 12 performs a bit better overall on my engine. Minor shift.

[Yadkin]

Yes it's a stock 1985 2litre except for the Allison electronic ignition. Sea level. Shouldn't the vacuum be best at the engineer designed 10 degrees?
Tx for advice.

10 degrees BTDC at idle is with the vacuum advance disconnected. Reconnection the vacuum should then increase it to 26-28 range, depending on the vacuum advance settings in the ignition system.

The vacuum advance should be connected below the throttle plates for full manifold vacuum. That way you have lots of advance for smooth idle, lots of advance when cruising and little or no load. Both of these conditions, the fuel will be metered out lean.

The fuel mixture will meter out richer at WOT, and a rich mixture ignites faster. Because the throttle is open, vacuum drops, so advance is reduced to ignite the mixture later and prevent pinging.

[Nikkum]

Ok thanks everyone. Looks like it's 10 degrees BTDC! Or thereabouts.
I understand vacuum advance but this is surely not relevant if I'm using Computronix ignition system.
By "vacuum" I mean the vacuum generated by the engine measured with a vacuum gauge. Many say that the engine is best performing when vacuum is maximum . The problem is that when the engine is performing best at idle 850 rpm with max vacuum, timing is about 25 degrees not 10! I can't resolve this dilemma.

[Yadkin]

What is the input into the Computronix system? Is it supposed to be connected to vacuum source?

Manifold vacuum is inversely related to engine load. The higher the vacuum, the lower the load on the engine and vice-versa. So your electronic module may require that for an input. Either that or it uses a MAP (Manifold Absolute Pressure) sensor, which is effectively the same thing as measuring vacuum.

An old school trick in the age of distributors was to time the engine with the vacuum connected, tee into the source for a gauge, and advance the timing until the vacuum increased to maximum, prior to levelling off. Then you'd test drive it up steep hills from a stop to test for ping, and usually had to back off the advance a small amount.

[Nut124]

This is what i recall and has worked well for me:

10 deg BTDC at idle with any vacuum adjustment disabled/disconnected.
38 deg BTDC by 4000RPM with or without vacuum connected
38 deg BTDC 4000 and up with or without vacuum connected

No more than 38 under high load.

Once the vacuum advance is connected it will add advance at idle and partial throttle and improve response and power at low throttle.

At full throttle, high RPM, the advance should increase from 10 at idle to 38 by 4000rpm or so.

The fiat mechanical advance produces a total of about 28 degrees of advance. That is why they wanted you set it at 10 deg BTDC at idle (zero mech advance) so that the result would be 38 at high RPM. Less than 38 -> low power. More than 38 -> possible detonation, engine damage.