Vacuum advance

[partsetal]

I know the curved arm can't be used in the straight arm distributor but have no experience using the straight arm advance in the curved arm distributor.

[Frango100]

I just took the vacuum advance actuator from the distributor. I have the curved arm actuator and i doubt that a straight arm will fit in there. I will have a look for a curved arm actuator here locally, or order one from Rockauto.
The vacuum advance actuator has a possibility for adjusting the amount of vacuum when it starts advancing. Is there a procedure how to do this? I know on my old actuator it had several shims (rings) under the spring, as adjustment.

[Frango100]

Received and installed the new vacuum advance actuator and put three of the previously used 5 washers on the spring in the actuator. While I adjusted the ignition to about 7° without the vacuum connected, it went to over the 20° with it connected. I understood that the vacuum advance should give max 10°, is this true? I put the other two washers in there as well and the advance went back to just below the 20°. (at idle). When I revd it up it went to just over the 30°.
I was reading some literature about vacuum advance and the guy over there said to NOT use the constant vacuum port, since at idle you don´t want a lot of advanced ignition. I now have two opposite stories, so what is the best to do?

[scumdog]

. When I revd it up it went to just over the 30°.
I was reading some literature about vacuum advance and the guy over there said to NOT use the constant vacuum port, since at idle you don´t want a lot of advanced ignition. I now have two opposite stories, so what is the best to do?

Use any port that eminates BELOW the throttle plates, you DON'T want to use 'ported vacuum' which is from above the throttle plates.

At a light throttle opening this will give you maximum timing advance, this is when you want all that advance.
When you hit the gas pedal hard the vacuum will drop - as will the advance, this again is what you want ( well what your engine needs to be precise - otherwise it will ping like a Jamaican steel drum band!)

At idle all that advance from the high vacuum doesn't matter, your engine has no load on it at that time.

[simplyconnected]

I'm with Tom. Use manifold vacuum not ported.

When cranking your engine there is NO vacuum (or very little). This helps the starter motor. As soon as it starts, your vacuum actuator adds lots of advance because the engine is not under load. Set your initial timing around 6° with the vacuum hose off.

Total Advance includes the initial advance. For example, if we start with 6°BTDC at idle with the vacuum hose off, and slowly increment the speed in 500-rpm increments, the timing marks should steadily advance to ~36° at 2,500-RPM and hold there with no more increase as RPMs raise further.

Vacuum advance is totally separate as it performs a completely different function. Cranking, we should expect to see very little vacuum advance. When running, as the throttle opens under load, vacuum goes down. When climbing hills there is very little vacuum. So vacuum advance is designed for steady throttle conditions and it adds to Total Advance.

For this reason, the vacuum module 'dithers' at a light. When you add more pedal vacuum decreases and so does advance. When you lift off the gas, vacuum resumes and it adds more advance.

If you experience pinging (usually under load), set your timing back or restrict the vacuum module so it cannot add too much advance. This is why I buy adjustable (with an Allen wrench) vacuum advance modules. - Dave

[Frango100]

Thanks for the replies guys. I can adjust my vacuum advance as well, but with washers. Should the maximum vacuum advance be 10°?

[simplyconnected]

The direct answer is, it could be more or less depending on the load. As said, you need to try it out on your car. A distributor machine can recurve but it cannot ever simulate a load. So, start a ten. If you experience knocking, back off. If not, give it more advance. The only caution here is, when you hear knocking, back it off.

Modern engines have knock sensors that tell the computer when to back timing off. It's impossible for anyone sitting at a computer to say what setting is perfect for your car because there are too many factors including fuel, octane, vehicle weight, engine rpm, etc. A different way of saying this is, put a Mustang engine into a Crown Victoria and all the settings change. - Dave

[Frango100]

Ok, thanks Dave. I drove around today and the idle is much "stronger" then it was before with the old, not working, vacuum advance. Before, I was putting the transmission in N when waiting at a traffic light, because it started to idle erratic, but now I can leave it just in D.
I didn´t go on the highway yet, but while accelerating hard in the city, it performs very well. No knocking of any kind and it seems that the engine temperature stays lower now as well.

[scumdog]

Ok, thanks Dave. I drove around today and the idle is much "stronger" then it was before with the old, not working, vacuum advance. Before, I was putting the transmission in N when waiting at a traffic light, because it started to idle erratic, but now I can leave it just in D.
I didn´t go on the highway yet, but while accelerating hard in the city, it performs very well. No knocking of any kind and it seems that the engine temperature stays lower now as well.

Sounds like you're on the right track!

If you feel brave you could advance your timing another 2 degress and road test it - and keep doing that until you hear 'pinking' - (that's what we call it in New Zealand!) and then back off a degree.

[Frango100]

Sounds like you're on the right track!

If you feel brave you could advance your timing another 2 degress and road test it - and keep doing that until you hear 'pinking' - (that's what we call it in New Zealand!) and then back off a degree.

What is the advantage of keeping the ignition timing as close as possible to where it starts knocking? Maximum power and fuel economy?

[simplyconnected]

Tom is suggesting you advance until you hear knocking, then back off the advance. Engine knock is a bad thing. Severe knock can cause piston, cylinder and head damage.

The idea is to allow fuel to burn with the most efficiency which also renders the most HP.

This is a trial and error test where the burn starts long before the piston makes it to the top but maximum pressure is produced just as the piston descends. All this happens very quickly when RPMs are running fast.

A mechanical load on the crankshaft will resist the piston from descending. If the spark happens too soon, the piston skirts will slap the cylinder walls hard and produce a knock. - Dave

[scumdog]

What is the advantage of keeping the ignition timing as close as possible to where it starts knocking? Maximum power and fuel economy?

Right on the money! It will give you optimum fuel economy AND power.

As an aside; if the timing is too far retarded/not advanced enough it will waste fuel, cost you power AND cause overheating, possibly enough to make your exhaust to glow!