Ignition timing advance - vacuum vs mechanical

[Sam_C]

Can someone explain to me the interaction of the vacuum advance and the mechanical advance on the distributor please? When I set my timing using a strobe light, I do it to give 34 degrees advance at 3,000 RPM as by this point the advance should be "all in". This in turn means that the timing at idle falls where it will and that's OK as you are more concerned about getting it right at higher power settings.

You time it with the vaccuum hose off the distributor, presumably so that it doesn't affect your readings and only the mechanical advance is producing the 34 degrees advance. Let me know if I'm OK up to this point...

So if you now connect the vacuum hose, is the idea that the timing will immediately advance to 7.5 degrees or whatever the figure is mean to be at idle, or is there not enough vac at this point to actually move the plate? Does it only start working when you increase throttle to combat flat spots whilst waiting for the mechanical?

And finally...once the mechanical comes in, does it "override" the vac or add to it? For example, if you had your dynamic set at 34 degrees at 3,000 rpm and then connect up the vac, will you still have 34 degrees or will you have added the 7.5 from the vac to give a total advance of 41.5?

I now sit back and and wait for somebody to point out the bleeding' obvious that I have missed

Thanks!

 

[jonbills]

They're completely independent of each other.

The vacuum is greatest with a closed throttle and tapers off quite quickly as the throttle opens.

So yes, on a closed throttle at > 3000 rpm you'll have 41.5 deg advance.
On a light throttle, cruising at 3000 rpm you might have 38 deg advance.

Full throttle at 3000 rpm you'll have 34 deg advance.

I can't remember the science, but with low load you want more advance to get a more complete burn and thats what vacuum advance does for you.

 

[Sam_C]

Many thanks Jon!

 

[johnymd]

This is why a road car should have the vacuum advance connected as it will run much nicer and more efficient when cruising. On a race car it is not used as you don't cruise around a race track.

Modern cars can run over 50 deg advance when cruising on light throttle.

 

[AliK]

Just found this from Mr F. Who wrote it way back in 2005 - thought it may be of interest:

http://zclub.net/forum/archive/index.php/t-4371.html



Original manufacturer's recommended setting for twin carb and manual points distributor was 17 BTDC at idle, vacuum disconnected. 280ZX original setting with electronic distributor was 10 BTDC. The former assumed the old "premium" 4 star leaded, the latter assumed "regular".

Vacuum advance controls driveability at part throttle, but for full throttle operation (and in installations where the vacuum advance is not being used, the centrifugal mechanism in the base of the distributor will keep adding advance at the rate of about 6-8 degrees per 1000 rpm from zero at a low idle speed (about 500 rpm). This will be "all-in" at about 3000rpm. Ideal "all-in" figure for ignition timing is generally acknowledged to be approx. 34 degrees (+/- for varying applications, camshafts etc., but near enough).

Distributors vary and many may be found to have worn, or worse, broken springs on the centrifugal weights thus affecting the rate at which centrifugal timing is delivered. Also, the spring rates can be varied to alter the ignition curve. If I make a basic assumption of 6 degrees per 1000 rpm centrifugal advance, then static / idle timing needs to be 16 degrees BTDC to give 34 degrees all-in. If I assume 8 degrees per 1000 rpm centrifugal advance, then static needs to be 10 degrees.

So, the figure that works best for you will vary with:
1. The type of distributor you are running
2. The quality of pump gas you are using and the state of tune of your engine (compression ratio, cam profile) - this will determine the maximum advance you can use.
3. Whether your distributor is in a proper working condition with no sticky or broken springs, loose weights etc.!
4. If I dare raise it again - whether your crank / cam / oil pump / distributor set-up has been done properly!

In conclusion, don't just check your static / idle advance, check how this sytem is performing at (and up to) at least 3000 rpm.

Also there is a great spreadsheet here ...

http://www.classiczcars.com/applications/core/interface/file/attachment.php?id=25509

 

[AliK]

I've often wondered why mine "dislikes" Shell Optimax especially as it's supposed to be cooler burning / better; this is something I will experiment with ...

http://classicmechanic.blogspot.co.uk/2011/05/ignition-timing-with-modern-fuels.html?m=1

 

[Sam_C]

Some good info there, thanks! Couple of questions - if you set the timing dynamically is the static timing value irrelevant? I presume that you only need it to get the timing something like when starting from scratch, say after a rebuild?

And...you set the timing dynamically with the vacuum advance pipe off and plugged. When you then reconnect the pipe what reaction would you expect from the engine / RPM increase or no change?

 

[jonbills]

Some good info there, thanks! Couple of questions - if you set the timing dynamically is the static timing value irrelevant? I presume that you only need it to get the timing something like when starting from scratch, say after a rebuild?

In the olden days when the likes of us didn't have timing guns, "static" meant you set the timing when the engine wasn't running. You'd manually rotate the engine with the dizzy cap off and see when you get a spark across the points as they open to set the base advance.
The static base figure given by the manufacturer was calculated as the target full load max advance minus the amount of advance the mechanical advance system could add.

And...you set the timing dynamically with the vacuum advance pipe off and plugged. When you then reconnect the pipe what reaction would you expect from the engine / RPM increase or no change?

So dynamic means the engine is running. You disconnect the vacuum advance so you dont mess the setting up with the vagaries of how open the throttle might be.
Then you're setting a base advance at idle, perhaps 700rpm. And the base advance figure offered by the manufacturer was defined at a set idle speed.
I think it was common practice to ignore the difference between static and idle setting, so if the manual said 10 deg at 600rpm, you'd set it statically at 10deg. I did anyway
So what would you expect when you've set at 10deg and plug the vacuum line back in?
If the throttle plate is perfectly shut, then no change, because the vacuum port is behind the throttle plate. If the throttle plate is slightly open, then quite a lot more advance, and perhaps the engine will speed up which results in mechanical advance too. As the throttle opens wider the vacuum falls away and the vacuum advance subsides.

As soon as you start modifying things (swapping dizzys, fitting triples etc) the careful calibration of that base advance setting goes out the window and you have to adjust for that 34ish all in mechanical advance figure, or run the risk of excess advance under load, which is bad.