First set your dwell to 30*. Then set your total timing without vacuum advance to 34-36* at whatever rpm it stops advancing. Let the initial fall where ever it winds up and note it for future tuneups. Then limit your vacuum advance to 10-12*.
Shanvhere, timing is one of those things that requires some messing with, but you can get it exactly right, with a bit of testing. Too little timing, and your car will be sluggish with bad fuel economy and hot exhaust. Too much, and you will not only lose power, but risk engine damage.
Your distributor's springs and weights make it change timing from what it's set to at idle (usually 6-12 degrees, as you said) to something a lot higher as the engine revs. The springs should be soft enough to let it be 'all in' (not advancing any more) by 2800 RPMs, and should stay stable after that.
When it's "all in", as Redstang said, it will probably be around 34-36 degrees. However, depending on your combo and heads, it could make its best power a little higher or lower than that. The old K-code 289 heads were pretty bad, and typically made best power around 38 degrees if I remember right, due to their slow burn. A good set of GT40P or aftermarket aluminum heads will have a much better chamber, and may be happy even as low as 28 degrees.
So how can you know what works for your car? If you have stock heads, it's pretty easy: Get the engine running, and go test it. When your engine's warmed up, take it out for a spin. Start conservative with timing, and turn it up until it pings a little at wide open throttle under acceleration. Then turn it back down a couple of degrees, so you don't have to worry about variations in temperature, or gasoline quality giving you trouble later, and to account for any ping or detonation you didn't hear.
With good heads, sometimes you can turn up your timing well past peak power without causing detonation. GT40P heads, for example, just flat out won't ping until they are way past the point where they are happy. Your best bet for those is to take your car to a dyno, and have them help find its peak power.
Your engine's maximum mechanical advance is the most important part of setting your ignition timing, because it is what your motor experiences at wide open throttle and peak power conditions. You would be very hard-pressed to hurt any engine at part throttle operation, and would certainly notice a problem if the timing was way off.
The timing at idle is also important, because if it's too low, again, your engine will be very sluggish, and will not accelerate well at wide open throttle. You must set your baseline timing with vacuum advanced disconnected (and plugged so you have no vacuum leak to the engine), while your engine is warm. Too low will make your idle speed slow, and will also send a lot of unburnt fuel into the exhaust, causing high underhood temps, and lousy economy at cruise.
Too high, and you run into problems with hard starting and again, possibly ping or detonation.
It's important to note that while setting your mechanical advance, if you have the peak number correct, you can't just turn the distributor to get the idle timing correct too. Different distributors have a different amount of mechanical advance available, and usually to adjust the number of degrees of total advance, you have to take out the weights and springs and reassemble things in a slightly different configuration, or even possibly file or bend things to make it work right.
Your mechanical advance numbers are considered your car's 'base' timing, and they are the correct settings for your engine at Wide-Open-Throttle. However, at idle and cruise, your engine will have a lot of vacuum in the intake, and the air-fuel charges being burned will light very slowly in the combustion chamber. In order to allow peak cylinder pressure to happen at the right time, despite this slow-burning mix, your car has a vacuum advance system. When the throttle blades are closed and vacuum is high, your engine will need to light the mixture much sooner than 6-12 degrees. That's exactly what vacuum advance does.
So, once your mechanical advance is set correctly, and you've done some test drives to verify that it's good at wide open throttle, you can hook up your vacuum advance and smooth the idle out. Many vacuum advance canisters have an allen screw inside that is designed to adjust how 'sensitive' the advance unit is (and that's helpful for engines with low vacuum), but that won't necessarily change the number of degrees of advance. With a stock distributor, you may have to file or otherwise adjust the mechanism inside to change how many degrees of advance it can achieve.
With a really hot cam, oftentimes vacuum advance is not very useful, because there's not enough vacuum in the intake manifold to allow it to work very well, and because the idle surges anyway, which can cause vacuum advance to also fluctuate, creating some really weird idle and driveability problems. For engines like this, it's often best to leave the vacuum advance disconnected, and set your base timing considerably higher (maybe even 14-16 degrees) in order to help reach a compromise with driveability and performance. It will be much harder to start the car, and mileage/idle quality is still not going to be the best, but should still be manageable. Some people find that using ported vacuum can help with this too, as it's "off" when the throttle blades are closed, and only provides vacuum once you step on the gas. I have never had much luck with such a setup, personally.
There is always a lot of debate about ported vs. manifold vacuum, but the truth is, ported vacuum was used by manufacturers mostly as either a crutch to work around certain driveability issues, and more commonly as an emission control scheme to lower peak cylinder pressures and ensure combustion in the exhaust with air injection.
If you're going to use vacuum advance, generally speaking, manifold vacuum is best.
In short... what ever the engine likes!!!!! Between the difference in fuel that was back then compared to today combined with your engine mods, in reality, the oem specs are good for a starting point but that's about it!
A forum community dedicated to Ford Mustang owners and enthusiasts. Come join the discussion about performance, builds, modifications, reviews, engine swaps, classifieds, troubleshooting, maintenance, and more!