Resident guru
Oh dear! That does make me sound like I know what I'm talking about...! Truth is, I just like taking things apart and thinking about them too much!!
RoverRocket - you have pretty well the same setup as my engine, which I built purposely to the late (72/73) German spec. But I can't add any more to what Dave has already said. Fuel is a very different animal these days, with substantially different combustion characteristics. For starters, unleaded - even super - combusts at a slower rate than older fuels.
The basic theory is that there is an optimum point at which the piston needs to have the full force of the explosion on it. That's when the con rod and journal are 90 degrees to each other, or about 20 odd degreess after top dead centre. Think of it this way - you would always set off on a pushbike with the pedals at 2-o'clock. That's the the optimum changeover point between downward force and rotational force. Any higher and part of the force is wasted by trying to force the spindle down onto its bearing (that's the sound of pinking in an engine). Any lower and the energy is expended over a shorter amount of pedal rotation and you don't get the full benefit from your legs.
But your legs push at a constant rate. Fuel doesn't. It starts as a small point of ignition, grows outwards in a ball and dies off again. You want it to be about halfway to the biggest point of the explosion as the piston gets to 20 odd degrees after top dead centre. From there, the explosion can really exert its force on the piston all the way down the stroke, dying out just before it gets to the bottom.
But there are variables and constants.
The constant is the compression ratio of the engine. The fuel is always squeezed the same amount, and always explodes at the same rate. You can't do anything about that, so the only way to control it is to vary the point at which the explosion is kicked off - the spark. At idle, it can be kicked off pretty late. If for example, the piston took 2 seconds to get to 90 degrees from TDC, and the explosion took 2 seconds after the spark to get to the optimum size, you're winning. But when the engine doubles in speed (climbs up the revs), it will get to 90 degrees in just 1 second, but the flame will still take 2 seconds to get to the right size. In other words, it'll be late and you'll have no power. So as the revs increase, the centrifugal weighs in your distributor move the cam relative to the driving spindle, so that the points are opened earlier and earlier as the engine speed increases. This kicks off the ignition earlier and earlier in the cycle to ensure (in principle) that the flame front always reaches the optimum point just as the piston reaches the optimum point in its cycle.
Easy enough so far, but...
There are three stages to that. Up to 2000rpm or so, the rate of advance is very steep (ie, the amount of 'earliness' that the spark kicks off increases rapidly as the engine gathers speed). After 2000 rpm or so, that rate flattens off, and it doesn't gather as much advance as the engine increases in speed - this is to stop the combustion getting too big at the end of the compression stroke. Ultimately, the advance will stop when the cam hits a pin to prevent it going further. Both of the above are governed by the springs and weights in the bottom of the distributor, and the size of the stop on the cam.
In addition to that is the vacuum unit. With the springs increasing the advance as the engine speed increases, you next need to worry about how to increase the engine speed if you want to accelerate. There's more to it than this, but think of it this way: As you accelerate, you mash the throttle, more fuel gets into the engine and thus will need longer to fully explode. So the vacuum unit reacts to the engine vacuum to vary the amount of extra advance (extra 'earliness') of the spark to give good throttle response.
As you can imagine, there are hundreds, if not thousands of different spring ratings, cam sizes and vacuum units, and therefore millions of potential combinations, only a handful of which are optimum for our engine. But even that is working on the premise that the fuel and engine condition is exactly the same as when the distributor spec was set in the 1960s/70s. The engine condition might be - the fuel most certainly isn't!
So you've got two real options. Straight away we know that the distributor will need to be set a little more advanced than normal to accommodate for the fact that unleaded burns slower. So 8 degrees doesn't sound bad at all. After that, your best bet is to keep advancing the ignition and road testing the car (plenty of full throttle in 4th gear) to find the optimum balance between decent pick-up and lack of pinking.
If you really want to go the whole hog, you will need to get a remapped distributor, which contains springs, weights and a vacuum unit which are more suitable to your engine and desired fuel grade - either supermarket or super - today.
I've done that on mine, and can get you the spec if you fancied it - it would probably be about right for your engine, and certainly a much closer approximation than any original item.
But I wouldn't concentrate all of my efforts on just that. As Dave says, the RP6C has recently published part 1 of a two part an article on restoring performance to the 2000 engine, which I contributed to. Part 1 addressed the cylinder head, and it would be time and (less) money well spent in that area before worrying too much about the ignition advance. It will continue running as it is, you'll just need to find a reasonably suitable position.
Hope that helps,
Cheers
Michael
