Restoring Rusty

[enaberif]

Originally Posted by Gregski

Thank you for that, however let me share with you an interesting observation, one that I have validated with a number of tests. The point that I will be making is that it is not that clear cut, Base Timing + Mechanical Timing, or in other words so far I have not seen such a thing as pure Base Timing, allow me to explain

It appears that Initial Timing (what you call Base Timing) always includes some Mechanical Timining aka advance. In my case my original Mechanical Advance Curve starts at around 800 RPM, so if my idle RPM is 850 I am already relying on mechanical advance in the initial 16*. This could explain why in my original configuration 16 Initial plus 24 Mechanical = 32* instead of 40* (that's because the initial 16 already has some of the 24 mechanical in it) hope I didn't just confuse the hiebie jibies out of somebody

If you look at the curve for the new weights, (pic above in the thread) it starts even earlier at 600 RPM, so for sure at 850 RPM idle we are using the mechanical advance. Now what muddies the waters a bit is crappy aftermarket inaccurate parts. Which I blame for 10 initial plus 8 mechanical = 18 (which really shows that their curve does not start at 600 RPM, lol)

Now looking at the GM HEI distributor part number 93440806 which I will be purchasing tomorrow (aka Pay Day) it's Mechanical Advance curve don't start till 1600 RPM - so we shall see how that all adds up

The springs you use is what determines your advance at idle. You should have a few different springs from a soft to a stiff spring. The softer the spring the earlier the advance comes in and vice versa with the stiff springs.

Depending on your cam shaft you may want your advance to start early or a bit later so find out your cam profile and where the lowest point of torque is made.