[hisgto]

In Sep. 2005 I decided to address the pinging during part throttle acceleration. After reading many posts, I removed my vacuum advance can to find out it was delivering about 20* of advance.

So I purchased and installed a Crane adjustable can. Before connecting it, I set the initial timing to where I would not get any pinging on WOT test runs up a long hill. Then I connected the vac adv can and adjusted it to where I would not get any pinging during part throttle acceleration. Performance was OK.

Fast forward to May 2006. After reading a ton more on timing (just a bit addicted to this forum ), I realized that maybe the better adjustment would be to limit the total vac advance with the provided cam plate instead of what vacuum level it comes in at. So I decided to start by mapping my mechanical timing curve. With a digital inductive tach, vacuum gauge and timing light, this is what I found.

Rpm...............Timing...............Vac
550 (initial).......5-6................11-12
700..................8...................17
825.................12...................18
1050................14...................20
1450................16...................22
1900................18
2150................20
2350................22
2500................24
2800................26
3000................26

So it looks like I'm getting about 20* of mechanical advance. Reading so many post that say about 34-38* all in by about 2800prm is best, I decided to set the initial to 15* to get a total of 35*. I also install the vacuum advance limiting plate and limited the advance to 0* as a starting point. Go for a ride and it runs pretty good, except when I floor it, it pings pretty badly. Hmmm....

So today I decided to remove the mechanical advance springs to allow full timing advance at low rpm to see what I was really getting. To my amazement, I am getting 40* of mechanical advance. With 15* initial, I read 55* at idle.

- Could this be correct?
- Is this a valid way to check for maximum mechanical advance?
- Do I have the wrong center plate and weight combo?
- Is my testing methodology valid?

I need to get my mechanical advance dialed in before I can even mess with vac advance.

After all the hours surfing this board, I know I can't forget the specifics.
1966 GTO
'72 455 stock long block (155,000+ miles)
Summit 2800 cam
Performer intake
Holley Avenger 670 carb
HEI ignition - 389 center plate - 139 weights - Crane adj vac can
Muncie 4 speed