Ignition timing set-up

[Johnny]

B.E. Herron wrote:
> 
> At 04:55 PM 9/1/97 -0700, you wrote:
> >B.E. Herron wrote:
> >
> >>
> >> Mathew is correct.  Idle advance (mechanical only) should be 10 to 12
> >> degrees, no more.  Idle advance (machanical+vacuum)) should be 20 to 25
> >> degrees.  Total advance (mechanical) can be anywhere between 30 and 36
> >> degrees for max power.  Your vacuum canister should be set to give total
> >> advance (machanical+vacuum) in the same range for total mechanical advance
> >> or detonation results.
> >
> >I'm not sure if I get this last statement. First of all, since manifold
> >pressure equals atmospheric pressure when at WOT, there is no vacuum
> >advance involved in total advance. What do you mean when you say "in the
> >same range for total mechanical advance"? Idle advance, or static
> >advance is just a function of where that happens to end up after you set
> >your total advance. Low and midrange rpm mechanical advance is just a
> >function of how much advance curve rate you can get away with
> >programming in at WOT before detonation, on the way to your total
> >advance at max rpm/max power. Vacuum advance setting is just a function
> >of how much part throttle advance you can get away with programming in
> >after you have set your total advance and mechanical advance curve,
> >without detonation.
> >
> >-j-
> >
> 
> I should have said "Your vacuum canister should be set to give total
> advance (machanical+vacuum)" at 3000 rpm".  You're right, at WOT, there is
> no vacuum signal, but at the above rpm there is.  The statement was made
> since he is using an adjustable vacuum canister and if too much advance is
> dialed in, his timing at 3000 could be in the 40's before he knows it.  Not
> a good situation if accel is part throttle and not full throttle.

There are situations where your actual advance will be higher than your
total WOT advance. That's the whole point of vacuum advance. Take my
camero for example. I have an Accel HEI in there, with the adjustable
vac advance, and "medium" springs on the weights. I run 38 total on that
engine. The mechanical advance is full in by 3800 rpm. I get another 10
or so degrees in vac advance when I have some vacuum. When I am cruising
down the highway at 80mph, I am running 4000rpm with about 1/4 throttle
(I need an overdrive). I have to be seeing at least 48 degrees of
advance there, because of the high vacuum. It's doesn't rattle because
manifold pressure is fairly low. If I floor it at that point, cylinder
pressure comes up with manifold pressure, and the vacuum advance is
gone, so I am back down to 38 degrees and no rattle. If I had EFI/DI on
that engine, I could probably program in even a little more advance than
that in that same light pedal high rpm cruise mode and squeeze another
mpg or so out of it, but you can only get so close with a dizzy doing
the thinking.

I have found it easiest to set up all the mechanical first. So, I would
load it down to 3000, with WOT, and set the mechanical so that it
doesn't rattle there, then add in the vacuum side of it so that it
didn't rattle anywhere. If the mechanical advance is such that it
rattles even before you do the vacuum, you will never get it set up
right. You will never be able to add the proper amount of vacuum
advance. That's why I do it in that order... Total, then mechanical for
WOT at all rpms, then as much vacuum as I can get away with last.

On a chev with 20 or so degrees of mechanical advance available in the
distributer, I usually end up with about 36-38 degrees total, which
means I have about 16 inital I guess, without the vacuum. If I find that
it has too much advance at idle with the vacuum, then I switch to the
ported vacuum. If it doesn't have enough at idle, the I use the straight
vacuum. Usually, if it has a bunch of cam, I use the straight vacuum,
because it can use more advance at idle, and the vacuum at idle is low,
so it works out about right. On a cam that has gobs of vacuum at idle, I
usually end up using the ported vacuum.

I pay absolutley no attention what so ever to what the static advance is
at idle. It just ends up where ever it ends up after setting up the
total advance. If I absolutely have to, I will change the total
mechanical advance available, but usually I don't have to.

-j- (dizzy's are dumb)