[AMC-List] Re; AMC 6 Ignition timing --Got It!

["John Elle" <johnelle@xxxxxxxxx>]

Tom and all
 
I thought it might be worth comparing to what the factory spec would be
for that year. Given of course that the engine is not a 1970 232 but the
Hornet is a 1970 and I have a 1970 TSM and that is the first year that
tune up specifications were considered in the light of smog criteria
which did make some differences and I loaned out my 1967 TSM and do not
have it back yet so!!!!!!!
 
Ignition timing chart, page 4-30
232 all transmissions
Initial timing @ 500 rpm w/vacuum hoses disconnected 3 =/- 1 BTDC
Maximum Degrees Advance @ 2000 RPM w/Vacuum hoses connected 33 degrees
=/- 4
Centrifugal Advance Engine RPM and Degrees, on the car. 232 I-6 
900rpm 4-6 Degrees
2100 rpm 16-20 degrees
Vacuum Advance Engine Degrees
232 I-6 Degrees Max Advance 22
 
Now the way I read this it seems to say that there is a possibility of a
maximum of 42 degrees advance available at 2100+rpm with 16 to 17 inches
of mercury at the vacuum can of the distributor. And of course as you
start pulling a load the vacuum advance will retard the spark at any
given rpm depending on the amount of load which translates into reduced
vacuum until you get down to zero at which point the only advance would
be mechanical and at 2100 rpm that would be 16 -20 degrees. 
Depending on the year of the car or the distributor, the recommended
vacuum source could be either ported vacuum or manifold vacuum, and it
just might be, depending on the year of the distributor the exact same
distributor for each source of vacuum leaving you with a distributor
that does not have as much vacuum advance available for use as ported
vacuum starts out at zero inches of mercury at idle rather than 17 or so
inches of mercury that manifold vacuum source does and then increases as
engine rpm thus air flow through the carburetor increases. With the
caveat that ported vacuum "can" be equal to but never greater than
intake manifold vacuum. It does have a tendency though to be self
limited to about 10 inches of mercury maximum under normal engine
operation. Using ported vacuum on a distributor that was originally
designed for intake manifold vacuum will cause the timing to run later
longer and that is good for smog criteria but bad for fuel economy and
over all performance. I believe in the very early years of smog tuned
engines that was the case. As to how far or how long that was done I do
not know as it has not been important to me to find out. Keep in mind
that much of the smog tuning though was done so to meet the 2 or 3 min.
warm up time that started when the engine was first started and then
through vacuum switching circuits were shut down and did not function
until the next cold start. At this time though many systems ran with
intake manifold vacuum to the distributor which would advance the timing
at idle to help keep the engine running, but when the engine warmed up
ported vacuum was switched to the distributor vacuum canister to run the
timing dynamically later. This caused increased heat of the combustion
chamber. Also the mechanism changed from year to year as the criteria
changed as did in some years different models had different criteria
there for different hook ups to make it work. For example in 1980 8
different configurations that could exist under the hood and this does
not include those Eagles that were smogged as light trucks for the state
of California or any 4 cylinder variations.  However one of the 1980
combinations uses a non-linear control valve that can supply as needed;
Full Manifold Vacuum
Ported Vacuum
And aprox. a 10 inches of mercury.
This gives a 3rd variation of controlled advance criteria.
 
So the question becomes, with Toms full mechanical advance, what is his
set up giving at 2100 rpm vs the factory spec for that year and engine
combination and what happens when the load becomes high enough to
require later or retarded timing and a rpm based advance can't supply
it?
 
Food for thought
 
Snip
 
I now have 32 degrees total advance -- with vacuum disconnected
-- and no pinging under load.
 
I will later this weekend plot a graph of the mechanical advance
curve, but I got it road-tuned pretty close today.
 
I'm now running 12 degrees static advance, 20 degrees mechanical,
by fiddling springs got it to come in about 3200 rpm. That did
the trick.
 
Snip
 
I too am interested in seeing what the advance curve looks like.
 
John. 
 
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