Hi, John...
I would go into equities now "but this
is not important". :)
The Cooper "S" advance curve was designed for the "enthusiast". So it IS
a WOT (wide open throttle) curve based
upon a compression ratio of around
9.75 to 1, the Cooper "S" cam, intake
valves of 1.4" and use of at least 100RON
gasoline. This WOT curve was also found suitable for the stock 1275
Series A which
powers the Sprite with lower compression
(8.8-1), smaller intake valves, and a slightly milder cam.
The operative words here are "WOT" and
100RON fuel.
The less throttle, the more advance you can run at any given rpm (as in
"vacuum
advance" on combo. distributors).
The higher octane you run, the more advance you can run, all other
things
being equal.
The stock Sprite 1275 Series "A" hi-compression ("H" designation) runs
8.8-1. For "enthusiastic" type of driving
where consistently high output is the name of the game, 97-99RON fuel is
recommended with this WOT mechanical
advance curve in mind.
The best you can do with
normal unleaded pump octanes is 93PON
which conservatively equals 96RON, just
outside the ballpark for the enthusiastic
foot (consistently over time) though fine for part throttle operation.
If you have Sunoco 94PON available,
that's even better for the enthusiastic
foot. Now you are IN the ballpark for
this advance curve, foot to the floor
forever (in theory, anyway!).
In terms of WOT operation, all you will
do by bringing the total advance in sooner
(all other things being equal) is reduce
your margin to pre-ignition detonation
when you put your foot in it at say, 3000rpm. If you drive gently,
you'll
get better fuel economy (this is what
vacuum advance does for you on the
street). Sure you will get more torque
when you put your foot in it....for a short
period of time...until pinholes
appear in the piston crown, anyway!
With the mechanical distributor designed
with a WOT advance curve, you cannot
have your cake and eat it too.
Thats why the combo. mech/vac. advance
distributors are recommended for the street. They give you a WOT
acceleration
curve and for light throttle cruise, use
vacuum either in the carb or intake manifold to give you more advance
under
reduced load.
The only place to come up with a proper
advance curve for your motor is a dyno.
However, if you have an air/fuel ratio
set up installed and start with one "known"
parameter, you can accurately set the
other to some extent at least, using the former as a basis for doing so.
Fr'instance, if you know your carb.
setting/calibration is correct for the
engine's state of tune, you simply
advance the timing a bit from stock
and run WOT in third gear up a medium
size slope (hill) of a mile or so...plenty
of time to get the engine really hot.
Note the air/fuel ratio meter reading.
It should show on the rich side.
Go back and try it again with the timing
advanced 2 degrees from stock. Where
is the air/fuel ratio indication now?
You can see where we are going with
this, obviously!! At some point you will
be at the "bottom" of the "rich" zone.
Retard the timing a bit from there to
be on the conservative side in terms
of combustion temperatures and that's
about it.
If you had a dyno, you would do this for
every 1,000rpm (or even 500rpm) and come up with an exact "power" curve
for your motor.
There are variations on this also. A colder
set of plugs will allow a little more advance than stock. Going from the
stock
"AN" needles in the twin 1.25 set-up to
slightly richer "H6" or "M" types will allow a little more advance as
well.
In a lot of cases, running more advance
than the engine can safely use at whatever throttle setting you care to
choose will result in a very quick engine
for a relatively short period of time.
While you can hear "pinking", you
probably won't hear "detonation" due
to general engine and exhuast noise
at high rpms, though you can see evidence of this on the spark plugs
with a high powered magnifying glass
which will show the metal particles
coming off the piston crowns.
You never get something for nothing
and so it is with the mechanical advance
distributor in question.
Cap'n. Bob (Ret'd)
'60 Frog
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