Here again, Bill....
Vizard devotes a page or two on the slight "alteration" of ANY
mechanical advance curve when replacing points with electronic. Why
this should be I don't know, but his measuring instrument picked the
changes up. The greatest change I can remember however, was only two
degrees at the crank.
So....unless you are putting together a motor wherein max performance
is THE priority, the foregoing is not something to loss sleep over!!
There are two things to consider...rate of advance and maximum
advance. The vehicle mfg. takes care of this for us in their testing
of the stock motor. All you need do is follow their settings, whether
"static" or "dynamic" and you're home free.
Once you start modifying the motor, the ONLY way to PERFECTLY match
the rate and total ignition advance (for max performance) is the dyno.
For the average street engine though, it just isn't a huge priority.
The main this is not to have too fast an advance rate or too much total
advance, depending upon compression ratio and octane utilized.
Fortunately, when going to hairier cams, a faster INITIAL advance rate
is the ticket, followed by a SLOWER secondary advance rate considering
the higher rpm cylinder filling capability of the hairier cam, which
tends to limit max advance to avoid pre- ignition detonation at the
high end of the rpm scale. This is in direct contradiction to what the
stock motor normally requires, considering it's maximum cylinder
filling capabilities occur in low to low-midrange in consideration of
the stock motor's milder cam profile.
Since normal carb originated vacuum advance only plays it's part under
very limited cylinder filling, you can normally leave this hooked up
for better fuel economy at light partial throttle cruise.
Since engines with real hairy cams won't really "go" unless your foot
is well into it, too much vacuum advance at partial throttle (partial
cylinder filling = less BMEP) is not a concern. Such engines produce
less vacuum at the low end than stock motors anyway. Or, you can
simply "cap" the vac. advance altogether on Series A motors with cam
durations over 270 degrees or so.
Now this really makes slightly modifying the stock dizzy for the
"tuned" motor pretty easy. You come up a couple of degrees on the
static setting, and using a spot of weld or a bushing around the
secondary weight's mechanical limiting "pivot", further limit
mechanical advance using a Sun distributor machine, dyno or a
programmable timing light.
Years ago when 100RON leaded fuels were available at the corner pump,
you'd run a total of 34 degrees even on a highly tuned series A, maybe
30 or so with compression ratios greater than 10.0-1. Today, Bill
Perry recommends nothing more than 28 total on the average mildly
tuned street car running no more than 9.5-1 on "garbage octane"
unleaded 91PON, maybe another degree or two if you've got Sunoco or
Citgo 93PON unleaded hi-test available.
You really want to go to Amazon and pick up a copy of Vizard's Tuning
BL's A Series Engine. He goes into quite a bit of detail on this
subject....it's a worthwhile read.
The Cooper S (#40819) 23D mechanical curve is a WOT (max.acceleration)
curve.... the Cooper S having been produced with the enthusiastic
"performance" minded driver....in mind!! :)
HOWEVER....even the later Cooper S camshafts were of a VERY mild
grind, (I know....I'm currently using one) wherein max. cyl. filling
occurred in MIDRANGE, aided by larger intake valves, performance
further aided by a point higher CR than the stock (1275) Spridget
motor.
This advance curve is not the curve you want for a Kent 286+ or a
SPVP3 camshaft.....far too much total advance towards the top
end. Matter of fact, this was the first dizzy used in the stock ('67)
Spridget 1275"s, the "racing" mechanical distributor (CHT-2266, I
think) being of the type of curve I've previously described for
hairier street or racing camshafts....fast initial advance, slower
secondary with "relatively" limited total advance.
Cap'n. Bob '60 :{)
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