Paul,
So that's why Vizards 1st edition says vacuum advance gives better gas
mileage? I assume the advance curves for the 23D and 25D could be the same
for the same engine, just the 25D has more advance when just cruising on the
highway at a steady speed? Is this correct? If all of this is true, why
did going from my old 25D to a rebuilt 23D improve my gas mileage by 20%?
Maybe I just got lucky and the 23D happened to have a curve that fit my
engine with better breathing than the original 25D?
Confused, but learning!
Coastie Bob
'74 Midget
'90 944S2 Cab
-----Original Message-----
From Paul A. Asgeirsson [mailto:Pasgeirsson at worldnet.att.net]
Sent: Wednesday, February 06, 2002 5:54 PM
To: Chris King
Cc: spridgets@autox.team.net; spridgets-mod@autox.team.net
Subject: Re: Recurving a distributor
Hi Chris,
Out of lurk/observation mode! I have one of these dizzy machines, vacuum
pump and all. Cool device.
I think that there is no calculation based on the actual physical weight of
the weights on the dizzy, but rather how much is the actual curve on the
nested weight and center piece and the effect of the springs that allows it
to advance. It isn't the weight that matters but what the weight was
machined to advance the maximum and when it occurs is the function of the
RPM and
the strength of the springs. That maximum amount is usually stamped on the
weights in the dizzy.
That said, you will still need to know at what RPM you want the advance
curve to start at and at what progression through the RPM range and what you
wnt it to end up at. If that is known, then the dizzy dizzy machine can
help you. It won't do the advance calculations for you, just the
verification of what it delivers.
About the vacuum advance, this is essentially useful only at high vaccum of
the engine, which usually is of no consequence on a performance engine.
That's the reason there is no vaccum advance on the 23D. On a street engine
the vacuum advance might be as high as 6 degrees which is on top of the sum
of static and advance timing and will occur only when the engine has vacuum
of usually more than 14 inches. To get that you are cruising with very
light throttle at light load. When you give the engine more throttle, the
vacuum drops and so does the vacuum advance (no vacuum, no vacuum advance
function,) so you will avoid pinging or detonation when it's pulling hard.
Running the engine with very light load and extreme advance timing usually
isn't
a problem, but when you put a load on it, the timing needs to drop back some
to preserve the engine.
I took an Allen tune up course on this machine more years ago than should be
admitted! Suffice to say it was pre Bug Eye!!
Paul A
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