If you haven't gotten a response yet, here goes (if I understand your
question).
Ignition advance is set so that engine is timed as far as possible under
hard acceleration without pinging, knocking or detonation (engine
breaker).
But, under part-throttle, dynamic compression is lower 'cause throttle
blades are making the engine "suck" to get air in. This equals Vacuum.
Since the (dynamic) compression is lower, the engine will tolerate more
ignition advance without pinging, knocking, detonating (again, engine
breaker). Using this vacuum source to advance the spark while cruising,
you
can get better mileage and drivability. But, which vacuum source you use
dictates How you affect ignition timing (****ted vs. Manifold).
-Manifold vacuum is high on idle, drops on part-throttle cruise, stays low
on wide-open throttle.
-****ted vacuum is low on idle, rises on part-throttle cruise, drops on
wide-open throttle.
Idle doesn't matter that much since you're not under load, so long as
it'll start and idle okay.
You want the ****ted vacuum to pull ignition advance on moderate/normal
driving and drop back to original under wide-open throttle.
Just because the engine idles fast doesn't mean it's what the engine
wants; it's just way advanced. It'll be retarded under normal driving
(not
what you want). You should go with ****ted vacuum for the vacuum advance.
Set the initial advance like it should be and you'll be happier in the
long
run. If it doesn't work like it should, then you may have to tune it a
little or look for another problem.
Hope I'm not preaching to the choir and this helps. If you'd like some
more advice on tuning the spark advance, let me know.
Best Regards,
Drink
"Nate Nagel" <njnagel@[EMAIL PROTECTED]
> wrote in message
news:ekdf8h026sp@[EMAIL PROTECTED]
> Message below is copy of message posted to alt.autos.studebaker - car is
> a '55 coupe but with a '63 Avanti engine (10.25:1 compression, longer
> duration cam than a "regular" 289) T-10 4-speed, and a new Edelbrock
> AVS, 3.73:1 limited slip rear, and stock exhaust with turbo-style
> mufflers. (I don't know how much of the above is relevant, but I
> figured it'd be better to post too much info than not enough) My
> question is more general than Studebaker-specific so I figured I'd post
> it here as well. I'm trying to start getting this car sorted out so I
> can actually drive it, and think I'm going to have to do some rod and
> jet tuning, but first I have to set the idle... FWIW "by the book" idle
> should be 650 RPM.
>
> (original message follows)
>
> subject says all... was just experimenting with my car today and it
> seems to idle smoother and faster on manifold vacuum... I can't get
> any more advance on the dist (it's already kicking back when I start
> it) and it seems to like it so well that before it is fully warmed up
> it'll idle at 800 RPM on the idle ****ts only (butterflies fully closed)
> I did look at a real R1 carburetor and the vacuum ****t appears to be
> tied to the transfer ****t on that side, so ****ted is "correct" - just
> curious why?
>
> My vacuum numbers still suck (or more correctly, suck weakly) at idle
> BTW but I have not played with static timing yet. They are slightly
> better with the advance on manifold vacuum however so I don't think
> there's anything to be had there.
>
> nate
>
> --
> replace "fly" with "com" to reply.
> http://home.comcast.net/~njnagel
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