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

[Tom Jennings <tomj@xxxxxxx>]

On Fri, 8 Dec 2006, John Elle wrote:

> 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

Yup, and I figured out where these numbers come from....



In the last month I learned a lot about ignition timing!

First there's things about vacuum advance that I now know are
technically verifyable facts, and reasons why vaccum advance
doesn't matter at all for performance tuning:

    One: The only timing setting that matters when the engine
    is under load, or WOT, is static + centrifugal. Vacuum is
    NEVER taken into account at this step. The reason? All vacuum
    advance units give ZERO advance when there is no vacuum! Wide
    open throttle == zero, or very low, manifold vacuum.

    Two: Ported vs. manifold vacuum. Ported distributor vacuum
    is a smog reducing modification only; it makes the engine
    run worse, not better. Ported vacuum lowers NOx at idle,
    by lowering combustion temperature by ruining spark timing!
    Fuel:air burns late, so it ends up in the exhaust; it makes
    your engine run hot at idle. If your car idles hot, switch
    to manifold vacuum, it will run cooler. 

    Three: High load/WOT means the fuel:air mixture is richest;
    you want to make a lot of power. This is when combustion
    is most sensitive to ignition timing: too early PINGs,
    too late BACKFIREs. Since WOT means no vacuum, there is no
    vacuum advance, so only static + centrifugal matters. Pull
    the hose while tuning!

And what vacuum advance really is doing:

    Four: lean mixtures (highway cruising), rarefied mixtures
    (high manifold vacuum means rarefied air:fuel), dry mixtures
    (decelleration) need LOTS of advance, because they burn
    poorly and the flame-front advances slowly. THIS IS ALL THAT
    VACUUM ADVANCE IS FOR. Cruising at 50mph flat and level at
    sealevel, say 2000 rpm, vacuum is relatively high. So you
    might have 10 static + 20 centrifugal + 15 vacuum advance
    = 45 total degrees advance. This is needed because the
    thin lean mixture takes longer to burn than a rich one
    at wide-open-throttle.

Those things above are technically verifyable and are true for
all gasoline 4-stroke motors. Only the numbers vary.

Though he writes only about Brand-C motors, Lars Grimsrud has
some EXCELLENT writeups on ignition timing.


> 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. 

Yup!

> 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. 

Yup! And this case (no vacuum) is where performance tuning
can be done. The factories were very conservative with these
numbers to save themselves a lot of work and ruined engines,
and gas was 35 - 75 cents/gallon...


> Depending on the year of the car or the distributor, the recommended
> vacuum source could be either ported vacuum or manifold vacuum, and it

I have some TSMs, I should go look, but I bet t wasn't until
late 1960's/early 70's they started the "ported" business.

> 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?

I will today make a chart of centrifugal advance vs. RPM.

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