Anything that improves combustion - like more compression, more cam, better
breathing, improved combustion chamber - means that less ignition advance
is needed. Anything that detracts from good combustion - lousy "squish",
bad chamber shrouding, poorer breathing, high octane gas in a low
compression motor - means that more ignition advance is needed.
High octane gas is resistant to knock, or detonation. What that means is
two things - first, it's harder to ignite, so if you have too high an
octane in a really low compression motor, you can have problems. Second,
it allows high compression without knock, so that you can get the benefits
of high compression and still have a reliable engine.
With that said, the best ignition advance is as much as possible, just
before you start to lose power. On a dyno, that's easy to find. If you're
guessing, it's impossible to get perfect (hence your question, I suppose.)
If it was me, I'd run 35deg total advance, and run gas that matched my
compression ratio to control detonation. Look for detonation as a sudden
increase in water temp, and as little black balls on the spark plugs. I
built a 948 engine for a guy up here, and set the timing for 32 deg. When
he had it dyno'd, they set it at 35 deg and found about 5 hp from just that
change. FWIW, I run my Martin/Ford engine, which has a remarkably poor
combustion chamber, at 45 deg.
Advance in race motors is always set as total advance. Your distributor
will have all of it's advance in by 4000 rpm, so just dial in the advance
you need and rev the motor till the advance stops moving - that's total
advance. On my Martin, I run a locked distributor so that I can set
timing at any rpm. The advance is there (on a race motor) primarily to
make the engine easier to start.
Detonation or knock, as I understand it, is when the mixture explosively
combusts rather than burning in a controlled manner. Pre ignition is when
the mixture ignites prematurely but still undergoes a controlled
burn. Knock sounds like guys with big hammers inside the engine,
pre-ignition sounds like guys with little tiny hammers going "tink tink
tink". Passenger car motors of a few years ago ran in pre-ignition
routinely in order to run lean enough to pass emission test (computer
controlled fuel injection and ignition fixed that). I'm told that
detonation can kill a motor in just a few seconds.
Hope this helps a bit.
At 12:09 PM 06/15/2000 -0400, Kendall F Jones wrote:
>Now that I've got a means to set my ignition timing correctly (TDC mark on
>top of the crank pulley & an adjustable timing light), I was wondering if
>anyone has learned the lesson of what works.
>
> Here's what I got - SCCA IT spec motor (very mild by vintage specs), .040
>over, stock head/cam/carbs. Electronic ignition with non-vacuum advance
>distributor, with 19 degree mechanical advance. I've always set the timing
>but just "twisting it till is sounds good" at idle, however, I popped the
>dist out during a warm-up and it has not been set back to what it was (cause
>I dont know what I started with).
>
>Any hints? Whats a good initial and total advance? I once heard 32 degrees
>was a good total advance, would that get me in the ballpark?
>
>thanks
>Kendall
Brian Evans
Director, Strategic Accounts
UUNET, An MCI WorldCom Company
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