could someone explain to me if you have a total advance of say 42 degrees why the spark does not jump to the next cylinder. the distributor has 8 terminals and they are 45 degrees apart.

Tony,

Timing degrees are measured at the crank not the distributor. When the advance is set to 0 (zero) deg's the piston is at the top of the cylinder (tdc or top dead center) and about to start down. When it is set at 42deg btdc (before top dead center) the spark fires just before the piston reaches the top.

It takes 360 degrees of the crank to move the piston from tdc down and back up to tdc so the spark fires when the crank has 42 degrees left to complete the full revolution.

I hope that helps...

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...Dennis
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[This message has been edited by DjD (edited 02-06-2001).]

maybe this will also help.....the rotor is still lined up dead on in the cap with whatever cylinder is about to fire....when you advance the timing it just means that the the cylinder will fire 42 deg. before the piston reaches tdc..as explained earlier, the rotor is still lined up with the terminal for that cyl and not 42 deg's closer to the next terminal....clear as mud, huh?

Well, not exactly. This may be true at the initial advance, but not after vacuum or mechanical advance is added in. Actually it will be true at just one point along the advance curve. The vac. and mech. advance (or computer!) actually change the relationship of the rotor and cap at firing.

But it is close enough the rest of the time because like DjD said, distributor degrees are half of crank degrees and 42 is really on 21 degrees at the distributor. Or to look at it another way, each terminal is 90 crankshaft degrees apart.

Some aftermarket distributors let you adjust where the rotor is relative to the terminals separately from the timing adjustment.


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Ray

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