Engine Timing - Page 4
Some information about the dwell and timing on a Chev six cylinder engine.
Some interesting things that might help you when trying to set your engine
performance. You may already be aware that changing your dwell,
changes your timing. But do you know why? and do you know by
POINT GAP - DWELL
(1) To begin with, the ignition points and dwell are
DIRECTLY related. Actually they are INVERSELY related. As you INCREASE the gap between the ignition
points, the dwell REDUCES. Decrease the gap and dwell increases. Here's some test
findings that we did:
Note that this was with our "uncalibrated" meter, so these readings are
not trustworthy, but it is sufficient to show the relationship.
DISTRIBUTOR ROTATION - FLYWHEEL
(2) The distributor turns ONCE for every complete firing of all six
cylinders. Since the distributor is direct geared to the cam shaft,
the cam rotation equals the distributor rotation. Since the crankshaft rotates TWICE for every camshaft
rotation (eg a 2 stroke cycle), then the distributor also rotates ONCE for every TWO times that
the crank turns. Distributor Speed = Cam Speed = 1/2 Crank Speed.
Naturally since the flywheel is mounted to the end of the crankshaft,
they turn together. Therefore, since the crank turns
twice for each distributor turn, if you turn the distributor by 2 degrees,
then this will alter the crank timing by 4 degrees.
Now you know why, when changing the dwell, it also changes the timing, and the
fact that it changes by twice as much tells you that you need to be
careful of introducing too much change in dwell (ignition point gap) without
going back and setting the timing again.
DWELL - TIMING
(3) The purpose of opening the ignition points is to allow enough time for
coil time to build up a charge. When the points open, the high
voltage charge is released and the plugs arc to ground. (This is
when you see the "spark".) Since the
highest RPM of these engines is relatively low, the 6 volt coil has more
than sufficient time to reach it's full charge. Therefore, the actual
dwell setting is not all that important. It appears that being 4 to
6 degrees off the recommended amount will not be noticeable. But you
adjusted the timing so that it is fairly accurate. When the ignition
points wear (and pit), it's not uncommon for the dwell to change by a few
|CONCLUSION: Check your
timing as the LAST setting since it can be changed by gap (and resultant dwell)
settings. The timing at the moment of when the spark ignition
occurs is the most
important setting of all. Changing the dwell changes the timing, but
changing the timing does not change the dwell.
|DISTRIBUTOR SHAFT PLAY
Remember the thrust washer that is between the distributor gear and
the distributor housing? In effect this is a shim between the gear
and the housing to take out the end play. The oil pump thrust
washer is there MAINLY to keep the weight plate from turning on the
distributor housing thus wearing a round groove in the housing. The oil
pump thrust washer pushes the distributor gear and shaft up therefore, the
weight plate on the distributor shaft moves up to give clearance between
it and the inside of the distributor housing.
RISK: Here's why you want
to be careful with this tolerance. Some Up and Down movement of the
distributor shaft is normal, but unfortunately too much can cause
problems. Since the distributor drive gears are helically cut, when the
cam gear turns the distributor gear, this may cause the distributor shaft to move up and down
slightly, resulting in a timing change.
The thrust washer is meant to take most of this unwanted motion out of the
ADDITIONAL INFO ABOUT END PLAY: From a Corvette Magazine: The correct clearance for most distributors is 0.010 inches of shaft-end play. This keeps any variation of timing due to end play to less than one degree. To calculate the timing changes due to a sloppy shaft clearance (or missing thrust washer) - simply apply one degree of timing for every 0.013 inches of end play.
distributor shaft should not move up and down very much.
Approximately .008 to .015 clearance should be more than sufficient.
|IGNITION ADVANCE - BTDC
The reason for ignition advance is that the air/fuel mix doesn't burn
instantaneously - it takes a little bit of time from the instant that the
spark is set off to the moment when the peak pressure in the cylinder is
reached. During that time, the crankshaft keeps rotating. So if you ignite
most of the fuel mixture at top dead center (TDC), the piston will be well
down the bore by the time the peak cylinder pressure is reached, and
you'll get less horsepower and lots of unburned fuel out the tailpipe. The
solution is to ignite the air/fuel mix BEFORE the piston reaches TDC, so
that the peak cylinder pressure is achieved at just the time that the
piston has reached the top of it's path and is positioned to take full
advantage of it. This is called ignition advance.
Now, why do we need any ignition timing advance at all? Why not light the
fuel when the piston is at TDC? The answer is that it takes the flame some
time to grow and fill the whole combustion chamber, and during the time
that the flame is growing, the piston keeps moving. The only way to get
the flame to finish burning (where it has produced the most pressure on
the piston) and time this so that the piston is ready to be pushed down
the bore, is to light it off early, while the piston is still moving UP
the bore. That is why advance ignition timing before top dead center (BTDC)
How much advance do you need? Naturally, it depends upon how fast the
air/fuel mix burns, and how fast your engine is turning. Roughly speaking,
if your engine is turning faster, you need more advance. This is the
why distributors have mechanical advance in them. This causes the
spark to occur earlier and earlier in the cycle as the engine rpm's climb.
Once you reach a high enough rpm, however, the air-fuel mixture begins to whoosh
into the cylinder with so much velocity that it becomes quite turbulent, and
consequently the flame spreads faster. Increase the rpm, and the
mixture becomes even more turbulent in direct proportion, and the flame spreads
even faster. This means that once you exceed a certain high rpm, the
mixture tends to burn at about the same number of crankshaft degrees, no
matter what the rpm is. Now you no longer need the ignition timing to
keep advancing with increasing rpm, so the distributor is designed to
level off the advance above this rpm.
Advancing your spark at low speeds can make a large improvement in the
performance of your engine.
|Thanks to Dick Olson and Skip
Geear for providing information on these pages.