Let's think about the events in the four stroke cycle engine as used in our old Fords.
Intake
When
the piston goes down, with the intake valve open, the atmospheric
pressure pushes air in. This air goes through an air cleaner,
hopefully, then the carburetor. As it goes through the carburetor, it
picks up a load of gasoline. This gasoline is in both vapor and
droplet form. This air/fuel mixture then goes through the intake
manifold to that cylinder when the intake valve is open and the piston
is going down.
Compression
Just after the piston starts back up in the cylinder, the intake valve closes.
As the piston continues up in the cylinder, it compresses the air/fuel mixture to over 100 psi.
Power
As
the piston approaches the top of the cylinder, the compressed mixture
is ignited by a spark jumping across an electrode gap in the spark plug.
As
the air/fuel mixture burns and the temperature in the cylinder
increases, the pressure goes up rapidly. This pressure pushes the
piston down.
Exhaust
The exhaust valve opens as the piston nears the bottom of its
travel. The noise we hear from the exhaust is the residual pressure in
the cylinder coming out when the exhaust valve opens.
As the
piston goes back up, it pushes most of the burnt air/fuel mixture ahead
of it, out the exhaust valve. The spent gas mixture then goes through
the exhaust port and out into the manifold or header.
Before the piston gets to the top, the intake valve starts opening to start the
next cycle. Below are diagrams of these events.
Diagrams #1, Intake valve open
Diagram #2 both valves closed
Diagram #3 both valves closed piston going down
Diagram #4 exhaust valve open
Note
that each up, or down, stroke is 180 degrees of crankshaft rotation,so
that one complete cycle (four strokes) total 720 degrees. This is two
full turns of the crank for each cycle. It is worth remembering that
only one cylinder is doing any of these things at the same time. In
our V-8's, there are two pistons that come to top center at the same
time, but only one has compression and is ready to be ignited. The
other is on the valve overlap top center.
Also
note that the cam turns only one revolution while the crankshaft turns
two revolutions. A diagram of valve timing might look like this.
Diagram #5, piston top center, piston bottom center
So why do we care about all this?
Let
us say that we have a 49-53 Flathead V-8 and we are ready to put the
distributor in. How do we do the initial timing? One way would be to
remove #1 spark plug and turn the engine in the direction of normal
rotation. With your thumb plugging the spark plug hole, you should feel
suction, then pressure. As the air flow stops coming out, the mark or
ball on the crankshaft pulley will be coming up to the pointer on the
front cover. Stop turning the engine when the mark is aligned with the
pointer.
Then,
install the distributor so that the hold down tab is to the rear, and
the rotor points to the #1 plug wire contact. Note that the rotor will
turn as the helical gear moves down in engagement with the cam gear.
When the distributor is all the way down, turn the distributor housing
counter-clockwise to just open the points with the rotor still pointing
to #1 plug wire contact.
If
you come out with the rotor pointing to a different plug wire contact,
turn it back past #1 and try again. If the distributor housing or
vacuum advance hits something before the timing is right, pull the
distributor housing back out to disengage the gear and try a different
gear tooth position.
When you get it right, tighten the hold down bolt into the head. You can do the final setting with a timing light.
I'll give you more applications for using this information next issue.