Thread: Z1 on pulley
View Single Post
cole930 cole930 is offline
Registered User
Join Date: Mar 2000
Location: Auburn,In. U.S.A.
Posts: 2,449
Sometimes we need to get back to basics to eliminate our own confusion.

Any four cycle internal combustion engine requires each piston to travel 2 complete crankshaft revolutions, or 720 degrees of rotation, to complete the 4 cycles required to produce (1) complete and successful combustion cycle.

With the piston at the top of the cylinder bore (TDC) after the exhaust cycle the first rotation of 180 degrees starts with the intake valve open & the exhaust valve closed, the piston rotates downward which draws the air/fuel mixture into the cylinder (intake cycle).

After the first 180 degrees of rotation the piston is at the bottom of the cylinder bore (BDC) 180*, the cylinder is full of the air/fuel mixture drawn in by the intake cycle and both the intake and exhaust valves are closed, trapping the air/fuel mixture in the bore. The piston now starts the second 180 degrees of rotation back up the cylinder bore which compresses the air/fuel mixture between the top of the piston and the closed intake and exhaust valves in cylinder head.
When we reach the top of the cylinder bore we have now completed the first 360 degree rotation of the crankshaft and we are again at (TDC) 360*, our air/ fuel mixture is tightly compressed and ready to be ignited by the spark plug.(compression cycle)

We are now starting the second 360 degree rotation of the crankshaft required to complete a combustion cycle, just as we pass (TDC), and the piston starts back down the cylinder bore, the spark plug ignites the compressed air/fuel mixture that's trapped in the cylinder and the ensuing explosion powers the piston back down the cylinder bore and reaches (BDC) 540* (power cycle), completing 540 degrees of crankshaft rotation of a combustion cycle.

The inertia of the power cycle propells the piston past (BDC) and with the intake valve still closed the exhaust valve opens and as the piston moves back up the cylinder bore the burned air/fuel residue pushed out the open exhaust valve
exhaust cycle and we again reach (TDC) 720* which completes the 2 revolution combustion cycle.

The reason I went through all this is to try and clarify the four cycles of a complete combustion cycle; intake, compression, power, and exhaust. To demonstrate it takes two revolutions of the crank shaft to complete one combustion cycle. And to point out that there are two (TDC) sequences in one combustion cycle. There is one (TDC) at the top of the compression stroke and one (TDC) at the top of the exhaust stroke. Your Z1 mark will be lined up with the split in the case for both. This is sometimes confusing when determining distributor placement and timing. Some balancers have a Z1 mark and some do not, some balancers have several marks and none of them have any designations.

There is only one way to find (TDC) for distributor palcement and timing.
Find cylinder number one. Left side front cylinder. ( drivers side cylinder on the fan end of the motor) Remove the number one spark plug, put a 1/2 inch ratchet and a 17mm socket on the balancer nut. Place your finger into the head where the spark plug was located and rotate the crankshaft in a cloclwise direction untill you feel a rush of air come out the spark plug hole, STOP. Take a pencile ( Thanks Jim) a long screw driver, or a dowel rod and insert it into the spark plug hole. While holding the end of pencil, screw driver or dowel rod against the top of the piston rotate the crankshaft back and forth untill you can feel the highest postition of the piston. Now look at the balancer marks, if any exsit. If you have a Z1 it should be very close to lining up with the split in the engine case. If there was never any mark to start with now is the time to file a line in the balancer and put a dab of white paint on it.

Once you have determined where to mark your balancer still remember it can be at (TDC) two times in a combustion cycle so always pull the plug and check for compression then adjust the mark to the case, that way you always know your on the compression stroke (TDC).

Cole - 80 930 "The Old Sled"
Mods: TurboKraft Custom IC, 934 Headers, GSX 61, Zork, Port Work, SC Cams, Air Mod Fuel Dist Relocated, Water Meth Injection, BL WUR, MSD 6530, Greddy EBC, Synapse Bov, Short 2nd & 3rd with 8:37 R&P, Wevo Shifter, Coupling, and Mounts, MTX-L SSI-4, Big Brakes, Rebel Coilovers, Bilstein Sports.

Last edited by cole930; 05-07-2010 at 02:52 PM..
Old 05-07-2010, 12:41 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #8 (permalink)