4 stroke engine theory
 

Basic question that I have always been a little unclear about. I am hoping the Gurus on the board can clarify.

I understand all 911 are 4 stroke engines. I understand that the four stokes are intake, compression, power and exhaust - in that order. My question is what is considered a stroke. Is a stroke from the bottom of the cylinder to the top and back to the bottom again? or does bottom to top equal one stroke, top to bottom equal another stroke and so on.

As I understand the piston travels DOWN the cylinder towards the crank on the intake stroke, UP towards the head on the compression stroke, DOWN on the power (ignition stroke) and then UP on the exhaust stroke completing the 4 stroke cycle. If this is correct, then bottom to top is one stroke and top to bottom is another stroke - I think.

DOWN = intake
UP = compression
DOWN = power
UP = exhaust.

Am I correct, or does the piston travel up and down for each stroke?

Second question. To static time a 911 distributor at TDC, I would rotate the engine CW until the intake value opens, then continue to rotate the engine further (another 360 I think) for the compression stroke at which point the piston should be at TDC and the rotor and the distributor body mark should line up. I think I should be able to tell the top of the compression stroke by attaching my compression guage to #1 and releasing the pressure as I rotate the engine, top should be when it stops blowing air.

The rebuild is done and the engine is in the car, it started but ran very poorly, I want to recheck everything before I attempt to start it again.

Thanks for the help. This is a great forum.

You're correct. Each stroke = TDC > BDC. Also, 180º of crank rotation. Also 720º for one complete 4-stroke cycle. Also, cam rotates 360º/4-stroke cycle.

To static or dead time an engine:
1) Rotate the crank pulley so the correct timing mark on the pulley is opposite the crankcase split.

2) Remove coil wire from distributor cap and hold close to ground.

3) Loosen distributor housing holddown nut.

4) Ignition ON (engine OFF)

5) Rotate dist housing clockwise (same direction as rotor rotation), then rotate counterclockwise. Stop rotation when spark jumps from coil wire to ground.

6) Tighten housing holddown nut.

You'll notice you don't really have to position #1 at TDC, end of compression. In fact, you could have positioned the engine to fire on #4 cylinder. Same difference as it fires every other rotation from #1.

Hope this isn't too confusing. You should be able to time it by ear and have the engine idle smoothly.

If the engine runs poorly, check compression (adj. valves?), check for vacuum leaks, proper firing order, etc. What year engine is this?

Sherwood

Thanks 911pcar - that helps. The car is a 1981 911SC Targa - USA production.

Will grounding the coil wire from the cap to the engine (not from the coil to engine correct?) damage my ignition system?

My plan to readjust the values, double check the cam timing and the plug wires then see if she runs better.

"Will grounding the coil wire from the cap to the engine (not from the coil to engine correct?) damage my ignition system?"

The path for electrons is from coil to center of cap to rotor, then to individual spark plug wires. Remove the coil wire from the cap, then position it (not with hand) close to ground. You could also do this with number 1 spark plug wire, but then you'd have to be sure #1 is ready to fire. With the method I described, #1 or #4 can fire using the same timing mark.

When checking spark, always make sure the electrons have a place to go, either directly to ground or by jumping a gap to ground. Otherwise, damage can occur to the electronic spark box.

Sherwood