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clarelaw clarelaw is offline
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Join Date: Sep 2003
Location: Vashon Island Washington
Posts: 99
Question 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.
1981 911SC - Targa
1981 Vanagon -
1996 Honda Accord (wife's car)
1973 914 1.7
Old 03-24-2005, 08:35 AM
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