TDC

Excuse me for needing to clarify such a bassic thing. TDC is TDC on the compression stroke correct? As there are two TDC relative to piston movement.

Thanks Bruce

Well, in the world of Porsche and 911's in particular, and MFI engines in particular, there are TWO TDC points in the two revolutions of the crank (720 degrees) ... the most common one is the one you are referring to, which ocurrs 360 degrees into the timing cycle of the crank. In the spec book for '72-'73 cars, the 'other' TDC timing event is referred to twice, once when 'Timing' the injection pump after installing a new cogged timing belt at the 'FE' mark of the engine pulley ... which is 40 degrees after 'TDC Overlap' which is that point in the timong cycle at 0 degrees and also 720 degrees on a cam lift chart. The TDC Overlap point is also used when setting or measuring the cam timing, and if looking at both exhaust and intake rockers, it should be quite clear why it is called overlap on our 'S' cammed engines ... both exhaust and intake valves are off their seats at the 0 degree point in the cam timing diagram ... both Intake and Exhaust cam profile curves 'overlap' at the TDC Overlap point!!! The drawing in the 2nd edition of Bruce Anderson's 'Porsche 911 Performance Handbook' on p. 130 illustrates this point quite well. My 'degree' notation is different than than Bruce's diagram, because I use the start of the intake cycle as the '0'degree point, 360 degrees as Z1 or ignition firing point, and 720 degrees as the end of the exhaust cycle.

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Warren Hall
1973 911S Targa

Yes. TDC is where the cylinder is packed with 150 psi of air, and possibly fuel. It's ready to fire.

360 degrees away, that same (#1) cylinder will be finished expelling exhaust, just about to begin to inhale fresh air. And another cylinder will be at TDC. #4 I think.

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'83 SC