911T cam timing

[TimT]

This is correct if the TDC marks were lined up and the rest unchanged.

You dont even need to see the other pulley TDC is just a point in time, as is the position of the cam, but the cam and the crank work in concert so I drew the crank. I was going to animate the drawing but it started to become a pain in the balls.

A simple end section of the cam and the specified position and other positions would have sufficed.

[snowman]

Now I think I see a point of confusion. It has to do with convention and the way Porsche does things. You have shown the cam and crank in the relative position that Porsche uses to set cam timing, ie near the start of the ramp at the intake/exhaust valve overlap point. This is NOT the conventional way to talk about cam timing. The conventional way always references to TDC or BDC of the crankshaft. The reason is that cam rotational position RELATIVE to crankshaft position is all that counts. TIME (seconds, minutes hours) is not used in any way. Timing is a misleading term as the actual time involved depends on the rpm the engine is running at the moment of interest. Cam TIMING is the rotational POSITION of the cam relative to the Crankshaft rotational position. Since the crankshaft ratates twice for each turn of the camshaft the position is divided up into 720 degrees of rotation of the crankshaft. Shorthand notations have led to talking about these things a little out of true context.

The lobe center of the cam, ie the highest point of lift, is a common point to refer to, as it is easy to find accurately. The TDC of the crank is also easy to find as it is when the piston is at its highest point. A typical number for the intake lobe center might be 110 degrees (always refered to crankshaft degrees) ATDC or After Top Dead Center. IF the manufacturer has stated that 110 degrees is the point it is designed to operate at, 110 degrees now becomes the point we referance. If the cam is advanced it means that the cam will now have its highest lift at more than 110 degrees. If the cam is rotated relative to the crankshaft so that it has its maximum lift at 112 degrees ATDC it is said to be advanced (112-110) two degrees. If the maximum cam lift were set to 105 degrees ATDC it is said to be retarded by (110-105) 5 degrees.

As this can be done with the engine off it has nothing to do with TIME, but is referred to as static cam timing. TIME is involved only when the engine is running and depends on RPM and will vary continuously. The relative POSITION of the cam vs the crankshaft does not vary at least with older motors. New motors actually change cam position relative to crank position or cam timing, on the fly! This is done with a mechanical arrangement with as little as two basic cam timing positions or on the newest cars infinite postions. electronically controlled. some have no cam at all, but individual, electrically controlled valves. Want a differen't "cam", just reprogram the computer!

[snowman]

Head up my own whatever correction, major correction. Boy it can get confusing when discussing essoteric things like cam timeing. Well everything I said was correct IF you reverse advanced and retarded in my last post. I think, maybe, just maybe I think, whatever.