[jamesm]

Originally Posted by arghx7

Keep this in mind: When your exhaust cam is near its most advanced point, and your intake cam is near its most advanced point, you have early overlap. By early overlap, I mean that as the piston moves upward and the exhaust valve is still open, the intake valve begins opening. This can cause backflow of hot residual gas.

The backflow occurs when the pressure in the combustion chamber is higher than the pressure at the intake port when the valve is opening. This often occurs in the cruising area of operation. In some situations the backflow is a desireable thing, but in other situations it just results in increased knock sensitivity and combustion problems.

So this is out of the BMW M5 service manual, but it's a nice illustration of a cam profile diagrams used during camshaft development and cam phaser calibration. If somebody had the proper equipment they could develop such a chart for the FA20 stock cams. Looking at the x axis units: we're used to thinking in terms of BTDC firing, like a spark timing of 20 degrees BTDC. The "0" number in the diagram is TDC intake, or 360 BTDC firing. This numbers at the top there, 115-125 , 60-55, are the centerlines. When we phase the cams the centerline positions change, but the profile of the cams do not.

The area I circled is the spitback area. The other extreme is between 0 and 60 degrees, when the exhaust is fully retarded and the intake centerline is locked at the default position. In that case, the piston is descending when the overlap occurs, and you don't get spitback into the intake port. Again, there are times you would want this and times you wouldn't, and it interacts with what hardware is one the engine.

awesome! thanks for the info, always much appreciated.