I was studying the Dimensions & tolerances book and I noticed there is no overlap between exhaust valve close and intake valve open. I always thought there was a brief period that the exhaust valve is still open and the intake valve already opening, short enough to prevent the incoming air/fuel can to exit the exhaust. The effect should more air/fuel mixture into the cylinder and less exhaust gass remaining. Why is this not the case in our beloved 928 engine? It seems that many years (1950's) ago this suction effect was already discovered, but apparantly not put into practice, This is also the case for the 911 engines,
In my engine, the exhaust valve closes at 2 degr before TDC. The intake opens at 8 degr after TDC. I would assume that a "fatter" cam lobe would enable the intake valve to open sooner so air can be sucked in by the leaving exhaust gas. Or is this theory about suction total BS?

 

The 928 engines, both 2-valve and 4-valve engines, do have mild camshafts with little overlap. Especially for older K-Jetronic engines overlap will upset the engine with rough idle and jerk and stuttering at lower rpms. You need ITB´s or Weber type carburetors if you like to install hotter camshafts with more overlap. From attached cam lift diagrams you can see the difference between standard and sports camshafts for the 928 S2 Euro 2-valve engine.
Åke

 

I presume from looking at the cam profiles that the 16V cam figures that Ake quotes are from the Euro S models that develop 300 BHP .

Ad's data is for the original relatively softer [240 bhp?] engines and clearly show no overlap from the data presented. As I understand the early models being quite soft are the non-interference engines but presumably the Euro S cams have interference at those lifts even though they are a 16 valve motor. Not sure what compression pistons the different models used.

 

Fred, the cam lift diagram is from a Euro S2 310 HP engine (MY 1985, LH electronic fuel injection). It is not a non-interference engine. I once broke the tap of one of the camshafts in this particular engine.
However luckely the damages were limited, some bent valves and damages to the valve guides. Nothing major to the pistons. I do not have a cam lift diagram for the 4,5L 240 HP engine.
Åke

 

Åke,

My guesstimate is the original cams probably have something silly like closer to half the lift of the Euro S cams. My S4 cams seem to fall off a cliff at or just under 6k rpms. I have a nice set of the early 32V cams- same profile as the GT but greater LSA - just figured it is probably not worth the effort of modifying them on an auto.

I was a bit surprised to see the data Ad posted for the early cams shows no overlap at all- just like Ad, that surprised me somewhat.