The V8 Owners Forum -

What role does camshaft duration play when setting the initial timing or is it completely indepedent?

Hypothetical.....Two engines, a 3.9 litre and a 4.6 litre. Both are fitted with post 1994 ten bolt heads. Each engine is running the same camshaft profile. The stroke volume is different between both units, and that is the only variable.

Given that the 4.6 require less total timing, that suggests that the larger engine is more efficient, thus the difference in stroke volume must be significant, but why?

Ron.

Hi
There a large number of things that effect spark timing on an engine that are not necessarily obvious. On an engine like the rover where there is relativly little squish area other factors tend to play a greater roll. The first one that comes to mind that is related to swept volume and does change in this situation is peak port velocity, this being higher will induce far more turbulance in the port and around the valve as the mixture enters the cylinder improving the quality of the charge. secondly there is a better surface area to volume ratio wihtin the cylinder, as residule exhaust gasses tend to stick to the surfaces of the piston/ bore then there is a better quantity of clean charge in the middle of the cylinder and a better quality flame front. Another one that again effects charge uniformity is that, as in this case, the stroke is greater but the bore is the same then the piston has to move faster up the cylinder on the compression stroke, again inducing more turbulance in the charge.
The term Efficient here is interesting as the larger bore engine will produce less power for its displacment but you are completely right the each BHP it produces will be from less fuel and air.
Mike