Cam Experts: Lobe Separation Angle?
Thanks for the inputs everyone, let's not argue. I left the 110 vs. 114 comparisons up as point of discussion, not to go into the details of why 114 was there in the first place. I was (still am) confused over how overlap (tight LSA) at idle can create an initial higher cranking psi. Case in point, I once had a 406 SBC that had a solid lifter circle track cam with 106 LSA, very detonation prone. I later changed out the cam to a milder hydraulic with 110/112 (long time ago) LSA, shorter duration, nothing else changed. That same motor ran fine on 89 octane unleaded. I guess that's partial evidence that tighter LSA can produce higher initial cranking psi. I don't have any power accessories on this car and it's a weekend only ride, so low vacuum and rowdy idle isn't an issue. Would a 106-108 LSA build up higher cranking pressure for my low compression combo?
Ok, I think I now understand what you are asking.
It’s the overlap triangle and where that triangle is relative to TDC (that’s the ICL).
Overlap is the Intake Valve Opening point plus the Exhaust Valve Closing point.
So adding duration changes overlap (that increases overlap) as does reducing duration (decreases overlap).
As the LSA gets tighter you can see on a graph that the overlap triangle gets taller. That means that the valves at overlap are open further.
If you spread the LSA out the overlap triangle will get shorter because the valves are not off their respective seats as much.
All of that dictates (along with intake valve closing) idle vacuum, it has some effect on minimum idle speed and it affects how well the engine breathes.
It should be clear that (especially at relatively low lifts like we have at overlap) valves that are open further will flow more air. That’s pretty rudimentary. Since that is the case, and since decreasing the LSA causes the valves to be open further, and being open further they flow more air.
If you have the intake valve closing correct then you’ll move more air/fuel into the cylinder at overlap, and getting the inlet charge moving sooner and moving more air/fuel you’ll make more power.
It’s about getting the air column moving and trapping the most of it you can at intake valve closing.
I hope this makes some sense.