I believe that when both valves are off the seat the same amount (what old people like me used to call split overlap or “straight up” back in the day) you are getting the best tug on the intake port from the header.
The amount of overlap doesnt change because it’s fixed by the LSA and the other events, but you are showing the intake port an exhaust valve/port that is open more at TDC overlap. How far off the seats the valves are at overlap changes the flow of both ports at that lift.
A properly tuned header will give a nice tug on the intake port to get things moving so where the valves are at overlap can change things. Overlap is one of the ways an engine can achieve VE higher than 100% on an NA engine. That and compression ratio.
The two above terms today are kind of obsolete because you can have a cam that is ground on a 109 LSA and it may get installed at 104 ICL and you would be at split overlap or straight up.
Back when I started this stuff you might have a cam ground on a 108 LSA and to get it straight up or in split overlap it would go in at a 108 ICL.
I never heard the term “centering the overlap triangle” until a read an article by Billy Godbold. Once I figured out what he was saying, I stopped using straight up or split overlap and started using centering the overlap triangle because it’s more precise.
To the young guys who have learned the term center the overlap triangle the other two terms might be confusing because if you look at my first example you can see that the cam ground on a 109 LSA that has the overlap triangle centered at a 104 ICL would be 5 degrees retarded if it was installed on a 109 ICL!
The math I use for cam timing almost always has the LSA and ICL different, but when I take those numbers and put them in the MGI Speedware program the overlap triangle gets centered even with an ICL that is different from the LSA.
For guys wanting to make the most power, especially if they have good headers centering the overlap triangle is how I install them, even if that is different from the cam card.
Every degree the cam is advanced from the overlap triangle being centered closes the exhaust valve a skosh and has the intake valve open a skosh more and that changes the pressure drop across the intake valve, which affects how well the engine breathes.
I believe that overlap on any inline valve wedge head is critical to making power. You can “over scavenge” the chamber but if you are on a dyno you can see that happen.
As you start rotating the valves around in the chamber like a 351C or the BBC, the intake and exhaust valve are becoming more inline from top to bottom rather than side by side. This promotes cross flow through the chamber and generally means you can useless overlap to get the same results.
The Hemi is the opposite of the inline valve head in that both valves are directly across from each other so overlap is real critical on that type of head. You can over scavenge the chamber very easily and then the power goes down.
So on the one end we have the inline valve head and on the other end we have the Hemi. The two extremes. Everything else falls in between those two extremes.
Edit: I just ran your numbers through MGI Speedware.
Interestingly, those numbers put the cam 5 degrees advanced from a centered overlap triangle. To get it centered it should go in at a 104 ICL.
That said, if the car is heavy, the converter is tight or the gearing is on the tall side (or a combination of those things or even all of those things together) you may be better off with the cam advanced. You will make the power curve shift down with the cam advanced and if the chassis and such aren’t optimal or close to it for the cam you may not be able to afford to get the overlap triangle centered.
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