Also how does lobe separation affect valve timing and overlap?
Lobe separation; Lets talk with a given cam, in a given engine, and installed at or near Split overlap.
the farther apart the centerline is, the later the intake closes. This reduces your Cylinder Pressure, and intake vacuum, which kills bottom end, and reduces your power.
On the Exhaust side, an early opening exhaust sends pressure into the tailpipes that could have been used to spin the flywheel.
When both are done, equally, the overlap is reduced. This reduces specific power output but does broaden the curve some.
Thus
cams with wide lobe centers are generally stuffed into grocery getters, etcetera.
For performance, we want longer intake/exhaust durations. But at a given Lsa, the durations start doing what the Wide Lsa cam does, so, we tighten up the Lsa, to reduce the crappy part.
This has the happy result of increasing overlap, which, if you have well-sealed headers, results in increasing horsepower leading up to and over the nose of the power curve. With the intake valve closing at an earlier angle, this keeps the pressure up, which produces low-rpm torque.
When going for performance, and chasing power, we don't much care about losing the bottom end, so not having an optimum exhaust opening event is of less importance than getting rid of the spent gasses.
Think of the Factory 340 cam. People worship that thing, but lemmee tell ya I got 4 of those, used but nice, sitting on the shelf here and have no intention of ever using them.
By the numbers, they look pretty good. 268/276/114, 44* overlap. What that means is installed at split overlap, the intake closes very late. Those cams worked in the factory 340s , cuz the were "high compression". But when you slide one into a smog-era 318, the cylinder pressure drops into never-never land.
But here's the thing, 340s never came with headers so the very modest 44 degrees of overlap was dead dead dead. Which resulted in a long broad power curve.
Finally, the exhaust opens very early so forget about fuel economy.
The thing is, split overlap occurs at an install of 112*, making the intake close, with a new chain, at 64*, a reasonable number, but this changes the exhaust opening to 104 ATDC, and the fuel economy was even worse. Plus that shifts the overlap centerline, reducing it's effectiveness, which, because there were no headers on that car, I guess was no big deal.
Now, for performance with headers, tighten up the Lsa to 108 with that cam, and now you could have a powerhouse, if the valves don't float first, which IMO, was the Achilles heel of the 340s, namely insufficient spring pressure, and subsequent lifter pump up.
Well, the thing is those cams were designed to last forever, so the lift was pretty modest and the intensity of lobes was off the wall slow.
Not a chance would I ever install one of those in something I want to flog.
Listen, I'm not real good at explaining stuff, so if I messed you up, I'm sorry.
The thing is that the cam manufacturers these days are putting out some excellent products compared to 55 years ago. You can always call one of them and explain what you are trying to do. To learn it all to a rudimentary understanding, could take maybe dozens of hours. and if you get it wrong, there you are with a used bumpstick and a used set of lifters, trying to sell it, probably here. I went thru that . I'm on my third cam. As for the first one, I pulled it out before the first summer was over. The Second one dropped lobes, but that was a real sweetheart. Now I'm on a third cam, only one size bigger than the sweetheart, and I've been waiting since 2004 for it to die so I can go back a size,....... but doggone it, wouldn't you know it, she's got nearly 100,000 miles on her now, and she just keeps on going.
Happy HotRodding
