To no one in particular,
and as applies to a streeter only;
Remember that when you move your cam, you are always trading extraction degrees and compression degrees, in a one to one ratio, and simultaneously swapping overlap from one side of TDC to the other, usually reducing the Effective overlap, in a ratio of 2 to 1. Therefore, advancing a cam by 4 degrees will change the Effective overlap by 8.
Therefor; if you decide to run a different ICA than the manufacturer recommends, you probably have the wrong cam, or the wrong compression ratio for it. The bigger the street cam, the more true this is.
Take the popular 340 cam
in at +4*/110 ICA the specs are;
268/116/104/276/44/40E ;but in at +8*/106 ICA,then
268/120/100/276/44/32E
intake/comp/ extraction/ exh/overlap/ Effective o/lap
SO;
in at 106, you gain 4 degrees of compression (120 vs 116) or about 6 psi cranking cylinder pressure;
but at the expense of 4* of power extraction, and
8 degrees of Effective overlap.
The 6 psi of additional cylinder pressure will improve performance below about 3000 rpm.
But the loss of Effective overlap is gonna kill a bit of power at the top and in the midrange.
And the loss of extraction will likely cost you fuel economy on an already poor-economy cam.
Those 4 degrees of additional advance are worth about .2 point of compression. And may drive your engine into detonation, that will require the next higher grade of gas to overcome. And so if you are already running best gas, now what do you do?
In this case;
you shouldda just started with more pressure in the first place; or a smaller cam, or a cam with a tighter LSA.
Lets tighten it up to 108, and install it at +2
268/120/112/276/56/56E, compared to
268/120/100/276/44/32E.
Thereyago, same 120* of compression, but now an extra 12* of extraction, and a huge boost to the Effective overlap , at 56* vs 32*. That's gonna make some power in Second gear! Of course whatever economy you mighta gained with the extra 12* of extraction, you will likely lose most of it by the longer Effective overlap. But the cure for that is to have a higher than "normal" cruise rpm.
That's the way uh-huh,un-huh.
You can easily go overboard with this idea tho, as the powerband gets more narrow while it gets pointy on top. Perfect for a close-ratio 4 or 5 speed, but not so good for an automatic on the street because the 1-2 split is pretty wide. The cure for that of course is a hi-stall. Until the stall becomes too much higher than the cruise rpm.
The cure for that,is a bigger engine and start all over.
