3 Degrees maximum change?
[QUOTE=The same amount? Three degrees is THE classic amount to change a single cam. You can't do it much more without running into troubles and if you do it much less it won't make much difference. The room for adjustment is SMALL. If you think about it, hopefully the need for a mid stroke arrived at TDC and the biggest degree wheel that you have room for will start making sense. The tiniest difference is a BIG difference! IMO rocker arm ratios and swing radii are best left out of the picture if possible! Spec duplication is key and super touchy! I have worked with people trying to time cams with the TDC determined at TDC. WAY too much crank to piston movement at TDC to even get close as far as duplicating cam specs! It's simple geometry!
Usually retarded for emissions? I don't know but it makes since to me to advance them. Why? Advancing them makes them more efficient at lower rpm. Emission testing is done at idle and cruise ie low rpm. Remember that it is easy to get all this stuff mixed up in your head when you are writing about it. Just one for instance: A BMW service bulletin that says that 336's had there timing retarded. Wrong. It's advanced.[/QUOTE]
I have been running my R1200GS cams 9 crank degrees advanced for 5 years and 25,000 miles. Doing so gave me more midrange power (which is what I wanted) with the possible expense of some top end. I rarely go 7,000+ rpm's anyway so I wouldn't notice.
Modern auto engines with variable valve timing, while not Airheads, advance cam timing as much as 7 degrees and retards it up to 45 degrees for a total of 52 degrees variability. Having been involved in drag racing for several decades I am very familiar with the effects of advancing, and in my case more often retarding cam timing to change power characteristics. In my experience cam timing changes of more than 3 degrees are commonly done but usually for reasons other than producing maximum peak horsepower.