So..spell it out in small words even I can understand?
Obviously higher lift with same cam, also obvious same cam that is properly clearanced for 1.5 may be hitting pistons at 1.6 or 1.7...no effect on duration though, right? Obviously less mechanical advantage at the rocker equals harder on valvetrain associated moving parts? (lifters, pushrods, etc.)
OK so why exactly would one go either way and to what advantage?
School's in!

Older Moparts Thread with Andy F. and Other giving Great Answers.

Actually duration at higher lifts will increase a lot with a higher ratio rocker arm. Seat duration stays the same. I use 1.85 ratio on the intake and 1.70 on the exhaust on my Indy EZ heads.

Effective duration and lift are increased with the same lobe with bigger ratio rockers, just like making the cam bigger Lots of talk on this in a lot of places, lots of confusion also The lobe should be designed to work with both ratios before you change the ratio , I have one cam that has what use to be considered BB Chevy lobes on it, it works real well with 1.65 rockers in my low deck with all kinds of different heads, race and stock

With higher ratio rocker arms, the pushrod contacts the rocker closer to the pivot point (rocker shaft), so you usually have to enlarge the pushrod holes for clearance.
The increased ratio also puts more pressure on the pushrods and lifters. An example:
If your valve spring pressure 300#, with a 1.5:1 rocker, the pressure on the pushrod/lifter is 450#.
At 1.7:1 rocker ratio, the pressure increases to 510#.
The flip side is you get more valve lift at all lift points on the cam.
So if you had a 0.500" lift cam with 1.5:1 rockers, now you get 0.567" lift with a 1.7:1 ratio.
The increase lift compounds the pressure issue.
Another example with mild springs:
Spring installed pressure is 120#, spring rate is 360#. With a cam lobe lift of 0.333" lift.
1.5:1 ratio the lift is 0.500", Open spring pressure is 300#, pressure on lifter/pushrods is 450#.
At 1.7:1 ratio, list is now 0.567", Open spring pressure at the higher lift is 324#, and lifter/pushrod pressure is now 550.8#.
The equivlent 550.8 psi using a 1.5:1 rocker would be open spring pressure of 367.2#.
So what difference does it all mean?
The main concern is the pressure on the cam/lifter interface. The increased pressure will cause increased cam/lifter wear, and if the cam lobes were not designed for the 100+# more pressure you could wipe out a flat tappet cam. It also means the pushrods will deflect more, or even bend if they were marginal to begin with, so you may need stiffer pushrods.

As for rocker ratio affecting duration; Technically, cam duration is measured from the cam lobe lift, not the valve lift, so "cam" duration is still the same, but the engines airflow does not "see" the cam, it just knows about valve timing and lift, and the higher ratio rocker arm reaches the same "valve" lift sooner per cam duration, the engine will respond as if the cam was slightly larger in duration and lift.
At 0.050" lift the valve (no lash) would be open 0.075" with a 1.5:1 rocker arm. The same valve lift with a 1.7:1 rocker arm would be at 0.0441" cam lobe lift, so if you measured the cam duration at the 0.0441" lobe lift figures that would be simular to the 0.050" duration numbers.

Here is the best way I can explain it. When you look at a cam card, you look at "lobe" numbers. For example, if your cam card says the duration is 230 at .050, this is telling you between .050 lift AT THE LOBE on the opening side to .050 on the closing side....there is 230 degrees of rotation happening there.

But....the engine could give a crap about the lobe. All it sees is what the valve doing, because after all....if the valve wasnt there opening and closing....really wouldnt matter what the cam spec looked like.

So while we are busy comparing duration at .050 lobe lift, the engine is actually "theoretically" seeing .075 valve lift with a 1.5 rocker(forget about any deflection or anything of that nature in the valvetrain that could change what the valve is really doing....we wont clutter it up wiht that).

So .050 lobe lift is .075 valve lift with a 1.5 rocker.

If you swap to a 1.7 rocker, .050 lobe lift is still .050 lobe lift, but .075 valve lift will occur sooner on the lobe. If we could measure duration at valve lift, since .075 will occur sooner, and happen again later on the closing ramp......this is exactly why the cam "looks" bigger to the engine.

Every lift point the valve shows the engine is bigger, and there are more degrees duration between those two points at the valve than with the smaller rocker ratio.

Thanks 451mopar and Mr. Moreira!
Between the two of you I (and the previous posts, not to slight anyone else)
Now believe I have a rudimentary understanding of the principles involved.I will cut and paste and make a word doc for future rumination(that's what sheep do).
THANKS!!!!
Greg, there is an old Moreira family here,two brothers and their Dad, all heavy car dudes.
I have heard their name (mis)? pronounced as Moo-Rayuh, but their Dad told my friend it's actually spoken Murr-rare-uh, which way is yours pronounced where you are at?
Aloha, C