Thanks big d. That helps a lot. Wrench/leverage analogy is good. So assuming I confirm clearances for push rods and pistons, do I need to do anything to mitigate the extra stress on the push rods? Any danger there? This is a street car that will be pushed hard from time to time...
And just to say out loud, I presumed that taking the lift up from 405 to 447 by making this rocker switch would have some noticeable performance improvements? I am not planning on doing any head work to optimize flow for large lift cam so felt like this was a nice compromise without having to change cam. Thoughts?
Performance improvements? Yes and no depending on what other matching engine parts you are using. Going to a higher lift may increase HP and TQ somewhat due in part to the faster lift (creating a little more overall duration) the valve will be opened by the 1.65 vs the 1.5's. Keep in mind that the faster lift of the valve with the 1.65's also means the faster the valve slams shut - and this could be a problem at higher RPM's and cause the valve to literally bounce off the valve seat, which is not a good thing.
.447" is a fine lift and it is said that about .450"-.460" lift is about where the factory heads stop flowing their best. So, IF you go with the 1.65's, you may also want to upgrade your valve springs.
The 1.65's as bigD
noted, may require additional side clearances where the pushrods pass through the hole in the head. The 1.65's move the pushrod cup closer in towards the rocker stud to get that 1.65 ratio and moves the pushrod closer to the side of the hole. This move of the pushrod cup puts more pressure on the rocker arm stud and can in some instances with high lifts and high spring pressures snap off the factory "bottle neck" rocker. This is why the BB Chevy ARP 7/16" rocker arm studs are used to replace the factory "bottle neck" studs. Changing to the BB studs means that you will now have to use poly-locks to secure the rocker arms.
Duration is where you want to get your performance from, this includes valve overlap. If you look at the Pontiac cams, the lift stays essentially the same, but duration increases as does the overlap. More duration also moves your power band up the RPM scale.
Depending on your compression, the Lobe Separation Angle (LSA) can be used to build additional cylinder pressure for those low compression engines or bleed off some cylinder pressure on those high compression engines.
So switching to 1.65's may increase HP & TQ a little, but may not be that big of an improvement by itself. Start adding a better exhaust system, dial in the carb, use an open element air cleaner, dial in the ignition curve and total ignition timing, and now you will see noticeable improvements.
If an automatic transmission, add a shift improver kit and to give additional performance, go with 3.55 rear gears unless you do regular highway cruising.
Just my opinion.