Re: 3.0 performance increase
There don't seem to be too many people on this board who have gotten deep into this motor beyond stock spec repairs and rebuilds. I've been trying to find out a little similar info on upgrading the 3.0 internally and getting little response, though the factory service manual provided by Don S wil be of great value at least for torque specs, etc upon reassembly, even if it doesn't answer my core questions:
http://forums.iboats.com/showthread.php?t=260723
I'm still hoping someone can answer my questions about valve sizes, since the service manual doesn't provide that info. Maybe I just need to spend a little time at a local boat salvage yard and look at the various heads myself...
As for roller rockers, I considered the same. I did learn from the service manual that stock rocker arms on the 3.0 are 1.75:1 ratio. So I'd be surprised if changing to 1.6's would gain anything, whether rollers or not.
I have found this company that obviously does get into 3.0's, and they sell them as complete units with an advertised 158+ hp.
http://www.remanufactured.com/Inboard_Marine_Engines.htm
Of course they won't share their specs, but from the photos it is obvious that they have utilized roller rockers in their rebuild, and they advertise a different cam, mild head work, etc. It's pretty obvious that nobody makes roller rockers specifically spec'd for the 3.0 Mercruiser, but these people sourced them from somewhere, so I have to assume that standard V8 GM-style roller rockers must fit. I'll know more this winter when I open mine up and see what's in there. I have some GM roller rockers sitting here that I'm not using so I can at least see if they fit, though mine are not the correct ratio (mine actually are 1.6's) so I wouldn't want to actually use them in this motor anyway.
I likely won't take my internal upgrades to the extent this reman company did, but I'll get in there and at least clean up the ports, will definitely do a 3 angle valve job, and maybe try to see if roller rockers will fit and maybe try 'em if I can find a good ratio. Even if rollers are in the same stock ratio, the reduced friction of rollers is always beneficial over stock ball/socket styles, and by nature of their design, they will actually provide a little more lift because they pivot only, instead of wasting some of their movement in a sliding motion. Rollers would give a true 1.75:1, whereas standard rocker arms waste some of their movement in a sliding motion...providing something probably closer to 1.70:1 despite their stated spec.
Whatever you (and I) try, we need to remember that a slow speed torque-monster motor like these need to be handled much differently from a high RPM high HP motor like most people are used to building in auto racing...so we must be very careful of creating drastic airflow changes in the head (like enlarging or reshaping ports, or high lift cams/rockers, which both will often cause a LOSS of low RPM torque). While such changes may increase peak HP at high RPMS (6000?), we have to acknowledge that these motors will NEVER spin that fast, so you'd gain nothing in your useable powerband and probably would actually lose some.
But simply massaging and smoothing airflow doing multi-angle valve jobs and port cleanup will usually increase low RPM torque, which is a much more important goal for a slow spinning motor like a 3.0 Merc. That's the course I plan to take with mine, and I'll do some good before/after performance testing to see if I gain anything. Look for results posted sometime next year. I'm not afraid to make tuning mistakes, so if I gain nothing or take a loss of performance, I'll gladly admit it. And if I do gain a little power and speed like I hope, I'll brag about it with facts.
In the meantime, I'd love to stumble across someone else who's tried to get into these 3.0's for performance improvements so I can learn from their successes and/or mistakes, but like I said up front they seem to be few and far between.
Good luck!