the 509s (Mercury 500EFI) that Cigrocket did with my heads made 620HP thru wet exhaust in full marine trim... some left on the table if bigger injectors and fuel system were used, so I'm quite sure your 600HP target is easily attainable...

If 4.5" will do then I guess there is no need for any bigger, just didn't know if there was any power to be gained by unshrouding the valves. Definitely gonna need a bigger carb!

http://rehermorrison.com/tech-talk-53-big-bore-or-long-stroke-which-is-better/

Good Reading, Thanks Buddy!

This piston in a World block 4.560" and a 741 looks good.. right around 9.1 cr. 6.385" rod 4" stroke.
338168 Product Detail Information - JE Pistons

What do you think?

If your going with a 4.560 World block, running 9.1 c/r, hydraulic roller cam and limiting RPM to 5500 I don't think you would be giving away anything longevity wise going to a 4.25 or even 4.50 stroke. The usable torque going to 555 or 588 inches with a 310cc head would be outstanding. You could be looking at 625+hp and 700+tq with the dock manners of a 500efi

At the beginning of this thread I wanted a nastier idle sound and higher revving engine. Things change and these builds will be based around the same reliability idle speed and quality of my current combo which is 454 with 500 efi cams. Should be a great combo for family friendly boating, it is plenty loud and does have a good tuned sound with the imco exhaust. Thanks for the input!

628HP and 626HP ...LOL.... plus it was 100 degrees. I had bigger injectors, (50# once I replaced them. we only used 70% of them.. Plenty of fuel too. Despite my through honed polished, port matched intakes, they still starved for air after 5500. I have the arizona speed throttle bodies too. They were not 509, by the way, they were 522. I was happy with the performance of Al's heads, and I reached my goal. My ultimate goal was 625, and that is exactly where I landed.

That's awesome! I followed your thread on the cylinder heads, those turned out great. What was your reason for going 522? I am thinking my build is gonna go this way, what CR did you end up at?

Thanks

Machinist on yellowbullet is claiming he takes them to 4.65 and maintains .250 thickness.

Who's gonna build a 4.65x5 with Dart 20s or SR20?