Does anyone know how far the block in an HP500EFI can safely be overbored? I am thinking to 4.5, but hoping for 4.6.

I'm sure you'll get lots of different opinions here, but personally I would not push that production block to 4.6. The difference between 4.5 and 4.6 is only 23ci on a 4" crank (509 vs 532), and 25ci on a 4.25" crank (540 vs 565). I doubt the difference in power would be noticeable, but a cracked cylinder wall would definitely ruin your day! I'd go 4.5 x 4.25 and make a 540 out of it. That's a nice proven combo on a production block that will make noticeably more torque than the 502.

-Brian

You can bore them to 4.6. I posted a sonic test here a while back showing good cylinder wall thickness.

Pfaff does this with his 700EFI Motor 565
http://www.pfaffengines.net/700na.html

See 502 Gen VI thread

I agree with Dog House.. and next time a freshen up is needed, you may end up buying new block.. Now the BowTie racing block is a different story.

Thanks for the replys guys. The reason I wanted to know was, the benifits you derive from the large overbore come more from the effect the larger bore has on unshrouding the valves rather than from the extra few cubic inches you gain. I ran a 540 in a Pro Stock style Super Gas drag car for years and everyone including my machinist allways said that when it came time to bore, to go straight to 4.6. The power gain was far more than the extra few cubes by themselves were capable of producing. Remember an engine is nothing more than a large air pump, more flow, more power.

I hear you on the large bore unshrouding the valves, but can't you notch the block when at 4.530 and help in that direction. If you want the most and not worried about the next rebuild .. go for it. I have seen scoring on the bores too many times, to not consider it. Although I am not sure which block the HP500 uses. Is it the same as the Chevy Bowtie??
At present I am at 4.605 bore on a Merlin block and the next step is 4.625, but after that????

The 500 is built with a stock 502 block.
The machine shop I work with won't go more than +0.060 for a marine application.