Those Merlin Rect ports are probably really close in flow to a GM Rect iron head. Basically, with a cam upgrade, a little head porting, youll have a HP500 Merc, and then some.

I ran the 741 crane cam in a 540 with standard stainless marine bravo tails, no reversion issues.

No the Merlins are about 18lbs/head heavier with just a little better flow. I sold mine last year and went with aluminum Darts.

So if they only flow a little more then my 088 worked over a bit in theory I can hit close to my goal of 600 hp n/a. If that's the case I'm happy. I can't wait to pull the motor and start!!!!


Thanks guys!!!

Some decent heads, and a larger hydraulic roller will get you to 600 on pump gas easily.

N/a...you should be able to reach your 600hp goal. Personally, I would run around 9.5 on the comp. Not really to big on the factory rods even with the 7/16 rod bolts. By the time you r&r them (keep in mind that if your going with new pistons you need to choose if you want floating pins or not......floating pins = more r&r to bush the rods) its not much more to by a set of hbeam rods.

As for valvetrain......go to rm builder. He's worth every penny!

Heads.....I wouldn't waist my time on a set of factory heads if your thinking about putting the work you have discribed into them. Wont be much more to pick up an aftermarket used set of iron Merlin's or darts and have a serface and valve job done on them.

You can also always go a little low on the compression now, have a cam ground but do a blower a little later.

The factory 502 crank is a nice piece and will hold 600hp all day long. I'm running one in my 502 with a b&m 250 blower....currently seeing just over 4 lbs of boost and 6200rpm.

Spend your $$$ on machine work, rods, pistons, a good oil pan & oiling system, aftermarket heads and have Bob set you up with a nice valvetrain.....you wont be disappointed!

I just went down this road this past winter and the results where very satisfying! Good luck! Mark

Also, if it may be of any help. I bought a set of merlin grumpy jenkens 345cc heads from a member here on OSO for $500, paid $200 for shipping (they're heavy suckers) then had a valve job and surface done. I bought them fully ported so I was stuck with what I got there (sometimes buying ported heads can turn out to be a bad deal if the company / person doing the porting doesnt know what they are doing but after having them flowed it was determined that the port work done was not all that bad.......I read that the merlin iron heads unported do not respond all that well when compaired to the factory head but with some port work they really wake up. We saw good numbers out of mine. The 345cc head is a bit much for a n/a 502 but seems to work very well with my blown combo.

I ran a set of dart iron eagle 308's on a flat bottom about 4 years back on a 454 I built that was n/a and was very impressed with them.

Thanks for the good info. I'm taking everything into consideration.