HP500's have 2.25 intake valves. And of course typically crane springs/retainers/locks. Some years may have had comp springs, but not sure on that. All OE Chev rect heads have 2.19 intake valves and gm hardware.

A thing not talked about with the performance (not replacement) EQ heads is that I believe they have a .500" raised exhaust port that could cause issues on exhaust alignment on some Bravo applications.

Been where you are in the past, several times. Learned something in the process. And that is, forget deadlines. Its just boating. Taking an extra week, month, whatever, to do the job right, with the right parts. Its much better, than slapping it together with whatever might be the most "convenient" thing. That usually ends up biting you in the azz, and , leaving performance on the table as well.

Engine quest makes the CH454A , which is a 320CC casting. it will fit your block, but it is a "aftermarket high performance" head. It uses 11/32 valves, heart shaped chambers, raised exhaust ports, etc, just like an AFR, Dart, etc. They flow very well and make good power. They are the old "RHS/Pro Topline" castings basically.

They also make a 299cc CH502A. This head, was a replacement casting for the GM rectangle port. It is made to accept 3/8 valves like the GM head, integral guides, and so on. It has standard location exhaust ports, and I believe performs a little better than the GM castings, but probably not nearly as much as the CH454A.

http://www.aamidwest.com/enginequest...ds/502-marine/
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http://www.aamidwest.com/enginequest...let-454-heads/

Mild Thunder excellent advice....thank you for the head explanation

Joe got it. Lol. These are on a gen 6 502. Just got em painted and ordered pushrods so I don't have dyno numbers yet. Hopefully in the next month

Just put some EQ 454a together
A few issues with gen 6 to make them fit nicely, the oil dipstick mount needs to have a 1/4-20 hole tapped on one head. All the accessory mounting holes are 3/8-18 tapped and gen 6 hardware are 7/16-14 and also need to be drilled and tapped. Stock 088 rocker studs and guideplates don't fit either. Assembling one engine tomorrow if the rain holds off.

The Gen 6 GM/Merc heads use a 3/8" rocker stud. The EQ 320's, take a 7/16 stud. With any aftermarket head, I would recommend a set of Adjustable guideplates, and getting your rocker geometry dialed in.

Yep got some comp cams adjustable guide plates. Drove to Trend performance yesterday and scored pushrods for 100 dollars less than from jegs/summit

Mercury sells the complete heads 100% setup for Marine use as a direct replacement for the hp500/500efi. Brand new castings, 2.25" intake valves, Manley inconel exhaust valves, proper valve springs with installed heights set up, all ready to bolt on. The price is not ridiculous considering they are direct bolt on, with all the parts that you need. Yes the same head can be assembled cheaper but by the time you buy all the parts separate and spend the money having someone set them up for you it's not to far off. Not to mention the time/hassle factor of ordering all the parts separately, dropping the heads off, picking them back up, etc. Some people value their time differently

How many hours and how much?

For the most part we are discussing upgrading from the Gm heads which don't flow all that well anyhow.