Does anyone have specs for the rectangular port heads (flow numbers, port volume, combustion chamber, etc...). I've found the specs for the camshaft, but I can't seem to nail down flow numbers for these heads. Are these identical to any versions of a standard chevy heads?

My engine is the 365hp carbed 454 magnum/bravo engine, Gen V, flat tappet cam. It's in a 93 formula.

Yes, the marine rect port open chamber heads are the same as auto rect port open chamber heads.990's and others have been used for many years.

, but to be honest with you, why do you need these flow #'s ? If for a camshaft, I'll disagree. A rebel,I know, right ?.

Ha, well, I need to know what I'm working with before I make any decisions, yes? I know the rect port heads are well loved, but I'm not sure I'm a huge fan for this application. There, I said it.

I take it you like the stock camshaft? The specs on the cam like so, as I understand:
224/[email protected]
.510/.510
115.5 LSA

Okay, so I found these specs for 990s:

Mercruiser Rectagular Port heads (same as GM 990 castings)

intake / exh valves: 2.19" / 1.88"
Material Cast Iron
Combustion Chamber Open 118cc
Intake Runner 325cc
Intake Port Type Rectangular
Exhaust Port Type Square
Flow: valve lift Intake Exhaust
.05 32 24
.10 69 53
.20 125 103
.30 183 140
.40 233 176
.50 264 198
.60 292 204
.70 - 207

What I'm considering, is an upgrade to AFR 265 heads.

Reasons:
* I think the runner volume on these heads is way out of line. It would only make sense if I ran this engine to 7K rpm, and it rarely runs above 5K, 5200 at most.
* Despite about 20% less volume, AFR 265s flow better than the 990s = win across the board. I would think much better midrange with the increase in port velocity
* Weight: this should shave about 70lbs of the stern.
* 112cc combustion chamber should bump compression slightly. I don't expect a huge return from this, but every little bit counts.

Likely an upgrade to stainless/aluminum exh manifolds as well while doing this, saving another 70 lbs or so off the stern. so there is like 5K of stuff. None of this will happen for a year, but I need to start planning/saving now. Currently an EFI conversion is under way.

This is a family boat on a big lake, so cruising performance is important, and we may do some skiing/wakeboarding now and then. Before I switched to a 17P 4 blade prop this thing would barely get out of the hole. Going flat out all day long is not a priority - it'll run near 50 now if i want to, that's all I need.

A cam upgrade is possible as well, but I'm not quite sure what should be run with these heads yet. Perhaps something that is similar to the cam that is in the carbed 400s/425s

You would be better off and money ahead to find a good low hour take out 502-540ci engine, especially if the engine you have has never been rebuilt.

Ok, Ill bite Scott 😁

Do you not feel flow characteristics of a cylinder head, play a part of camshaft selection?

What exhaust are you planning on running on this build?

I do not like the stock cam performance wise, but it will last seemingly forever since it's ramps/lobes are so easy on the rest of the valvetrain.

On a generic round table talking basis,sure....but On bbc's w/open chamber + 24or26* valve angles, For a mild build,or even most performance builds, no, not really.

bbc's always make more power with more lift, so we use a cam that has as much lift as possible (for the duration) while keeping valvetrain stable/ maintenance/breakage within where we want or are willing to accept. i believe most all of us here agree on this part. As lingenfelter used to say"BBC's love lift." even when referring to street engines.

To the OP, if you are staying hyd flat tappet, Crane has several marine cams that do real well. Some have even come stock in the higher hp black motors.
Not saying these are the only choices...just mentioning them.

Anyway, with such a mild cam stock (I am not referring tothe .050" numbers) I'm not sure how much power would be gained and where by just switching heads. Yeh, the AFR's are killer, but the stock cam is just so lame. I've done head swaps and keeping stock lame cams on SBC's but not bbc's. the SBC's really don't gain enough for the time and $$$ without changing cam too....so at this point I'd have to assume BBC is no different. Assume is key word so thus underlined...lol.

I think an excellent example of head flow ratios vs cam specs was in the LS build thread. The LS3 heads have a very poor IE ratio. In my opinion , not a candidate for a single pattern cam. Sure, in my cases, a single pattern can generate more low end torque. In that thread, ditching the single pattern "torque" cam, for a dual pattern cam with quite a bit more exhaust duration, netted a nice increase in hp. It also gave nothing up in the lower rpm torque range.

Had that been an AFR head, I doubt we would have seen a torque gain in the lower rpm from a cam with such a large split.

anyhow, back to the OP's build. Question is, whats a better bang for the buck. A cam upgrade with stock heads, and a nice exhaust like the diamond/eickert manifolds, or, afr heads , stock cam, and stock exhaust?