87 BFN Exhaust Manifold Porting

There goes Joe making perfect sense again.

Conjecture: would the water cooling of the hot exhaust be reducing back pressure?    If yes, would that effect be the same in all manifold styles?

Backfoot100 wrote: Gases on a PCM pyramid manifold have to go straight up several inches against gravity before the gases hit that wall of a riser.

I don't think that 800 degree, high velocity exhaust gasses are affected by gravity too much. Otherwise, no racers would still be using zoomie-style headers. Plus, as Joe mentioned, there's much more to it than just flow. I'm sure a good set of 1 5/8" shorty headers would probably flow around the same as a pair of 1 5/8" longtubes on a flow bench, but the longtubes would make more power on a running motor since the runners are closer to the proper length for each cylinder pulse. The runners are obviously slightly longer in a PCM than a log (almost like a set of block hugger headers), as opposed to a log-style manifold that is similar in design to an automotive cast-iron exhaust manifold.

nice, job, don't forget to check alignment with your gaskets too.
Did you bolt the manifolds up the the head and look down inside or just go by the exhaust soot marks as to wether they match up or not?

Wow, looks great.

So much for the 330 hp big block. That much restriction stock engines should make 350!

I would bet that this is pretty common with most exhaust manifolds on these boats. I would also agree that it has to make a difference in the breathing. The exhaust gasses hitting those sharp edges have to cause some power robbing restriction. Just getting rid of those sharp edges has to be fairly significant.
These were mine when I did my rebuild.
Before and after.

Tell your source to work on their reading comprehension… the people here are talking about matching the exhaust to the existing head ports, which is fine.   Going past that would in fact be not advised, but that’s not what’s being discussed. I doubt there is a ton of usable hp to be gained either way but done well porting the pcm manifolds to match the heads in my experience does at least make them sound better.

Those RHS Heads look bad ass.

Yeah you're right its not technically porting, but when you have a manifold mismatch as bad as this, with a 1/4" to 5/16" high flat wall on the short radius of a PCM manifold it can't hurt. The lighter grey area in the picture is all on the face of the manifold , none of it is on the inside of the port, and its a flat machined edge that the exhaust gas hits head on coming out of the cylinder.



Porting heads in a relatively low rpm operating engine, is not really going to buy me much anyways, Unless you really want to go crazy with modifications to the rest of the head and spending huge money on some custom cam work this "gasket matching" will at least bring a stock BBC engine back up to its 330 HP factory raating and at the moment with the engine still apart, its not taking any time away from using the boat and its relatively free so why not. My 820 heads flow relatively well for a factory cast iron head but by know means are they the best ones out there, these are typical numbers vs valve lift for these heads stock out of the box and the exhaust flow is fairly weak to begin with. no matter what kind of exhaust they are bolted to..
INT                 EX
.2-145--------132
.3-207--------166
.4-256--------193
.5-295--------201
.6-308--------214
.7-301--------221

The other thing to note is that in my case I'm going from peanut port 236 casting oval port heads that really suck on the exhaust side to the 820 heads. So if it was choking off exhaust flow before, it will really be chocking it now, so any little bit helps.

peanut port flows:
INT                 EX
.2-125--------87
.3-183--------111
.4-215--------130
.5-233--------144
.6-235--------150

You will have real trouble finding a reasonable (under $500) BBC reverse rotation cam with lifts much over .5" so every so you certainly don't want any extra material in your exhaust cutting down your flow unless you've got deep pockets.. The only thing i did to my heads was to lap the valve seat, put in new under cut stem SS valves, and new umbrella seals. The heads themselves had nothing else done to them.

Aren't we looking at the manifold here Gary? If so it's the head port that is larger and the manifold ridge is exposed.

Yes thatis a PCM pyramid exhaust manifold flange for a 454 Chevy not the ehaust port on the head.itself, the blue behind it is the rug in the boat, not the paint on the head. (Plus its a Chevy, it should be orange - I don't care what color the marinizers paint them)   The exhaust manifold has a way smaller port than the head has.