87 BFN Exhaust Manifold Porting

Good thinking Mark, I noticed that when we pulled the manifolds off Mark's footer. I bet its scabbing off 15 hp or so between all the cylinders. A smooth transition would make a difference I think.

820's have 113 chambers and will really benefit from 2.19 intakes and some minor massageing.Are you going to have block 0 decked? Be carfull with quench.Compression is going to north of 10:1 what kind of pistons?

Mark, Decal operations are down currently (don't know if I told you that). I will keep you posted.

I bet she's gonna run good.

Not quite as simple as adjusting gasket thickness to get desired compression .Top of piston at TDC to flat pad on combustion chamber "quench"should be 0.040-0.045 smaller the problem is obviously bigger you are inviting detonation.Cast piston will not live long in this build one little timing issue and back to drawing board.BBC'S are sensitive to there issues not trying to talk you into more power just like to see you do it one time.

The 330 has low compression less than 8:1 so cast is fine.I have a 85 330 hp with closed chamber 063 heads compression is almost 9:1 about as far as I would push cast piston.Don't understand what mag 502 has to do with what you plan on building

Can't hurt but stock manifold/performance two words don't belong in same sentence

oldcuda wrote: Can't hurt but stock manifold/performance two words don't belong in same sentence

Agree, but these are relatively new (2002 casting date) PCM manifolds, and are in very good shape other than choking off about 20% of my exhaust flow. I'm just trying to get anything out of the motor thats relatively free. I'll put in the time to clean them up some, but stock manifolds for the BBC are like 500 new so i can't imagine what "performance" ones cost. If I hadn't noticed how much of the ports are mismatched I wouldn't have thought anything about just bolting them back on. I'm just wondering if anyone else has done it before I spend 2 days with a grinder trying to open up the ports to match the gaskets.

I've already got the rods, so I'm stuck with whatever piston height I get when they go in the block. I've got the block as well but I don't know if its ever been cut, or if it has a stock deck height. I need to get the engine in the boat out and apart so I can see how the rest of the parts are going to fall into place.

It sure looks like the bottom of the exhaust manifold is restricted.
It can be BLENDED some with a carbide burr and a cartridge roll but not
much more than a bevel 1/2 inch into the manifold. The manifold will get thin fast if you go too far into the manifold.

If it was me, I'd invest a couple of hours on it   

[QUOTE=TRBenj]    Alan dissected some SBF PCM's and they actually looked really good inside.
QUOTE]

Long ago I sliced & diced my cracked PCM's. Alan wanted to see them and was kind enough to post pictures. I really don't remember but I think he paid the postage. Not everybody would send a 40lb. package of broken parts across the country just to see whats inside. I bet he still has it. I still have the other one.

   That disected manifold picture would go good in this post, but I can't find it (Tim) help a brother out.   

Andy for some reason I can't find that post and I don't have the photos anymore BUT I do still have the manifolds so when I get a minute I'll snap some new photos. I've started to toss those in the scrap pile several times but every time I look at the amount of work that you put into cutting them in half I just can't do it. I figured someday these will come up in a discussion again....


edit, THANKS TIM

TRBenj wrote: Those logs are super cool but unless the ports are significantly larger than the PCM's, I'm not convinced they'd offer a performance advantage. Log manifolds have to make relatively short radius bends coming out of every cylinder, and there's just no way to get the runners anywhere near equal length.

I respectfully agree and disagree Tim. I've often debated this subject between the angel on my right shoulder and the devil on my left.

Your theory definitely makes sense. The logs do indeed make a short radious bend into the log itself but the PCM's do the same into the runners. That point is moot but the PCM's do indeed have the runners which would have some advantage. How much is debatable.

I know I've read more than once that every 90 degree bend in an exhaust is the equivalent of adding 10' of exhaust pipe which obviously restricts flow. The path of least resistance wins.
With that in mind the riser of the PCM makes a crazy sharp bend well past 90 degrees (just guessing someplace near 110-120 degrees???) and runs parallel to the angle of the backside of the manifold then. The logs elbows (at least in this case) do have a bump to get around but still doesn't have the super short bend of the PCM's. Again, is that any advantage or not? Certainly debatable and if so, how much?

My thinking is that the path of least resistance is the path with the least amount of bends. Eliminating the "bump" risers of the logs and replacing them with a 30 or 45 degree elbow would be even more advantageous using that theory. Then again, the relatively large exhaust on our wet marine engines is huge compared to the automotive world and does that theory even apply then?

Just interesting debate fodder.

I personally wouldn't be scared of the hypereutectic pistons at that compression level with the lower water temperatures in a FWC engine.

My 67 is in the neighborhood of 10.5:1. It has 0 deck height and slightly machined heads with flat top Keith Black Hypereutectic pistons. On pump gas in the summer it detonates like a mother (I think the dizzy over advances past 4000 RPMS). It gets beat on like a rented mule. When I pulled it apart the pistons look like the day I installed them with no detonation damage. Just tune it so it doesn't detonate and it should be good to go. Anything has to be better than stock cast pieces.

Now we are getting scientific. Coors light being the standard.

Scavenging is one thing but conflicting flow fronts is another thing all together, the crappiest of logs dump right into a common tube the flow from each cylinder – which then then pushes out in both directions, not just in the way it should be going, where it will be met with another puff of exhaust from further down the line, impeding its flow and requiring energy to turn it around and send it back in the right direction. Its like bringing waterflow into a pipe with a t instead of Y, no es Bueno.   So even the short runners if they set all the gases going in the same direction can be very beneficial even if not to the point of reaching actual scavenging like you would get in nicely matched set of long tubes. The logs above feature swept runners, and possibly an expanding unidirectional design of the actual log, which wouldn’t be all that bad.   

An old interceptor log though.. that’s a hard son of a gun to push air through. The chyrslers or omcs that go on either side wouldnt be much better eitherI personally doubt that any of the logs offer any performance benefit over a port matched pcm pyramid with the 3.5 inch risers , and most of them I suspect are worse at everything except looking cool and sounding sweet.

Just checked and ports on the aluminum logs are much bigger than manifold.see if i send pic with phone.Ports where actually little bigger than gasket .