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Hey guys,
The pcm exhaust manifold and risers are NLA or crazy expensive.
There are aftermarket ones available from Barr, but still pricey.
There are manifolds for other marine 454's like mercuriser etc. Which are cheaper...what's involved with changing to a different style on a full cap off rebuild?
Thanks for your thoughts
So after researching around quite a bit here's what I've decided to do. I thought I would share in case anyone else needs it down the line.
Part # Chva184 - BBC manifolds
Part # 20-0097 - 3" spacer block
Part # 53-2027 - Barr/imdmar Riser with 3.5" OD outlet
This keeps the longer riser putting the water /exhaust mix further from the exhaust flange and keeps a very similar geometry to the original PCM manifold/riser. It also keeps the 3.5" outlet to match the original PCM riser which is NLA.
The total height difference between what I plan to go with and the OEM is ~ 9/16" measured from the center of the manifold flange to the highest point of the riser. If I remember i will post up a picture or two once the new parts come in.
I did also consider looking at the Barr risers but these put the water/exhaust mix closer to the block than OEM and they also come in either the 4" or 3" outlest vs the 3.5" OD.
pics would help with what you are describing.
Ok, here's some pics to try and illustrate what I'm talking about.
Existing PCM manifold & Riser, note the water mixes w/ exhaust at roughly the 15" mark.
https://i.imgur.com/11bx5bR.jpg
Here's the replacement Barr equipment, here the exhaust mixes at roughly the 12" mark. The Barr equip has 3 pcs, 1 = manifold, 1 = 3" block, 1 = Riser
https://i.imgur.com/jLEfAyE.jpg?1
The purpose of the 3" block serves 2 functions. 1 - Addresses a clearance issue shown below where the riser is contacting the manifold preventing a flush surface where it blolts to the manifold
https://i.imgur.com/M18KPDt.jpg?1
and 2 it keeps the total height from the center of the block exhaust port to the top of the riser to be within 9/16th of OEM.
https://i.imgur.com/hExvmQE.jpg
Instead of using this particular riser (53-2027) with the 3.5" OD output, there are 2 other options available. One is 20-0082 with a 3" OD, or 20-0100 with a 4" OD, however these two risers have the exhaust gas & water mixing at roughly the 8.5" & 9" marks respectively.
Reversion will not be an issue with any of those options if running a stock cam. Also keep in mind you should be measuring the vertical rise as well in the distance before mixing water, since some have spacers. Personally I'd be looking at performance manifolds or shorty headers if I ever have to replace my manifolds.
Kudos for getting the manifolds off the block. I replaced the gasket between the risers and the manifolds a couple years ago. Took some real patience and PBblaster to get those out. Some of them were a hot mess. My manifolds seem a little rusty on the bottom exhaust ports. I thought about removing them and cleaning up/painting, but they don't leak and the bolts look ugly to get off ... so I'm going to leave them well enough alone.