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Tell me about my manifolds

SneakyPete

SneakyPete

Member
I was looking at my manifolds today, and I saw the part number "816900-C" on the riser. This p/n was on the starboard riser, the port riser says "stainless steel" under the mercruiser emblem. Any idea if these puppies and the manifolds are good for performance or will I have to swap them out if I get a hot motor? I took some good pics, but I'm not sure who I need to send them to in order to get them posted. I want to upgrade the motor at some point, but I hate to blow most of the budget on exhaust components.
 
I was looking at my manifolds today, and I saw the part number "816900-C" on the riser. This p/n was on the starboard riser, the port riser says "stainless steel" under the mercruiser emblem. Any idea if these puppies and the manifolds are good for performance or will I have to swap them out if I get a hot motor? I took some good pics, but I'm not sure who I need to send them to in order to get them posted. I want to upgrade the motor at some point, but I hate to blow most of the budget on exhaust components.
 
Depending on your intensions, upgrading the motor may or may not require you to upgrade the exhaust. While upgrading the exhaust is a great idea regardless of what you do to the motor ( I know I would do that first ) it may not be absolutely necessary. The cam profile will be the major factor in determining your exhaust needs. The more valve overlap, the longer the risers you will need.
 
While I have not researched this, other than Imcos' M-73 stainless risers, I dont believe any other company offers performance risers that fit Merc manifolds. Imcos M-73 are good risers, but they are not any longer than stock. They wont help your reversion problems. Maybe someone makes a good set, i am just not aware of any.
 
here is an important note.. adding spacers to increase the height of the riser rareley takes reversion out of the equasion.. mixing the water back further towards the transom usually solves reversion.. dry pipes will end reversion, and are costly, but cheap compared to eating engines... check your local laws for noise restriction devices.. i have heard that in some areas, silent choice isn't enough.. that silencer tips are also required. if your going to hop up your engine... hop up your exhaust... your new engine will need to breath.. some places to look... imco, stainless marine, gill... there are others out there also.. perhaps some members have input on the exhaust they went with.
 
If I switched to dry pipes, theoretically couldn't I just use automotive shorty or midlength headers (provided I found ones that fit) and have SS pipes fabbed to run from the collector to the transom? I know they would be loud, but perhaps I could use mufflers similar to Mad Max's setup. Another question for you, are the temperatures that are seen in dry pipes potentially damaging to the fiberglass where they exit through the transom?
 
Camshaft Specification Table
Part Number 12-246-3
Engine 1955-1998 Chevrolet
262ci-400ci
8cyl.
Grind Number CS XE274H-10
Description

--------------------------------------------------------------------------------

Intake Exhaust
Valve Adjustment 0 0
Gross Valve Lift 0.487 0.49
Duration At 0.006 Tappet Lift 274 286

--------------------------------------------------------------------------------

Valve Timing At 0.006
Open Close
Intake 31 63
Exhaust 77 29

--------------------------------------------------------------------------------

These Specs Are For The Cam Installed At 106 Intake CL
Intake Exhaust
Duration At 0.05 230 236
Lobe Lift 0.325 0.327
Lobe Separation 110

--------------------------------------------------------------------------------

Recommended Valve Springs 981-16

Is the LSA to tight on this cam to avoid reversion with stock manifolds? The magic number is around 114, correct? As in you don't want to go any tighter than 114 degrees?
 
Forget that cam. While duration has a direct correlation to overlap, there is no way you can run a 110 lobe seperation with stock exhaust. You cant run dry automotive type headers on a boat where the engine is buried in a bilge. The pipes are too hot. I run full dry CMI headers on my 540. Remember, that still means that the headers are water cooled, the water just doesnt mix with the exhaust gases.
It was way too loud. I was getting pulled over for noise. The fix was to add a set of GGB Extreme mufflers. While it is still loud, the mufflers took the crack out of the sound.
 
I see. Thanks for the knowledge, I didn't know dry systems weren't fully dry. There is a set of stainless marine manifolds and stainless risers for sale on scream and fly, would those do the trick? Would those risers be long enough, and would they fit under my hatch you think?
 
Can you post the link for the manifolds? Or what are their part #'s? The trick is to get the water to mix as far back as possible. I had a bad reversion problem with my 540 that had Gils WetOffshore manifolds and risers. My cam is on a 114 * lobe seperation.
The water mixed with the exhaust all the way back at the tips, maybe 6" from the transom. The motor sucked water in real bad. I switched to wet CMI headers and it still sucked water in, but only a little. The next step was full dry tails, problem solved. I will never have anything but full dry exhaust on a performance boat again. That reversion could have cost me an expensive engine.
 
i suggest that you contact the manufacture of your manifolds of choice.. they will get you to measure your existing holes from your x dimension.. and build your pipes to fit your boat.. it would seem difficult.. but its not. a laser level and a tape measure is all you need , the manufacture will fax you the dimensions they need.. youll get a right and a left side pipe.. that you'll never have to fight to get in and clamp down.. << just my suggestion..
 
Those are definetly not what I had in mind. I would stay away from them. They look like stock manifolds to me, with stainless risers. They will revert water with any cam other than one with very mild lobes.
 
I know that this stuff is expensive, but you have to realize that a good exhaust system is a key component in a marine engine. While a good flowing exhaust is a plus, the main reason for a good aftermarket exhaust is to minimize reversion. Notice I said minimize. As I said before, the only way to be 100% sure you will not have a risk of reversion is to go for a full dry set-up. Take it from me, that was the best money I spent on my new power.
 
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