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I lost a good chuck on a lightly modified 350 SBC going from the Revolutions/GLM to stock OEM's but I lost even more from headers to manifolds. Something in the tune of 18hp or something. I have to find my old post.

Where at around the islands are you?
 

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From what I can tell... as much as 100 HP loss from DYNO to Boat.
With Headers, Circulation pump, power steering pump, alternator, ETC it really kills the top end HP readings from DYNO to "real world" numbers.

It really would depend on how much HP was on the DYNO.

If you are pushing 600+ at least 50 @ 800HP maybe more. 450 HP maybe 10-20. It all depends on how much more the engine flows over the capability of the stock Merc manifolds.

But.. ask anyone here what kind of gains they got from adding headers (perf exhaust) to their stock motors. They did not gain much more than 1 MPH from what I can tell. 1 MPH is about 20-25 HP depending on the boat.
The 496 is the only motor seeing better bumps in performance with the addition of headers/exhaust. Which tells me Merc is "choking" the 496 down a lot. (to keep it under a HP rating for some reason. Maybe? EPA or Government regs of some kind)


AS ALWAYS.....JMHO :(
 

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Originally posted by opie272
The 496 is the only motor seeing better bumps in performance with the addition of headers/exhaust. Which tells me Merc is "choking" the 496 down a lot. (to keep it under a HP rating for some reason. Maybe? EPA or Government regs of some kind)


AS ALWAYS.....JMHO :(
[/b]

Both the 496 & 377 [ 6.2MX] has the turbulators installed to combat reversion, also having the 4 Star Ultra Low emmission stickers on their exhaust manifolds :winker:
 

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Discussion Starter · #6 ·
I'm curious from a jetting angle.

my motors were set up on a dyno, with headers then installed in the boat with the stock cast exhaust, thru hub.

power runs were good, then more fuel was added to lower egt (to help the exhaust valves)

would make sense to me that some fuel could be pulled out and still keep an eye on egt.

first change (in the works) is to lower the power valve opening point.

hull is a heavy cruiser
 
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