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Just got my first jet boat (just a family cruiser) and it seems to have a very mundane BBC. My plan is to keep the current engine cover so is there a log/snail exhaust option for higher HP? My plan is a 540 with a large hydraulic roller cam and single four barrel.
 

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With the current engine cover there is no room for anything to come up the side by the valve cover like a header. Believe me I would love to have headers but there is no room.
lightning headers are double walled for cooling, and can be used under a hatch. but you're correct assuming they won't fit under the box you're currently using. as long as you want to keep that box, you're pretty much stuck with the logs. the other option is to remove the box and those two small seats, build a back seat/hatch cover that covers that whole area. and use lightnings. has been done many times. personally, i'd just stick with what you have and enjoy the boat...
 

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I haven't seen any test results, but this style riser should flow better than the conventional 90-180-90 snail. I have a set that I would sell. Outlets are 3 1/2".

The one in the picture is not mine, it's just one that I found online to show.

Also, you might consider by-passing (through dumps) most of the water that exits through the risers. You only need enough water to keep the hose temperature under its melting temp (plus a safety margin). I did a few by-pass systems back in the 90's and typically picked up 150 - 200 rpm on the top end. No promises though.............

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