The Gibson muffler tips arrived today. I don't expect a significant volume reduction from the straight through with baffle design, but it's similar to a glasspack you'd use on a car but using water instead of fiberglass to muffle the sound. They are very short at about 6-5/8" overall with only 3-1/2" beyond where they slide into existing 4" tips. They slide about 3" over the existing tips, and the inner baffle goes up into the tips 3-5/8". This means they won't fit the Hardin Marine tips with baffles installed as there is only 2-7/16" from the center baffle to the end of the tip. The slot where the two bolts clamp onto the tips is also too narrow to slip past the bolt holding the baffle into the tips.
I will modify these to fit by widening the slot to about 1/2" to clear the bolt holding the baffles, and cut down the inside inner baffle to 2-1/4" or less, maybe 2". This will result in 3/16" to 7/16" of open 2.5" exhaust (ID of inner baffle) at the end of the Gibson tip, plus the area of the louvered baffles. Much of the exhaust gas will have to make a 180* turn and the rest will go through the louvers mixed with water. I suspect this will result in fairly significant volume reduction and a much more effective muffler than either Hardin Marine Baffle or Gibson tip alone.
This may be the hot ticket to get it where I'm comfortable, though it may be too restrictive on the top end. If it is too restrictive at high RPM I will remove the tips and trim more from the inner baffle until performance is acceptable. Given the low HP per cubic inch of the PCM 454 I don't expect any significant performance change.
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