Very interesting about running glasspacks. I'm surprised they muffle well with the fiberglass saturated. Wet packing permanently reduces muffling performance on motorcycle mufflers, a problem when hosing down dirt bikes or on two strokes when the packing gets too saturated with oil. The constant flow of water probably makes up for it. I suspect 2" would be a little restrictive for a 454 though. Thanks for the details of your solution and experience.
I modified the Gibson mufflers today, shortening the inner baffle by about 1.25" (2" cutoff wheel on a die grinder) and widening the relief slot where they clamp on (3" cutoff wheel). I made a mistake measuring how far I needed to widen the relief slot for the bolt on the Hardin inserts, and did not wider the slot far enough. They are almost all the way on but seem pretty solidly on there and are clamped tight on both the bolt for the baffle and the pipe itself. I may lengthen the wider portion of the slot a bit to get them on farther. My other oversight was not realizing I needed to make a relief cut on the inner baffle to clean the bolt and nut inside the Hardin tips. I made this relief by cutting the baffle with a sawzall along each side of the slot for the clamp bolts and baffler bolt. I then folded the remaining inner edges of the baffle up (needle nose vice grips) to clear the bolt and nut for the Hardin insert.
The result idling in the driveway, measured same as before, 20' behind the boat and a few feet to the right is about 75 dB but spiked up to 76 dB occasionally and then in the driver seat is 72 dB. 5' behind the boat and a couple feet to right measures 82 dB.
No baffles: 87 / 77
Baffles: 81 / 72
Baffles+flappers: 81 / 72
Baffles+Gibson+flappers: 76 / 72
I'm very impressed with the muffling performance combining these two mufflers. It is very quiet at idle. I suspected this would be very quiet acting as a chambered muffler. I'm curious how it will perform on the water, I suspect noise reduction will increase with more water flow. If they perform as I suspect they will and they don't fall off, I think this will be the hot ticket for my needs. I'll probably be out tomorrow to test.
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