Formula Outlaw

Own an '89 311-Sr1, twin 454 mags. Regarding the "silent thunder", I removed the partitions inside "the box" and enlarged the circular holes and made them BIG. I gained 300 rpm on the top end. Has anybody done this and then removed the whole silent thunder system and what was the result? Also switched to Hydromotive Quad IV props (24 inch), what a difference over the Mirage (23 inch). Gained 5mph at 3200 rpm and 3 mph at the top. (gps, comparison three different directions, same day). Stronger out of the hole, and just pops up on plane. Would highly recommend the Quad IV's. Any info on the exhaust issue would be appreciated. Outlaw

Rambunctious

i cut big "doghouse" arcs ouf of the baffle plates through the exhaust tip with a long sawzall. took all but 20 second
i can peek in and clearly see the transom through hull flange and exhaust opening.

i did not see any gains though. same 4600 rpm with my 330's on a 272, 25" quicksilvers, 64.9mph gps with a tailwind

does sound a little nicer though, especially along the seawall.

Formula Outlaw

Rambunctious & Troutly, Thanks for the feedback. I got the 300 rpm gain by the exhaust mods only, still had the original Mirage 23 inch props on at that time. Sorry for the confusion. I cut the entire partition out by using a long drill bit to drill holes at the edges (through the rectangular exhaust opening) and then a saw-all to finish the job. When I enlarged the bottom "circles" I cut great big rectangular holes, as large as I could make them. I used a "reverse angle" sabre saw blade as to not "tear outward" the fiberglass but I chewed up five blades in the process. The holes are now big enough I worry about pelicans making a damn nest up there. After the exhaust mods, I switched over to the Hydromotive Quad IV's. They are designed to add transom lift, which my boat is ass heavy, particularly with that silent thunder platform hanging out over the drives like that. The boat attitude is much better, the way the props work it's almost like shortening the "X" demension of the drives. Less drag equals more rpm. I talked long and hard with the Hydromotive guys and they convinced me how well they would work. They certainly knew what they were talking about. I'll give them credit there. Overall with both prop and exhaust mods, I gained 8/9 mph if the GPS is correct. Doubtless the cheapest speed gain I'll ever see. If you ever get a chance to see Bob Russell's 42ft. OuterLimits, "Can You Hear Me Now" (at poker runs) CHECK IT OUT. Twin 1350 motors. Makes the race boats sound tame. I saw it at the OPBA Spring Poker Run (I was a card boat for them) and could not believe what I heard. Every window within five miles had to be rattling. I was in love. Anyway guys thanks for the info.

PhantomChaos

I removed my silent thunder box from my 312. I never checked for an increase in HP or speed because I did it since it kept having cracking problems, and it sounded better. Hell, it sounded faster!

C_Spray

I'm trying to work out a custom switchable muffler system to fit inside the box, and still use the stock oval-shaped outlets, but with polished tips instead of flappers. I want to cut all the guts out of the box, but still use it to hide all of the mufflers, linkage, plumbing and actuators. Hopefully, it will come out pretty neat. I'll keep you posted. (The boat's a 2000 382)

Formula Outlaw

2000-382- Your idea of putting a switchable muffler system inside the silent thunder box is pretty ingenious. Don't think I would have thought of that. Teague Custom Marine makes a switchable exhaust that has all the "stuff" located in the engine compartment with only the tips showing. That doesn't help you with your rectangular holes unless you had some tips made up that were rectangular and polished on the outside then flanged to a circle on the inside. That way you could use exhaust hose to connect the round tips from the Teague units to the round interior tips of your retangular exterior tipped outlets. That would make your install pretty easy. The hardest part will be clearing out the inside of the silent thunder box. Keep me posted on your progress. Outlaw

Audiofn

TTT\

C_Spray

Here's what I'm thinking. The whole assembly slips onto the existing tailpipe and clamps in place with a pinch bolt. The smaller pipe out the bottom also fits via a slip joint.

Formula Outlaw

"C", just what are you trying to achieve here? I am surprised that a stock silent thunder isn't quiet enough or do you have serious noise regulations to deal with? I would think it would be easier just to install some switchable mufflers in place of the "thru hulls". If you combined that, with the silent thunder, I'd think you'd be real quiet.

C_Spray

With both valves open, it's essentially straight-through. Shut the rear valve, and you have a muffled system. Shut the forward valve, and you have an external Captains Call - totally quiet. I do a lot of cruising, and would like to be able to depart the marina silently on Sunday mornings...

(I can't fit an internal Captains Call with my 600 Innovations, and they are VERY loud at start-up.)