silent thunder exhaust
 

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

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.:D

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.

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!

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)

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

TTT\

OK - No cheating now:
 

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.

"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.

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.)

I see said the blind man. That's going to be some kind of plumbing coming out of the bottom of that box. Are you planning on running it down between the outside of the drives and the K-planes? You could tell people it's a "water cooled turbo". :D

What do you get for speed with those motors? I'm seriously considering some Innovation's for my 311.

The "down pipes" will be smaller versions of the full-time underwater system on the PX MTI "Pair-A-Dice" if you saw that at the Miami show. I may get them powder-coated to match the paint on the transom to make them less obvious. Brownie (from Stainless Marine) runs the same setup on his little Donzi.

To try and keep it simple, I'd like to mount the shafts for the valves vertically, and connect each group together with a tie bar so one pneumatic actuator can close all four valves of each group. (Can't use electric solenoids outside the transom.)

My boat is "loaded to the gills" (12,000 wet) with genny, A/C, water heater, TV/VCR, 2 sets of ground tackle and enough of the wive's clothes to get by for two weeks, but we've seen 79.7 on GPS ("almost 80" in bar-speak). That's with DPX drives, since cruising efficiency is more important that all-out top speed for us. Love the engines: start flawlessly and never stall. (Did I mention they're loud, too?)

Can anyone post some pics of that cut out exhaust that you have been talking about? I would also like to do that but i dont understand that you all have done. Thanks

Two weeks of wife's clothes, LOL.....

Formula Outlaw - What rpm were you running with the 23 mirage props and what is your rpm with the 24 hydro's?

I am considering the same props on my new to me Formula 336 with 454 mag's which now has 23 mirage plus props.

Thanks-
Dave

Silent thunder exhaust tips
 

I am redoing a 1988 223ls mint, been in side climate controled storage for 11 years 245 hrs, I'm powering it with a 500 efi. I've read every ones ideas for thunder box mods and pretty much combined a few Ideas into this build. Ive removed the silent thunder box replaced the plywood and drilled 2.5, 3, and 5 inch holes in the bottom behind the stock drain holes, let the water in and out quicker, and let engine breath"Gooder" I am in desperate need of the stock rectangular tip, I am wanting the baffles to function as designed. If anyone has a set of these tips I need them bad, or if anyone has put round tips on and kept the baffles let me know how that worked out for you.