silent thunder back pressure
 

Has anyone done any modifications to there silent thunder swim platform to relieve back pressue. I saw a thread that said you could drill additional 3" holes beneath the platform on the radius that goes from each side to side. I am wondering how much the silent thunder affects the performance of the motor.

sheikman

I read someplace here that Formula did extensive testing and found it didn't hurt performance at all.
Most of the modifications done are to the internal baffles. Specifically removing, or taking a hole saw to, the one that's visible looking inside from the rear.
I found that one of the baffles in mine has come loose and should be easy to pull out. I'll have to look at the other side and see what that one's like.

It will only be louder when the box is in the water. On plane = no differance.

if your plugs are in the bottom of the silent thunder then i think it will be louder on plane too. because the exhaust doesn't have to blow under the baffle plate through the water.

i removed my baffles last year. sounds better all around, although noticed more at idle. still has the "flowmaster chamber-like" sound of formula though. I like it.

do a search for my post on it here on the formula page. i even inserted a section drawing to show how easy it is to remove (ie cut away)


all it is is one fiberglass plate "glued" in on the top and two sides with a 1" gap to the bottom of the chamber. the chamber fills with exhaust water, and the exhaust is forced through the water and under this plate.
i used a sawzall and they were done in 5.75 minutes!!!! well 15 minutes anyway;) I cut a doghouse opening arch from the bottom right up and around to the bottom left of each baffle plate, then broke the piece in half to remove it out the square exhaust opening.

you need a long sawzall blade to do it though. sorry if you know this already. i did not see any speed increase. but is sounds faster:D

Does anyone know the impact of taking out the 4 plugs on the underneath side of the swim platform? Will it make it louder?

Mine are out. bought that way
don't know how much louder, but the water can't fill to the top of the sqaure exhaust tip and run out as designed when they are removed. supposedly they are deisnged to be removed when winterizing ( ie drain the water)

in the driveway, the water runs out the holes fast enough to not fill the chambers completely as designed.

Bustin,
Probably so:) The four plugs hold water in the "mufflers". By removing them you allow water to drain while on plane. At an idle they we stay full and quiet. Keep in mind even though the drains are open , you still have a constant supply of water from the exhausts.
I got mine three years ago without plugs and never gave them a second thought. Simply because when you pull it out to go home , you're carrying around more weight and I couldn't be bothered pulling them all the time.

BTW , Once on plane it does seem to get a little "throatier" but not to the point you have to SCREAM at the person next to you:cool:

hey guys

we shoud invent "select Thunder" ( i trademark that right now.....)

it could be a pneumatically controlled flapper that replaces the fixed one inside the silent thunder. use it just like silent choice. only integrated into formula's system.

open for on plane. closes adn partially submersses into the water resorvoir for idle, could actually be designed for more back pressure because you use it mostly at idle

hmmmmmmmmm

boiler daddy, what do you think.....

I can't believe that you said that. I was just thinking about doing this to my silent thunder this morning.
Good job

sheikman 540

I'll put you on the patent:D


the hardware exists, just bolt the flange of a switchable tip to the baffle plate instead of a transom

TTT for Formula

I have an 89' 311 with basic 454 mags. I cut my baffles out using the specialized Porter Cable saz all, then I enlarged the circular holes at the bottom of the box into hugh friggin rectangular holes. It sounds better at idles, when the boat comes up on plane and the box comes out of the water gets much throatier, and spinning the original 23 inch Mirage props, gained 300 rpms on the top end with the modifications. I've read where some people have done this and did not see any more rpms but all I can say is I did.

I've got this system on the drawing board, but it has to be done in SS tubing. I'll put a sketch together and post it tonight or tomorrow. It's a 3-way system: Straight-through / Muffled / Underwater. Brownie at Stainless Marine is helping me with it.

I removed my Silent Thunder exhaust on an 89 272SR1 an picked up nearly 7mph on GPS to a top speed of 90.2mph. I am running 454Mags with Dart Pro1 CNC'd heads and Whipples so I was probably producing more back-pressure than normal. In addition to the speed increase my boost actually dropped 2lbs.

Hey guys I am new to Formula I have a 1992 242 LS with a 502 runs great only400 hrs if I take out those rubber plugs in the bottom of the swimplatform the silent thunder exhaust will sound alittle better

Prop Question I have 1992 242 with a 502 takes long time to come on plane with 5 people running 23p s/s prop should i go to a 26p to get out of the hole quicker or will i lose alittle top end

lv92780

taking the plugs out will make it a little louder, but mroe water flows than can drail out the small holes under full throttle.

if you wanted to make it sound better, you could cut out the baffle plates that are midway inside the silent thunder box.
i did this early on and it sounds nice. search under my logon and i even have a simle scetch on what it looks like inside

a long bladed sawzaw and in 30 seconds you are making more noise :D

regarding prop pitch, the number equates to theoretical travel (in inches) a blade makes in one revolution. ie the boat travels either 23" or 26" in one prop rotation.

so a 26 would be more pitch, making it more difficult to get on plane ( like starting in 3rd gear)

the key is getting on plane in sufficient time , and running full throttle at your engines peak rpm to get the top speed

so with the 502 you shoudl be at wide open with your most common amount of passengers and fuel, and be at about 4800 rpm

now if your engine is spinning up too much rpm with the full load, then maybe a bigger diameter prop may be needed. I assumed the engine was just lugging with 5 people....
hope this helps

Ramb

Is the 23P a 3 or 4 blade prop?