Glen-L.com

Hi All,

I need to adapt my 4" muffler spigots to 3 1/2" Glen-L Chrome trumpets (BEAUTIFUL objects!!!!!). There seem to be readily available various reducers made of silicone material stregthened with fibres. Since they will be near the waterline and of course contain harmful exhaust gases I wanted to be sure they will be ok.
Heat is not a problem, they are rated higher than standard rubber type ones. Salt water shoudl be fine, Oil maybe???? Vibration cracking???
Does anybody have any knowledge of issues or lack thereof in using these on hot salt water exhaust?

Thanks,
Glenn.

_________________
By the time I have built a boat, I'll be ready to build a boat....

I can't speak to the silicon.

I used these guys for fittings (and mufflers)

"designed for purpose"

http://www.centekindustries.com/fittings.html


I went from 3" manifolds to 3-1/2" muffler to 4" trumpets...

_________________
Bruce.

~~ Do what you love, and love what you do. ~~
~~ To me - only my boat is not yet perfect. Everybody else's is to be admired for I know the path they have walked (Dave Lott, 2010) ~~
Dow's Monaco Project

Hi Trackhappy,
Not sure on the silicon but under no circumstance should you reduce the size in your exhaust line from engine to outlets. Increasing is fine though.

Wayne

_________________
Building the Bonanza

As to the last comment,on cars it was common practice to reduce the pipe size somewhat the farther you went from the engine (tailpipe after the muffler)....reasoning I had heard was as the exhaust cooled,it lost "volume" and didn't need the larger size.

Wonder if this would be OK with boats too????

_________________
Boat building can best be defined as an endless series of
tragedies obscured by the occasional miracle, followed by a good bottle of beer.

Don't Dream Your Life, Live Your Dream

I think the answer is "it depends". If you start off at 3" with a 350 Chevy, you sure don't want to drop down without expecting extra back pressure and extra heat in the run. In my case I have started off at 4", which is the diameter where the water outlet surrounds the exhaust outlet. The exhaust outlet itself is more like 3", so I figure that I am in good shape going to 3 1/2" right at the outlet. The engine manufacturer advised 3 1/2" is fine. I have noticed on the Mercuriser that the elbow is 4" to match where the water comes out, reducing to 3 1/2" (or it might even be 3"') where you connect the rest of the exhaust. Maybe this is the "mixing area" for the water and gas. I also note that the 327 Corvette in the shed only has 2 1/2" exhausts all the way so given that I am not building a hot rod, we should be in good shape.

_________________
By the time I have built a boat, I'll be ready to build a boat....

Most of the back pressure will come from the risers, and mufflers. I am assuming you have a wet design. The cooled wet volume of gasses will be far less than the volume leaving the ports. A half an inch isn't anything to worry about.

A proper exhaust design is far more than hanging the biggest pipe you can find on the collectors, and letting it rip. In most cases any SBC running stock heads, and a hydraulic tappet cam that actually idles without sounding like half the plug wires are missing, is going to need a mild amount of back pressure to run properly. 3-1/2" is more than enough to vent an engine that is only producing a static volume of 228CuFt/min at 4500rpm.

More back pressure produces more low end torque, and higher fuel efficiency, within reason. 12:1 compression, a cam with a big lift, overlap figures that look like the national debt, valves the size of manhole covers, and a 7000 rev power band needs a well designed, tuned system... a lake boat with a 350 hp SBC does not need an exhaust the size of a sewer pipe.


Silicone is great... just don't let it bask in the sun's UV's for extended periods. degrease the metal parts, and use a splash of isopropyl alcohol inside the tube to lubricate the hose when you push it over the metal parts. Double clamp the fittings. Silicone slides off easily - not the ticket to happiness.

_________________
Some plan to stroll through Saint Peter's Gates, I plan to go through them at 150mph... backwards... in a screaming ball of flame, with a glimmer belt wrapped around my head, and a NOx button in my hand.

Thanks for the replies. I have worked around the issue by getting the outside of the clamping area of the trumpets turned true in a lathe and getting two pieces of bronze tube turned to be a close fit onto this area and 4" OD. They will be coated with sealant inside, then slipped over the trumpet and three countersunk stainless steel machine screws added to fix them in place. That gives me a 4" to 3.5" adapter without adding extra length and the 4" tube can bend happily in that distance. Once they are complete I will add a piccie in my main thread to show what has been done.

As for the back pressure, I can only now hope that the mufflers will do the necessary along with the 3 1/2" trumpets. I have too much invested to back out at this stage.

Cheers,
Glenn.

_________________
By the time I have built a boat, I'll be ready to build a boat....