Silencer in exhaust

I have a mongoose fitted (full system) with de-cat but as most of you know it is rediculously loud. I'm getting a bit fed up with everyone giving me evil looks and looking like a complete tosser. It is very loud!!!
It looks as if there is a silencer built into the mongoose system but I'm thinking this maybe just for looks to fool VOSA or something? lol.
My question is, if I weld this into the system would it reduce noise much?
http://www.ebay.co.uk/itm/Jetex-Uni...arts_Vehicles_CarParts_SM&hash=item564fee1f24
 

Mr B

Member
Best way on mongoose is a custom large rear box with exit repositioned to the N/S as allows best pipe run that way.
Can also be done to keep exit on O/S but pipe run has tight bends & u-turn.
Is a couple of threads on here with pics so worth searching a bit.
Done one myself some years ago & result is good if you use a decent silencer & spend the time to plumb it in nicely ...
 

mreguest

Member
I have full 2.5" mongoose system with decat pipe, as with all exhausts the silencers degrade over time and as such get louder. To attempt to make things a bit quieter I fitted a decat silencer.

The decat silencer is about 250mm long and 8" x 5" oval shape. The eBay silencer is 249mm long and 125mm round (I see there is a miss print saying 625mm long?). The two would probably have similar performance, but, the oval decat will work a little better due to increased circumference.

Noise measurements with decat pipe:
79dBA @1m Lmax - idle
102dBA @1m Lmax - 3/4rpm i.e. 5625rpm

Then with an oval decat silencer (maybe hks?) fitted:
78dBA @1m Lmax - idle
95dBA @1m Lmax - 3/4rpm

This was all measured professionally by me with an £8,000 sound level meter doing full frequency spectral analysis! (I work here www.hanntucker.co.uk) I've seen exhaust silencers bigger than a car :)

http://www.power-technology.com/contractors/noise/pennseparator/pennseparator1.html

Without getting mega technical you don't lose much dBA at idle because the low frequency sound dominates the value and short skinny silencers can't get rid of it.

At 3/4rpm making 7dBA better with a relatively small silencer is quite a noticeable difference. but 95dBA is still pretty loud... Certainly very useful when you look at track day's noise limits.

At 3/4rpm the low frequency noise still dominates but the increased mid/higher frequencies sound caused by higher rpm that you don't generate at idle is quite nicely attenuated...

In layman's terms you won't notice much difference at idle, and revving the shit out of it will have a noticeable difference but is still pretty loud.

Length and circumference is key to making less noise and jetex/simons are pretty well made silencers. The thickness of the casing is quite important, thicker the better for a car driving by. If the casing is too thin sound energy is lost through the side casing. The result will be quieter at the rear tail pipe but listening/measuring the car side on you will find some sound energy is 'booming' out from underneath.

If it will fit put something like this in your decat:
http://www.ebay.co.uk/itm/Jetex-Uni...-5-Mild-Steel-Case-Length-315mm-/290616385668

As said above at idle and slow driving not much difference, so if it's still too loud replace that long cat-back section with something like this if it fits:
http://www.ebay.co.uk/itm/Jetex-Uni...tlet-Stainless-Steel-630mm-Long-/370741978168

And if still not satisfied take off the rear tail pipe come 90degrees across the rear with another silencer and out the other side...
http://www.gtiroc.com/forums/showthread.php?64916-large-exhaust-box&highlight=silencer

You can get a massive oval one in the void under the boot.

Later this year (summer time) I'm testing the noise levels of different silencers on my car. My plan is to get it as quiet as possible and will report on my findings...

Good luck!

Adam
 

Jon Olds

Well-Known Member
Nothing the matter with Mongoose'e IMHO IF you fit the silenced decat. Put one on my rally car and passes noise test every time. BTW I have a new Mongoose system spare if anybody wants one half price. SR6
 

littlewimp

Member
the jetex silencers are pretty good, only thing ive found is that the Aluminised Steel isn"t the best stuff to weld.
 

red reading

Active Member
It's like galvanising just use a soft pad grinder and take it off, then when done cover the weld in galvafroid or similar.
 

MartyMG

New Member
The Goose sounds terrible without the decat silencer imho. I have the 5" tail on mine & I'm sure a silencer box could fit in its place to quieten things down some more.
 
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