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