[vex]

You're going to need a lot of muffling devices. You'll need to reduce the pressure differential (which is directly tied into performance) prior to the exhaust exit. That's been my finding at least... Let me clarify.

When gas leaves the engine it's very hot and under pressure. At the tail pipe you'd idealy (sp? Stupid spell check doesn't work on this browser) have atmospheric conditions when it comes to pressure and temperature. The result is that you would need to have to balance the conservation equations (mass, momentum, energy). We have hot and high pressure gas coming into the system and we want cool, atmospheric gas leaving. The farther away from ideal the louder the system will be to the outside world. The way to minimize the difference is to provide areas of expansion (lowers pressure, and reduces heat, but creates noise from the energy difference). To remedy the noise issue from the sudden expansion of gasses deadening material is shoved into the volume to absorb that energy--producing heat. Since there's a pressure differential still the gas will exhaust that chamber and then enter the y-pipe. There, pressure is reduced and therefore temperature is reduced as well. The temperature and pressure are still greater than atmospheric, thus additional chambers need to be installed after the y-pipe.

As a general rule of thumb, the larger the volume installed in the system the greater the sound reduction so long as there is appropriate sound deadening material. There are nuances associated with exhaust components and disrupting harmonic resonence, but that isn't really necessary to be discussed at this juncture.

In your case, the addition of more sound deadening devices would be necessary. Installing the largest RB universal presilencer helps a lot (it's what I run). Shoving it as close to the headers as possible without damaging the deadening material (it doesn't like extreme heat). IF you have room a catalytic converter can help in sound deadening as well and help you pass emissions (LOL)--though another muffling device might prove more beneficial. Instead of a y-pipe you could use a single in- dual outlet muffler to further reduce the noise. Large mufflers at the rear will also help. Remember that the larger the exhaust pipe is, the louder the noise (on average). So if you need to lower the sound the pressure needs to be lowered at the points the gas exits to atmosphere. This means smaller exhaust outlets will reduce noise effectively as the gas has to speed up and therefore reduces in pressure once the flow is incompressible.

If you're more concerned with performance however--you want to get the exhaust out as quickly as possible and damn the pressure differential.