I too have an exhaust question, I recently bought a used monte its a 2001 SS as a gift, my brother bought two after market mufflers that look like they belong on a tuner...I love how they sound but i actully have lost some power....The debate had been going on for a long time about back pressure good or bad...I know this, I have come to the conclusion that I need more back pressure...I tried adding a straight through design glass pack and it didnt make any difference. This may sound stupid but After doing some homework, Back pressure is back pressure...Right??? So i came up with the idea to use a C clamp and some steel stock and basiclly squeese a section of the exhaust pipe to create some more back pressure... seeing how no one i know knows about cars i havent had anyone give me any feed back on the idea.... i havent done it yet but want to soon...I want the power back... any thoughts....I know this is a really low budget attempt to gain some more power but thats the boat im in....

 

the back pressure you are talking about is called exhaust scavenging. Look it up on wikipedia or sumthing. No backpressure is only needed in turbo applications so the gas can be recirculated properly with no restriction. Ur idea vailiant an effort as it is, wont work right. You need the backpressure (in your situation) from the mufflers so that it can scavenge throughout the exhaust system. the best i can guess for you to do, is throw a louvered glasspack in to replace the resonator. They do really sound nice and dont kill HP at our levels of power, and create what you want, backpressure. Redhots make the part your looking for, its cheap and sounds amazing. You probably arent losing as much power as you think anyway though, since a magnaflow muffler is designed like a ricer muffler in terms of straight trhough, but sounds differently due to the shape and size of the holes. The killer lose of low end torque is from changing to a larger downpipe than your car needs, which throws off the flow from the start. Its like trying to push a lil water out of a massive straw, vs blowing water out of a skinny straw (one requires minimal power to do so, the other would need a lot of power to make it work right) My flowmaster mufflers are actually really ****ty at flowing compared to straight pipe due to the 2 chamber design. They do provide maximum scavenging and I think flowmaster rates them up to 600hp in terms of what they can handle until they will cause serious restriction. This is my understanding of flow, im sure phenoix will come in and add some or just own me lol.
so to sum up, if you still think that your mufflers are killing u, dont add different diameter pipe, buy a lovered glasspack and replace the resontor if you would like to add some backpressure and make your car sound beast. ("flowtech" red hots glasspack)