I bought a Flowmaster 40 series to replace my main muffler (which I removed a few months ago) but it was a bit to big. I have to buy another 40 series so I can replace my dual mufflers. I'm looing for a deep, aggressive tone. In the meantime I have straight pipes. It sounds raw, especially when it idles, but after any aggressive acceleration if I take my foot off the gas too quick, it backfires (humiliating). What causes this, and how do I fix it before I get my other 40 series (might be a while)?