Exhaust: Max.HP with Min.dB - how?

[Apolan]

Hi, i'd like to know how to make an exhaust with minimal restriction in flow (in other words, an exhaust that would yield as much HP as possible) but still keeping the loudness (not sound, but dB) stock/oem (or as silent as possible). I know countless calculations would be needed to make an exceptional compromise, but what are some of the general properties of such exhaust? Engine in question is a standard 4age..

I've heard fujitsubos and they keep the noise down, but give a gain in flow/power/torque. But they are bloody expensive. So for a homegarage inventions/improvisation guy, what is the best take?

regards,saso