I added the 4.6is exhaust on my 03 4.4i, it appears that the 4.6is has 2.5" tubing vs 2.25" from resonator to the mufflers, Side by side, visually it appears to be a larger pipe leading up to the mufflers. At any rate, it does seem to make my car purr a little bit nicer when i step on it going down the highway, not sure if there are any hp gains, but the sounds makes me feel like it does!

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2003 BMW E53 X5 4.4i - M62 engine (Non Sport with Premium and Cold Weather Package)
Color: Steel Gray Metallic
Mods:4.6is Exhaust (Pipes, Mufflers and chrome oval tips) BMW Sport package Taillamps, BMW Tire Valve Stem Caps
Medium tinted front windows
JVC HD Radio
Alpine 4x100 channel Amp
Orion 600W Mono HC Amp
Three 12" Sony Xplode in subwoofer enclosure