whats the most basic mods for our cars?

Ape Factory

If you're daily driving any distance, you'll want something that errs on the side of quiet (but still louder than stock) with no droning. The HKS would be a good choice in that regard because it's dead quiet at neutral throttle and when cruising on the highway but it'll scream nicely when at wide open throttle. I found the FI cat-back to be too loud honestly but it's a personal choice.

The other consideration is rasp. Some combinations of testpipes/high flow cats and certain exhausts produce rasp which many think sounds "cheap" but it's definitely not as pleasant-sounding as a smooth, deep exhaust note that sounds like ripping silk towards redline.

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'13 Audi RS5 Misano w/mods-The Red Mist
'13 G37 IPL-SOLD