1. Working by hand, with no wind tunnel or other aero instrument, it's pretty hard to reproduce the Porsche design.
2. However, the cooling improvement by
leaving the rotation of the forward wheel to suck out the air from the intercooler bay has been tried and proven by thousands of people ever since 1999, when the Golf Mk4 platform with side intercooler became widespread.
3. It only works since a design quirk placed the intercooler very close to the wheel
and separated from the engine bay with a plastic wall. Some insulative material on the wall and behind headlight in the upper engine bay, and the IC bay is thermally separated from engine bay entirely.
4. The car lacked some aero features which had become ubiquitous after 2004. Being designed during 1998-1999 crisis, when crude oil dropped to 10 USD/barrel, there was little incentive to cling to the smallest efficiency improvement. Official Cd had been 0.32
5. There is fitted from factory a partial grill block just behind the S badge,
as visible here. Side upper grills may be covered on the inside in the same manner, but there is little benefit into doing so. Airflow for cooling is driven by the
pressure differential across the radiator, more than by straight flow into the radiator bay.
BEFORE / AFTER
No deflectors before front wheels
/ Boat-prow deflectors before front wheels;
Half undertray below engine
/ Full undertray from front lip to the rear end of the front axle
Parachute-like shape of the rear bumper on the inside and rear axle exposed
/ Full undertray from rear axle well to the rear edge of the bumper;
Nothing behind rear wheels
/ Boat tails behind rear wheels
Big and ribbed muffler
/ Flat aerodynamic muffler
Lower side of the intercooler open towards ground
/ Side undertrays left and right beside engine undertray
Exhaust tunnel made a big bend before fuel tank
/ Flat aluminum plates around the exhaust bend (but not covering it)
Sheet steel front control arms
/ Control arms plated over, like fish flippers parallel to ground
Useless gaps in the front bumper, leading nowhere
/ All gaps which can't steer the airflow into intake or some radiators closed oveer
Gaps in the bodywork
/ All gaps in the bodywork closed with rubber gaskets
Audi has found out by experiment that smoothening the underbelly as much as possible may give a delta Cd around 0.024.
If deflectors before front wheels really give a delta Cd of 0.01, this drops the overall Cd from 0.32 in the 0.28 range, which is actually very good for a daily-driven car.