UPDATE
As of July 2020, the underbelly of the car had been modded as such:
1. Plastic undertray from front bumper lip to just after front axle subframe, at the front end of the catalytic converter. It can't be flat, due to engine oil sump and gearbox, what matters is that it's quite smooth, no bits and bobs dangling in the airflow;
2. Aluminum sheet rear edge on the undertray, where it approaches the hot exhaust and catalytic converter;
3. Full undertray from rear bumper forwards to the well of the rear axle;
4. Flat plating as tightly as possible around axle well, leaving the exact space for the axle to move up and down;
5. Aerodynamic exhaust muffler (part of the Milltek exhaust.
Milltek muffler is oval in section and with a smooth surface, unlike suitcase-like and ribbed factory muffler);
6. Aluminum sheet flat plates around the exhaust bend, below rear seat. Around the pipe, not over it. Exhaust needs to radiate heat away. There is half-inch of free play between plates and the pipe;
7. Aero (boat prow like) deflectors before front wheels,
as the above said SLR McLaren "boat prows";
8. Boat tails behind rear wheels;
9. Front control arms plated over, to achieve smoother lower surface. Golf Mk4 / Leon Mk 1 sheet steel control arms are pretty good for this,
since they are smooth from factory. Audi TT Mk1 / Golf R32 cast steel control arms are not smooth, and they are also heavier, with bigger and softer bushings, and they shouldn't be used;
10.
Factory flat plates over rear control arms have to be left in place. They are officially called "stone guards", but they are too flexible to claim to protect anything from stones. They had been designed to flatten the surface of the rear axle around the fuel tank and keep the airflow straight.
Materials
All trays have been made from 2mm Coroplast, only the parts near exhaust from 0.8mm aluminum. They were assembled by screws or small bolts wherever possible, glued with strong Mamut Glue where there was no way to screw. Glue hardens itself like RTV silicone, holds even better than small sheetmetal screws and dampens vibration better than silicone. Obviously it has to be used on those parts which are hardly probable to ever be removed for mechanical repairs, throughout the car's life.
Exhaust tunnel has to be left alone. Milltek runs much cooler than factory exhaust, but still it needs air to cool itself. Factory muffler surface may heat itself to 140-145ºC (284-293ºF), as measured with a multimeter and a probe. Milltek may be touched unpleasantly by hand after a highway run, may burn hand after city run, but it's nowhere near factory.
Ground effect
Car is pretty low to the ground, minimum ground clearance is in the 80 mm (3.15 inches) range, in front, higher in the rear, with deep side sills. This means, if the highway is very smooth,
there is some stability gain once the entire belly had been smoothened as quoted above. Since the highways are often not too smooth, this is not very important.
Cooling
Engine coolant temps in the 85-88ºC (185-190ºF) highway in cool weather, 90-93ºC (194-199ºF) hot weather, 93-96ºC (199-205ºF) city driving, occasional jumps in the ranges where fans go into full speed (over 102ºC/216ºF) in stop and go traffic. Intake air temps as read at the manifold 13-15ºC over atmospheric temps.
Comparisons
As Audi had found out by experiment on an A4, smoothening the underbelly as much as possible may give a delta Cd around 0.024. I've left alone the effect from flat plates front to rear when adding up, on the sides of the exhaust tunnel, since the floor had been already smooth there from factory. Also, their A4 car had already small plates before front wheels. Leon didn't have them from factory, when a Cd of 0.32 had been calculated and published. SAE paper 2004-01-1307 says no deflector before front wheels contributes 13.1% to total car drag, this is why all post-2004 cars have them. Also, there was no quote on how much delta Cd we can gain by closing all useless bumper and body gaps. By useless I mean "which obviously do not feed into various radiators of the car".
This means I might have been lucky enough to achieve a delta Cd of more than 0.025, and therefore cross the barrier of "very aerodynamic cars" set at Cd 0.30.
Road testing
Stability at highway speeds has proven itself pretty good until now. By moving all unnecessary parts rearwards, the front/rear weight ratio has improved by 55kg (121.4lbs), which contributes to superb turning ability, besides suspension and tracking mods.
Milltek exhaust produces less vibration and noise than factory exhaust. Engine airflow measured by mechanical pressure gauge and Torque 7% better than before, which should give a peak hp over 250. Not bad for a factory turbo.
Fuel economy may be in the 24-28 mpg in fast driving, mixed city-highway with moderate traffic, 29-30 mpg in highway driving at constant speed.
(If doing everything possible to hypermile, it may be 36-37 mpg, but this defeats any reason to buy a turbocharged gasoline engine in the first place.)
PS Mr. Julian Edgar gave me the idea to make the aero mods, when I started to test the aerodynamics back in 2011. Nowadays, compared to 2011, I'm 20hp higher, 16lb/ft higher, but also on the second fuel pump, second alternator and third clutch in the car's life
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