Functional flat underbody and rear diffuser design

[VWindahouse]

Very true. Here is a link to some wind tunnel testing for the Bluemotion 9n testing.

POLO BM IN THE WIND TUNNEL - YouTube

[blackbetty]

i did some reading up on the weekend about undertrays and general aero tricks. to make a one piece under body tray i reckon is over kill. the general consensus is to tackle individual sections such as a cover for the rear suspension, the rear boot - spare wheel - fuel tank etc. this is a very (time) expensive experiment. but it would be easier if tackels with a section at a time.

some general tricks to improving your top speed (and i mean top speed only):
keep the front of the car as close to the ground as possible (street cars obviously cant do this) - front air dams, fron wing extensions etc
wheel discs - but this will mean ur brakes get very hot
cover anything that could couse drag - the grill is prob a big improvement area. enough air enters below the number plate for cooling, so you could block the grill off totally BUT please keep an eye on your temp gauge
give the car a slight rake (rear end higher)
vent the pressure that builds up in the engine bay - that could mean a couple of lovers in the bonnet, or even lifting the rear of it up in relation to the front just enough to allow the air to get out. venting the hot air also helps with natural airflow passing through the radiator so in effect helps with cooling.
remove anything you dont need that could cause drag - wipers, mirrors, badges.
Tape up any seams Mcgyver style.
Keep the car very clean and waxed (NASCAR have proved a cars top speed is best when the cars are spotless, something like 3 to 5 MPH at 180+ MPH).

Also, keep in mind that HP and top speed is a "to the power of 3 factor"...for example lets say it takes 100 HP to go 100 KPH, then it would take 400 HP to go 200 KPH, 800 HP to go 300 KPH. So to improve drag/airflow even a tiny bit to gain that little bit extra top speed, is heaps cheaper (and more reliable) than pulling the extra HP.

to illustrate, my torana with a stock 179 six cylnder had a top speed of 145-146kph with a 3.36 diff gear and 3 speed automatic (super high tech stuff haa haa). after some of the aero cheats above (the single biggest gain was in grille and headlight covers), the car just touched the max of 160 on the speedo. But you cant drive around with a blocked off grille on a 40 deg day.
Plus it looked sh*t.