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      08-06-2009, 09:15 AM   #51
The Cthulhu
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When I see 1/4 mile, I stop reading... My last car was a 2007 Exige S and my current car a 2009 135i 6 speed w/ sports package, the two cars do not compare on a track in terms of handling. I'm talking differences of 7 seconds on a 1:20 lap time between the two cars, that's huge! Go to a track, and not an oval ring. Lighter cars smoke heavier cars, if they don't, add power and reduce weight and they will. This whole "heavier cars can be faster" thing is nonsense! Have you ever seen the inside of a racecar? Do you know anything about Collin Chapman? What about Lee Noble?

When I say "faster" and you say "quarter mile" it sounds like you only know how to stomp the accelerator and shift in a straight line. AutoX times say a lot too. When I got my Exige, I was amazed by the amount of downforce the car put down. At 40 mph, wiggling the steering wheel tossed me side to side, at 140 mph, it felt like it was on rails. Nothing but wind noise and downforce.

Quarter mile times include launches, which really don't apply to downshifting and flooring it on the highway, even. 60-120 times are a different story.



"Even best case scenario with the Exige, it's shaped like a barn (like an Atom or Caterham), so over 120, it's pretty much dusted by most cars."


The Exige is shaped like a Caterham? Like a barn? O...K... I can honestly say I have no clue what you're talking about there. Generally, you need a lot of horsepower to overcome the force of drag for even a small frontal cross-sectional area. Top speed doesn't have much to do with weight, weight determines how long it takes to get there. For every unit increase in horsepower, there is a cube root increase in top speed.

I posted the following on N54tech, recently:


Force of drag:

Fd = 0.5*rho*v^2*A*Cd

where rho is air density, v is velocity, A is cross sectional perpendicular area, and Cd is coefficient of drag (0.31 for e92 M3).

Power needed to overcome drag:

Pd = Fd * v

Let:

rho = 1.2kg/m^3 (~20C at sea level)
v = 89.4 m/s (200 mph)
A = 2.6 m^2 (frontal area of an M3)
Cd = 0.31 (for E92 M3)

Fd = 0.5*1.2Kg/m^3*(89.4m/s)^2*2.6*0.31
Fd = 3865.11 N (3.9E3 N w/ proper sigfigs)

Pd = 3865.11*89.4m/s
Pd = 345.541 kW (463 HP)
Pd = 460 HP (w/ sigfigs)

You need 460 WHP to maintain a speed of 200 mph at 68F at sea level in an E92 M3.


BTW: 200 MPH on GPS... 218 MPH on the speedometer (doesn't go that high).


The thing that really limits the Exige is top speed, because for top speed, you need horsepower. You can't reduce cross-sectional area and coefficient of drag enough. I almost purchased a Noble M400 with 600 WHP for $53,000, instead of my 135i. But after test driving one, I knew I'd lose my license of push the car too hard. It weighs 2300 lbs and puts out 600 WHP. I guarantee a 900 WHP vette can't touch a noble on a track.

Low curb weight is so much more important than horsepower. You can change horsepower, but it's nearly impossible to drop 1,500 lbs of weight from a car and keep it safe.

If I wanted to build an insane track car (for under 200K), I'd probably start with an Ultima GTR chassis and add a cosworth F1 engine that weighs 200 lbs and puts down 1200+ HP (even though they need constant rebuilds).

Say it with me: "our cars are luxury sports coupes, not track racecars"

Buy a miata with head work and have fun Light is beautiful. Nobody likes a fat chick.