Louie928

Hi Adam,
I think what you are wondering about - wing or spoiler - is that you can use just about any surface to create downforce. A flat plate would do it. It's the airfoil shape that will produce downforce with the least angle and drag. The S4 "wing" is there to cause separation of the air at the rear of the car and decrease drag. It normally does not produce any downforce. However as Mark and others have shown, raise it more into the airstream and give it a little more angle and it'll make downforce. A sharp turned up angle at the trailing edge of a wing, or Gurney strip/flap, will increase downforce at the expense of a little drag. It acts to effectively increase the camber of the airfoil. Too much angle on a wing, or more than 3/4" tall Gurney strip, will increase drag without increasing downforce.

hacker-pschorr

Meh, this is all nonsense, just build a bigger motor

Aerodynamics are for people who can't build engines.
-Enzo Ferrari

That's how Porsche dominated Can-Am

dprantl

Haha, that must be the dumbest thing any person working for a performance car company ever said (let alone a person owning one).

Dan
'91 928GT S/C 475hp/460lb.ft

mark kibort

The net net of the "real " wing vs S4 wing, was that i could get near the same downforce with the "real" wing, as the S4 wing, with 10 degrees less angle. (so less drag) the drag is such a small factor, that I wouldnt worry about it.
200lbs of downforce will be less than 20lbs of drag at 100mph. That equates to about 5ft-lbs of engine torque and HP. (and only while youre at 100mph or faster. and of course, this goes up with the cube of the speed)

mk

mark kibort

This is a great calculator!

http://www.gtechprosupport.com/support/AeroDragCalc.php

dprantl

So my car would be able to break 200mph with 2.2 5th gear

Dan
'91 928GT S/C 475hp/460lb.ft

jorj7

Sorry to say, that drag is only part of the story, though it's the largest component.
Roll resistance also needs to be factored in. Then the part many people forget
is that you need to be making that much power at the RPM the engine will be
turning at that speed. Making 475 hp at 6200 RPM when you're hitting 200 at
5600 RPM won't cut it...

dprantl

I think I would just make it. 200mph requires 385.9hp and I put down 395hp to the wheels at 5,600RPM (that's including all losses and rolling resistance). It would probably take me 5 minutes to get there from 170 though

Dan
'91 928GT S/C 475hp/460lb.ft

mark kibort

I think we have already proved this with the Devek machine. 420rwhp got 200mph. AND, Marc was at altitude. At 6000ft, thats roughly 20% less air resistance, but 20% less HP so it does work out.

mk

mark kibort

most of the rolling resistance is measured on the dyno. (at least the rears only, including rear end and torque tube losses. They might be even more than on the road, as it is strapped down. Rolling resistance is fairly constant.

Since most of our 928s make peak hp in the 5600-5800rpm range, the 2.2 is pretty optimal for running 200mph as RPM would be near 5800rpm



.

jorj7

I guess a lot depends on your tire size, real Cd and frontal area. For my car I use 26.6" diameter
tires, .32, and 20.5 sq-ft. For a stock S4 (2.20:1), probably I'd estimate 25", .34, 21 sq-ft. Using my
calculations that would get you to 200 mph at 5925 RPM and require 478 rwhp. Using the numbers
for my car I get to 200 mph at 5567 RPM and require 446 rwph. Of course variables like elevation,
slope, barometric pressure, temp, and wind all will have an effect on the actual speed and power.

mark kibort

your car looks a lot like deveks and they ran 200 at 420rwhp. why do you have it up around 475rwhp for 200mph. by th way, what tires are you running. im running 305x35sx18s and they are almost exactly 26" tall. sure, smaller, and you would need more rpm. But the point is, the 2.2 gets you near the 5800 to 6000rpm where most have peak HP anyway.
elevation is a huge deal obviously, so we have to count on a flat incline, and all the barometric conditions pale by comparison to the elevation effects and are somewhat of a trade off with power vs aerodynmamic drag which is the dominant force you are trying to overcome.
If a stock S4 is .34Cd, I would certainly think that .32 or lower was possible with all the tape and mirror delete. Heck, all you need to do is look what has already been done. 172mph with 290rwhp (approx) and 180mph with the same hp but on a track at near sea level. (Nardo). Then, work the numbers backward. The good news is, Devek already has shown us the way. 420 to get to 200mph and wasnt it like 440rwhp to do the 209mph?

mk

mark kibort

A GREAT story of what it takes to just go 173mph on the salt, even with 600hp!!!!

http://www.edmunds.com/insideline/do...opanel..4.*#22

RKD in OKC

Didn't Devek completely mask off the front air inlets and take in air at the rear of the hood by proping it open and making a custom filter thing to get air from the back of the hood? Also I remember after making his 212 mph run he could go a little faster with narrower tires.

jorj7

Mark,

I'm running Michelin PS2 275/40ZR18 (91Y) on the rear, they list 26.6" tire diameter on Tire Rack's
site.

Not to take anything away from Marc and Susan, they did great things for the 928, but DEVEK made
the run in the White Car on a nice cool morning on the downhill run of the Pony II with a tail wind.
All the factors were in their favor. Recently Tim has had to have over 500 rwhp to hit 200-208 mph.
That's usually in the afternoon heat with a crosswind, if not a headwind. I topped out at 199 mph
against a headwind when the last few dynos had me over 475 rwph (up to 531 max) on the same
dyno that Tim used. So I'd say not all high speed runs are equal, and you don't know for sure you'll
hit 200 until you get there. Also, for ORR, you have to hit 200+ twice in one event in the designated
speed traps and finish the event without disqualification. This adds a little spice to the mix, can't
just make your run then go home. Of course, that's not necessary to just say you've done it...