View Single Post
jyl jyl is online now
Registered User
jyl's Avatar
Join Date: Jan 2002
Location: Nor California & Pac NW
Posts: 20,050
I am sure the cosmetic damage to your air dam won't make any discernable difference to performance.

As for the effect of the air dam and the rear spoiler, I looked this up in The Porsche 911 Story by Paul Frere.

Porsche factory test data on an H-series Carrera:
- With front air dam and rear spoiler: Cd 0.414 Ct.f 0.010 Ct.r 0.025
- Without air dam or spoiler: Cd 0.423 Ct.f 0.112 Ct.r 0.246
- Without front air dam, with rear ducktail: Cd 0.414 Ct.f 0.113 Ct.r 0.062

So the air dam and spoiler (or ducktail) together reduce drag a little bit and reduce front and rear lift to nearly zero. And a rear spoiler (or ducktail) with no air dam means the car has much more lift at the front than the rear.

I don't know how you convert from Ct.f and Ct.r to pounds of lift, but the book also says that with air dam and spoiler the total lift at 152 mph is 38 lbs, without either it is 397 lbs.

For a C-series 911 (1970-ish) without aero aids, the lift at 143 mph was 183 lb in front, 255 lb in rear.

And when a deeper front air dam was used, it actually created slight front downforce (Ct.f = -0.055) but increased rear lift (Ct.r = 0.113) by pitching the car forward.

There's more data in the book, but anyway it seems to me that at 150 mph your front and rear aero aids need to be well thought out and balanced.

Nowhere does the book say what the effect of these aero aids is at speeds less than 143 mph, which is a bit more pertinent to me. Perhaps lift is proportional to speed - no idea if this is so - any pilots to help?l
1989 3.2 Carrera coupe
1988 Westy Vanagon, Zetec conversion
Really want to find a clean E28 . . .

Last edited by jyl; 02-03-2004 at 08:16 PM..
Old 02-03-2004, 08:13 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #21 (permalink)