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Under Bumper Trim Function?

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With Driver’s Ed and Autocross season almost here, I decided to remove my front bumper this week and clean the radiators. Not much, if anything, new to report above and beyond JeTexas’s excellent video DIY. But among other things, I found I was missing both RUBBER LIPS (996-504-503-00) found on the underside in front of the wheels .

The Pelican Parts catalog describes these as “expendable.” Since Porsche tends to do things for a reason, I am curious what these pieces do. Since they are beneath the radiator cooling fans, do they help extract more air, simply direct it, or what? Do they provide enough benefit to justify the cost (sub $50)?

Also, for anyone doing the bumper removal for the first time, I would recommend 4 things:

  1. Have a few extra screws for the bottom of the bumper ready. I was missing 3 of them.
  2. Also the fender liner fasteners may be old and brittle, so I would have a few of them ready as well. I just replaced all of mine.
  3. The screw in step one that is described as the toughest one to reach is not bad if you go ahead and pop both fender liner fasteners out.
  4. And finally, have an 8 inch or better extension ready to get to several of the torx screws, especially the ones holding the inner plastic scoops in place.

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From the MY99 Tech Book:

"To improve underbody air-flow, the vehicle has been given the flattest possible underbody. The design of the floorpan complemented by a underbody paneling comprising several sections has made this possible.

Just in front of the front wheels, a longitudinal diverter fin with a small, flexible spoiler lip is attached to each front fender inner liner. It can resist wind pressure even when the vehicle is traveling at top speed, but can bend back to avoid solid obstacles


A large and extremely aerodynamic underbody panel at the front of the vehicle protects the front axle and the fuel tank located above. Apart a reduction in aerodynamic forces, the middle panel also provides the pipes located above with protection.

The rear cover was designed in such a way that it carries the underbody air-flow to the rear end of the vehicle without limiting access to the engine, transmission, and exhaust system or restricting the required dissipation of heat.

Furthermore, the vacuum in the engine compartment produced by the rear cover also improves the efficiency of the engine compartment fan used for temperature reduction. At the lateral connection point, the cover is also adapted to the spoilers in front of each rear wheel which also considerably reduce the aerodynamic forces. The bottom 10 mm of the spoilers have been made of flexible rubber material so that damage is avoided even in the case of extreme road conditions. All of these underbody measures reduce the drag coefficient and the rear-axle lift.

Together with the retractable rear spoiler, a reduction of lift forces as well as optimum harmonization of the lift balance and thus a significant improvement in driving stability have

been achieved by the aerodynamic optimization of the underbody."

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When I bought my car both were missing or damaged badly. Looking at where they fit, and the air chutes near them, I tend to think the fin diverts the hot radiator outlet air down and outboard of the front wheel and away from the brakes.

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