Well, again, as per my post above (number 9), my situation may be a bit different. Whatever it is that slows down the C4S at high speed (whether aerodynamic drag or mechanical drag) does matter to me - based not on theory, but on experience. If I were focused on how it drives on track, I'd go the GT3 without giving it a second thought - but I'm mainly focused on how it drives on the road.

 

I'm pretty sure it's more mechanical drag that makes the C4S slower on the top end. AWD has a lot more parasitic drag than any of the subtle differences in aero between the listed bumpers. Even a GT3 wing wouldn't affect top end that much. From my experience with aero on my S2000 race car (different splitter lengths, GT wing at different AOA, underpanels) it will not affect your top end that much. You would have to be running data acquisition on a track to notice the difference imo. The most noticeable effect is downforce with a proper wing and/or splitter. Even in your case (hitting high speeds on the Autobahn) downforce is a more welcome benefit than less drag. Sure, it's primary benefit is increased corner speeds but even on a long straight stretch of highway extra downforce will make you feel much more comfortable at speed. You will have more confidence in the car and be able to sustain higher speeds for longer periods of time. It is a benefit you will feel even in a straight line at speed.

 

Per your situation, I'd go with the GT3 aero. Even that is not really extreme but it is a good balance between street / track or in your case, high speed highway runs. Proper GT air foils will put a stock GT3 wing to shame in regards to downforce and less drag. I wouldn't want to run that big a wing on the street though

Edit: Actually go with the GT2 wing. I think it looks better - sleek and not as high as the GT3 wing.

 

i was looking for a bumper that would send more air to the rear breaks. I often taken curvy roads at high speeds all while trail braking. This causes my pads to heat up beyond belief.

 

Are your pads fading? I wouldn't rely on a bumper to send air to rear brakes. The proper way to do it is with brake ducts. I think that is overkill on the street though. I'd just go with a little more aggressive compound if you are fading your current pads.

 

I was also thinking that the main thing that slows the C4S down is mechanical drag from the 4wd. Then I realized how I could find out - check the official top speeds of aerodynamically identical C2s and C4s. Much to my surprise, my source gives the same top speed for the 3.4 liter C2 and C4 (280 kmph, or 174 mph), and the same speeds for the 3.6 liter C2 and C4 (285 kmph, or 177 mph) (although there was no 3.6 liter C4 in the US, there was in Europe). So much to my surprise, in fact, that I don't fully believe it. I found this on autoscout24.de. Tomorrow, if I remember, I'll check check the owner's manual in the car. I remember it includes performance data for all 3.6 liter, non Mezzger 996s.

 

Anyone know what the GT3 rear bumper vents vent? Brakes? Wheel wells? Muffler heat? I found myself with nothing to do while standing next to a 997 GT3 RS today, and I took a close look at the side vents in the rear bumpers. And although the car was parked on a light floor in a reasonably well lit place, peer though I might, I could not detect the slightest glimmer of light coming through them. Surely Porsche wouldn't put fake vents on a GT3 RS, would they?

 

On second thought, I may have forgotten something. Can't the 996 4wd system direct all torque to the rear wheels? And if so, wouldn't there then be no extra drag? Perhaps the entire drive train for the front wheels disengages. Or perhaps most/all of it keep turning, driven by the front wheels themselves, but turning mass alone isn't drag? Rather, it's like a heavier flywheel, or heavier wheels, which dent acceleration, but not top speed?

 

Factory aero,Third radiator kit.Accusump,and a Motorsport AOS,you should be covered for cooling and oil starvation........all it takes is a few $$$$...LOL....good luck.

 

That's probably your traction control. It uses your brakes. High speed curvy road trail braking will get it going crazy. You can't fully defeat it with the push of a button.

 

Thanks for your post, including the pictures - well there's certainly no doubt that the factory aero kit looks good!

Does it make any significant contribution re. cooling?

Also, did it come with the car, or did you add it? And if you added it, did you notice any effect on high speed acceleration? Above 120, for instance?

Thanks!

 

I thought I should update my hypothesis here, because it isn't quite right. On a C4S, and I'm pretty sure on a C4 as well, at least 5% of power always goes to the front wheels. So whatever impact 4wd has on mechanical drag or inertia, it's always there. Still, given that the C4 has the same top speed as the C2, it still seems that aerodynamics plays a big role in the C4S's weaker top end.

 

Why not just stay with the regular factory front bumper? I assume you have not experienced any overheating issues currently? If you have not then the stock setup is doing just fine. Don't fix it if it ain't broke.

About the only modification I would advise would be to add the 3rd radiator but many cars get along just fine with the 2 radiators.

By keeping with the setup the car came with you don't have to worry about upsetting the balance by installing an overly aggressive front bumper then trying to come up with a suitable replacement at the rear to regain the balance.

BTW, regarding the AWD system and the 5% torque minimum to the front axle is theoretical. In the real world it can be 0% to something around 5%. Standard allowable deviations due to normal manufacturing tolerances probably account for the difference. FWIW, my reference has the 40% (max.) coming in at around 150mph.

 

So it is sending 40% of torque to the front wheels at 150? Wow, that would be a surprise; I'd have thought it would send it there to help acceleration at low speeds, then back off at high speeds. This could explain why a C4S has noticeably less acceleration at high speeds than a C2 - provided the same is true of a C4. (I presume the C4 and C4S have the same drivetrain.) (As I may have mentioned (last visited this thread a while ago), while the 3.6 liter was not sold in the US, it did exist in Europe, probably elsewhere too.)

 

Undobtedly good advice, and based on what I've learned, I would not change the bumper just for cooling. But since I have to replace it anyway, and since I would not mind C4S or even 996.2 GT3 level lift/downforce if the speed penalty is small (and I can accelerate better than a C4S at high speed), I'm hoping there might be a solution that works for me (for both the front and the rear).

As for the 3rd radiator, I'm kind of inclined to pursue other options as I cam across a post saying there is a "well documented" increase in drag (though no more details than that, and I haven't done any searches related to this topic). Also, it adds a bit of weight. Maybe lift too? (No idea.) So I'd lean towards other options, like waterless coolant, maybe a low temperature thermostat, and not doing too many laps in a row.