Drag Coefficient (CD)????

OK, this *is* getting beyond anal-retentive, even for Germany! ......................... Carrera Cayman S Boxster S Drag Coefficient .... 0.28 .... 0.29 .... 0.30 Each time I wash my Cayman S , I wonder about the center front, blanked off blind insert. What's it for?.....(see attached pic). I know very little about aerodynamics, but .... being a bit of a Machiavellian I wonder if the purpose / design is to generate additional drag, positioning the Cayman S exactly between the BoxsterS and the Carrera ? Any aerodynamic experts lurking??? Maybe they can shed light on what might happen to the CD if it was covered over? Would it decrease Coefficient of drag (maybe at least 0.01)? Now wouldn't that be interesting! Also, what impact would lowering the CD by 0.01 do for performance? Does anyone have other input into what purpose it serves? Inquiring mind wants to know! graham

I've never understood why the Cayman and Cayman S have a blanked-out indentation where the functional opening is found on the Boxster S . The third grille opening on the Boxster S is an opening for a central radiator. I wouldn't expect a base Cayman to have such an opening (just as a base Boxster does not), but I would imagine that the Cayman S would have a functional central grille opening, as befits its larger, more powerful engine. It's not like Porsche to dream up a stylistic device that is strictly for show. It seems like the one fake element in the car's exterior design (actually, there is one more--the "blank" above the fog light spears). Am I missing something here? 987f

Interesting question. It is clearly "dead space" and does contribute to the overall frontal area of the vehicle. I suppose that if it were open and ducted underneath or to the side of the car it would be less of an impact but I think that would bring in other variables that are difficult to determine. The simplest way to approach drag is the frontal area. The aerodynamic equation for drag is the (density of the air) multiplied by (the velocity squared divided by two) multiplied by (the coefficient of drag) multiplied by (the surface area). The coefficient of drag can be reduced by aerodynamic techniques and of course the Cayman is probably near optimum in this respect. The coefficient of a flat plate for example is 1.28 (from an aerodynamics textbook) and the Cayman is about 0.29 with a manual and 0.30 with a Tiptronic. This data is from the Porsche website on Cayman specifications. So if you consider the overall effective drag of the Cayman it is possible to assume that the area you pointed out is not significant and likely would not contribute to performance. I found it interesting that the drag coefficient changed 0.01 because of the transmission (and I don't think we need to go there based on other posts. ) but it is the value you mentioned. I can only assume at a constant speed that there is no real difference provided by 0.01 except perhaps at top speed and then not much. Hope this helps.

I can not confirm this, but I believe that center opening is not blocked off on TipTronic cars and instead contains a third radiator. This would also explain why the Tip cars have a different Cd. Sounds like blocking it off is better than letting air flow through it.

My understanding is that the central block-off was for a transmission cooler for tip cars. The real question is how to get the drag coefficient lower than the 997? Beyond shapes, the friction coefficient also comes into play. Maybe some sort of polymer coating? Does a clear bra slow the car down or speed it up?

Gator Bite is right - the Tip cars have a 3rd radiator in that spot and it actually has a higher Cd. It may seem counterintuitive, but it's true. Cd is determined by lots of things, not just frontal area - it's also determined by how the air flows over, under and around the body. With the center opening open, the air that enters has to be disposed of someplace - with the Tip cars the air flows over the center radiator and out the bottom and it creates a small bit extra turbulence under the car contributing to slightly more drag. If you were to open the spot up and have nothing there, you would actually create a high pressure spot under the front of the car, giving you front-end lift at speed, and even more turbulence under the car - and a still higher Cd, neither being a good thing - especially the lift at high speeds. If you look under your car, you'll notice it's flat and smooth almost all the way to the back wheels - it's done that way to eliminate turbulence - you don't want to add any. The cup also adds some down-force to the front end at speed. You'll notice on the GT3s, the center radiator disposes of it's airflow through a vent between the front bumper and the front of the hood. This has actually been discussed quite a bit on the CC - do a search and you'll like find some other opinions. brad

I should have explained my concern / point a little clearer, re: aerodynamic flow / drag . THe center "opening" is not simply blocked but is a " blind indented pocket". I was thinking more about laminar flow as opposed to frontal area. Would the blind pocket create a disturbed airflow around and over the frontal area / hood.... ? I know manufacturers spend a great deal of time and effort in wind tunnels getting the air to flow up and over the hood etc..... does this pocket disturb that flow? And would blocking off the pocket create more slippery design(lower CD)?

Thanks, Gator Bite and Beez. If that is true, then Porsche is in fact letting form follow function! 987f

Or does the indentation act like a dimple on a golf ball?

Something to keep in mind is that Cd (Coefficient of Drag) is a units-less value attempting to characterize the drag of a particular shape. "units-less", in this case, meaning without regard to size, velocity or air density. The actual drag experienced by a car depends as much on the car's frontal area (as well as air density) as on the Cd, and depends even more on its velocity. Therefore, a car with a low Cd may still have a higher drag than a different one with a higher Cd. For instance, Tour Buses usually have a very low Cd (on the order of .22 to .25), but very high drag values (the low Cd is due to the high proportion of overall length to frontal area). While Cd is a nice thing for marketers to brag about, it's not as useful for comparing vehicles in the real world as Cd times frontal area (e.g., a vehicle with a Cd of .25 and a frontal area of 4 square units will generate as much drag as a vehicle with a Cd of .2 and a frontal area of 5 sq. units). Coincidentally, I never see marketers bragging about frontal area....

That would explain why the Cd appears lower on the 911 despite it's obviously larger size and larger tires, both of which should create more drag.

There is a lot to drag than meets the eye. What happens underneath, how the radiator air is let in and out. How the rear of the car is shaped all have a big input on the drag. I would think that the air hitting the center opening would just stagnate and the air that would hit next would just be pushed up or down. Then was the drag measured on a rolling road wind tunnel or a normal wind tunnel, makes a big difference.

Folks: This may seem simplistic, but has anyone asked Porsche why they have the nose of the Cayman S configured the way they have? I suspect that if someone who had connections with Porsche could inquire, we could stop the speculating ( although a fun exercise) and get some answers.

I found this online Drag Coefficient calculator sometime back. http://www.processassociates.com/process/dimen/dn_dra.htm and had to read this to get more educated http://en.wikipedia.org/wiki/Drag_coefficient

So it was mentioned a couple times that Cayman tips have higher Cd. Could someone tell me what exactly is the Cd value for Cayman tip? And where was this information published? Thanks.

kimchang - see the Porsche website for Cayman S "specs and features". It is listed under a tab entitled "body".

What is really frustrating is that the extra radiator is used only on S Tiptronics, base Tiptronics don't have one. Three out of four types don't use the radiator, yet we have to live with the ugly monstrosity. Has anybody seen a good block-off kit or something to cover it?

I hit a piece of rubber tire on the freeway a couple of months back and when I got home the only "damage" was that the center cup had been pushed in on one side. It was just held on by a couple of clasps that clipped right back in. Has anyone removed this on their car and replaced it with something else?