rs10

Does someone already make one? I’ve seen in the 991 forum a fair amount of interest in this for 991s. Here’s why someone should make one for 996s and 997s, and why many of us should buy one:

The 911R diffuser is very different from all the aftermarket diffusers for our cars I’ve seen – indeed from all the other diffusers I’ve seen for any car – with many advantages:

Why get a diffuser at all? Why not just get a big wing?
• But a wing will almost certainly create more drag, especially if you tilt it to not just reduce lift but generate downforce.
• It will be expensive. The good ones seem to start at close to $3k and go well up from there.
• It will be heavier. Yes, you can get a carbon fiber wing and engine lid, but you still save little if any weight versus a stock carrera, and you may raise your center of gravity. Whereas if you want to save weight, you can get a carbon ducktail for much less, and combined with a 911R style diffuser, you might get roughly the same downforce as with an aerokit wing less weight, hopefully less cost, and hopefully less drag too. (I think a duck tail doesn’t add any significant drag versus the stock, raisable spoiler, but I’m not 100% sure.)

Here’s a thread with good pictures of the 911R diffuser.
https://rennlist.com/forums/991-gt3-...iffuser-2.html

It’s different from most diffusers because it doesn’t extend all the way back to the bumper. Because it is closer to the wheels, despite being lower, it is less likely to get banged up over speed bumps, steep driveways, when reversing, etc. (Though one still needs to be careful. In the above thread, some have had bad experiences.)

It should be inexpensive to make because it is just some plastic fins, attached to a sufficently wide underbody plate. The optional Porsche motorsports plate which protects the oil pan is light (~2kg) and cheap (under $100 or close). Though to be as effective as the R diffuser, one would need a wider, and perhaps longer plate. As for the fins, the R’s are reportedly made of plastic, probably because they need to be flexible. These should also be light, as they are small. Indeed, Suncoast was apparently offering the OEM R diffuser for 991s and according to someone in the thread mentioned below, even this part wasn’t very expensive. (Alas, the link leads nowhere, so I didn’t find the price.)

The only thing that concerns me is whether, if the plate needed to be much wider and longer, enough heat could escape. I guess most of the areas that would not already be covered by the optional Porsche motorsports plate would be covered by any of the existing aftermarket diffusers. So if they don’t cause heat problems … . Also, it seems to me that if the R diffuser works with a 4 liter engine reving to 8500, it should work with our engines too.

Finally, as for downforce, on the R, together with a small spoiler (smaller than the Carrera’s?), it generates 35kg of rear wheel downforce at 186 mph (300 kmph). It may not seen like much, but it’s similar to a 996GT3, and that big wing pays off not just in lap times but also in high speed stability. Many have noted in this forum how effective aero mods result in an increase in stability that you can really feel.

So please, someone make one of these!

tekkie

i fail to understand how that adds downforce but obviously it does

making something like that out of stainless steel or aluminum for looks wouldnt be that hard but making it functional to add downforce would be another story

Rvca3112

For the sake on conversation, I hve the oil pan protective skid plate on my car. It mounts up to 3 holes that are open on the bottom of it. I think this could be easily done with the proper r&d. Even for a purely aesthetic view point, if I was to use a spacer between the shield and the bolt holes to lower the pan a little (my car is stock ride height, I don't really feel the plate is needed, but I bought almost every accessory I can get for the car) and added the fins to it it could do the job. This may be my winter project.

tekkie

Thats exactly what i was thinking, it should be easy to make the plate. The fins maybe a little more difficult depending on the angles but worst case welding them should be easy

Rvca3112

i was thinking of using some type of sheet metal rivets into the current lines already on the pan itself. We'll see.

tekkie

yeah that should be pretty easy by the look of it

johnireland

And what is going to be the performance improvement as you drive to the market and over the speed bumps? Even on the highway...even at 90 mph on the way to work...you are never going to need or get more downforce than you will get from the stock Carrera design. Is this just a cosmetic choice to make the car look "fast?"

rs10

No, not cosmetic. I kind of thought that would be clear from my initial post, but never mind. At high speeds, 996s with less lift or a bit of downforce feel much more stable, in a good way. The front end and steering feels much more tied down. This was made quite clear in a few threads I read on European forums, and while many strongly prefered the feeling with more aero, I don't recall anyone expressing the opposite view. (My experience is similar, though my experience from well tied down 996s at high autobahn speeds came from the first time I was considering buying one, many years ago, and I can't remember what kind of aero they did/didn't have. On the other hand, I can confirm that my car does not feel very tied down at autobahn speeds.)

Of couse, a diffuser won't help the front. But if one reduces lift at the front, by all accounts one must reduce it at the rear too, especially on a 911.

rs10

That depends where you drive. Mine has a few battle scars showing it's doing its job. The one time when I know it definitly hit something was crossing RR tracks with a shape clearly designed by someone who hated Porsches. Just as the rear of the car car was coming down from the first impact, the second one came ... . And my car was not lowered, though I do have 17s, which may permit a bit more upward and downward motion.

rs10

Unfortunately, I'm not sure it's so easy, not if you want it to work, anyway. The underbody plate on the R is longer and wider. It's curved up to the rear and the fins curve up with it. I believe (not sure) the fins curve slightly outward, which might caue an aero conflict with the stright indentations on our skid plates. And welding on metal fins, if that was the idea, seems like a bad idea. They'd quickly get banged up and be useless or worse.

I think what is needed is larger plate, possibly with additional mounting points, and flexible, impact resistant (and easily replaceable) fins.

The Radium King

if you look at a wing, in it's most basic form, it is flat on the bottom and curved on the top. given the curve vs flat, the upper surface is longer than the bottom. as the wing cuts through the air, the air that goes over the top has a further distance to go than the air that passes underneath. conservation of mass requires that any air that starts at the front at the same time has to get to the back at the same time. this means that air traveling over the top of the wing "stretches out" to cover the greater distance in the same time as the air passing underneath. this 'stretched' air is at a lower pressure than the air passing under the wing (pv=nrt from chem class). this pressure difference creates lift.

look at the 911 shape - flat bottom vs curved upper. the car is a wing and will start to lift as airflow over the form increases. you can offset this in two ways (1) create downforce (wings and canards that push down to offset the lifting effect) and (2) reduce this lift.

to reduce lift you can do several things:

- reduce the amount of air traveling under the car - front spoilers and splitters, side rocker panels
- kill the airflow over the top of the car - gurney flaps, etc.
- give the bottom of the car a reverse curve - make it less flat. this is what a diffuser does - it curves upwards as it exits the rear of the car and extends out the back to make the distance air travels under the car the same as the distance traveled over the top. if you loook at the 911R unit, it is designed to work with the muffler and rear bumper cover to form a continuous upwards-curving surface. it's one of the things folks don't consider when they install smaller aftermarket exhaust - they are impacting the aerodynamic characteristics of the car.

probably the simplest would be to take a piece of cardboard, create the shape you want and locate the mounting holes, buy a sheet of 1/8" aluminum off ebay (or wherever) and bring to a machine shop to cut for you. the one bend should be easy enough to make, but makes the fins difficult (you can buy universal fins, but nothing to fit a specific bend like that). you could make templates and cut out of lexan or rubber, mount with small 'L' brackets.

if we are talking the 996, however, porsche fcuked the car from the factory - understeer is designed into the system. look at the front three radiators - all of them vent down. the faster you go, the more air that is pushing down and lifting the front. not only does this lift the front, it introduces air under the car which makes the whole car lift.

pelican has a how-to on how to vent your two side rads into the wheel wells instead of downwards. note that this was done on the gt3 rs cars and introduced by porsche into all the 997 cars. 6 speed has a how-to on how to install gt2 ducts onto a car to vent the centre rad upwards.

808Bill

http://i.ebayimg.com/images/g/fQAAAO...Ze/s-l1600.jpg

9964runner

I don't think there's near enough room to build a tunnel that would generate enough of a venturi effect to create any amount downforce without a major rework of the back end. Thinking a total rework of the bumper, cats would have go in order to a curve up the top of the tunnel, and the fins would need to go tight with the ground to stop sucking in outside air.
If your truly concerned with reducing your cars lift, lower the ride height to reduce airflow underneath.

rs10

Unfortunately, unless everything I know about diffusers is wrong, that's purely cosmetic. It's completely different from normal working diffusers, as well as from the R diffuser.

rs10

Seems to me there's roughly as much room as under an R. With the cats there, it can still curve up, I believe (though this is from memory, I haven't looked under the car specifically to check this), and hopefully roughly as much as with the R. And the fins would not need to be any lower than the R's.