911R style diffuser for 996s and 997s?

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!

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

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.

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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

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

yeah that should be pretty easy by the look of it

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?"

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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.

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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.

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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.

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.

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

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.

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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.

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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.

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A chin spoiler is the clear easy win for the front, but I'm looking for a solution for the back. By all accounts, Porsche has been careful to more or less balance front and rear lift downforce for good reason, so I'd like to do the same. And for various reasons, touched on above, I'm not sure a wing or just a ducktail would be ideal for me.

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Interesting. Venting downwards is generally bad, but then, so is more air entering the wheel wells. The 911R got huge aero benefits by adding wheel well vents to get the air out. And I has heard this was done on GT3s, but I thought it was for brake cooling. Are you sure it helps the GT3s with lift and/or drag? Or has this mod been well tested on Carreras?

pressure analysis of wheel wells shows highest pressure in the upper front quadrant of a wheel well. if you look at where the dp cars, sport racers (and yes, 991 gt3) vent their fenders, this is where it happens. a cool thing if you can do it, as it converts lift to down force. if the air isn't vented then it exits the fender in around that area - look at how water exits your fender when driving down the hwy in the rain. out the side is better than down and underneath. the gt3 rs guys took holesaws to their fender liners to vent the rads into the wheel wells, they called it brake cooling because most regs don't allow aero mods but they do allow cooling mods. the gt3 rsr completely re-did the rad layout to vent out the side instead of downwards. the gt2 and i think the turbos, as well as the 9x7 cars, all vent to the wheel wells. the third radiator venting upwards showed up on the gt3 rs and the gt2. i can't recall where i got the info from, but the excellent handling of the gt3 rs was attibuted in large part to this upwards venting of the third rad. again, now it is pretty common on the porsche track-oriented cars. they don't do it to their street cars as it takes away from the understeer engineered into the chassis (porsche have been afraid of oversteer since the 930 litigation days).

I could use more down force, I often drive 70+ on the highway

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Thanks for the very helpful info! One interesting note about the third radiator venting upwards. I thought so too re. the RS's handling. But according to SportAuto, this only reduces lift by 3kg at 200 km/h. Still, that may in fact have been a significant benefit, as otherwise it would have almost as much front lift as a Carrera due to the huge rear wing pushing the front up.

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This mod is something I'd definitely like to know more about, and I don't want to highjack the thread (even if I started it), so I started a new one: https://rennlist.com/forums/996-foru...l#post13847239

Seems I was right that it doesn't cost much to make the 991R diffuser. Suncoast is selling them for 991 GT3s for just $560. Here's a link with good pictures: http://www.suncoastparts.com/product/SKUGT3DIFF.html Surely there's a market for something like that for our cars!

Man, I would have probably destroyed that thing last track season. I have the skid plate below my 3/4qt deep oil pan and its got a million nicks and scrapes.

I ordered one to try to fit to my 996. I don't have have a deep sump but, I do plan to run one.

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You ordered a 991R diffuser to fit on a 996? If so, please keep us posted!

Just received it yesterday and this thing is very stout! It's a lot more rigid than I had expected. Going to look how I'm going to go about mounting it this weekend.

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Thanks for the keeping us posted! If I may ask a few questions to follow up on your initial report: When you say it's rigid, is that just the undertray, or the fins too? And if the fins, do they bend at all? And as for stout, do you mean heavy? In any case, it would de great it you could weigh it and let us know! Thanks again!

One good speed bump and it's done...

Both. It's as thick as the oem 996 sump pan guard Porsche offers. I believe the 911r metal is steel. I never magnet tested it but, guessing by the weight. Both the fins and undertray are stout/thick. The tray is metal and the fins a think hard plastic. The fins do not bend. I used the oem 996 sump guard to mount the 911r diffuser. In the picture I have it hard mounted. Will prob add more mounting points soon but, need to get my gas filled for my welder to weld brackets/mounting tabs for the other hole locations on the 911r diffuser. The rear bumper is a demon speed gt3 rep. The deck lid is a 996 cup lid with 996 uprights trimmed and welded to fit a 991 gt3 wing. In the front I'm running a 996.2 gt3 front bumper fitted for a 996.1 with a joe toth splitter. Going to add his fender foils to help eliminate the trapped air in the wheel wells. 83E8DDB6-95E0-498B-B58C-BF16148ECC75_zps9wgntafj.jpg 59EBA085-0FEA-48D7-8203-1BB0F60A8678_zpspuaayebh.jpg Ps don't worry she's going to the paint shop to have the front paint matched better, rear bumper painted and a solid roof skin put on.

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So you attached it to the sump pan guard? Or you just used the sump pan guard mounting points? And any rough idea how much it weighs? Sounds like at least several times more than the sump pan guard, no? (Which I think weighs 2 kg, but I seem to recall it's aluminum.)

Wow, that actually looks much better than expected. What mounting points were you able to use? The sump pan guard weighs next to nothing. It is aluminum.

This has got me thinking, i built a front splitter for my Sti before i sold it to get the 911, didn't get a chance to build a rear diffuser, looks like it could be a fun project

Yes. I mounted it straight the pan guard. Not the pan guard mount points. The pan gave it a more rigid base to mount to. The weight all together is prob in the ballpark of 5lbs but I never weigh anything.

Thanks! So have you already driven it with the diffuser? It will be very interesting to hear how it works out! (And indeed, to see how it works out - if you have a chance to post some pictures when the car is back on the ground.)

D1320B35-8B5A-403B-AF8E-D1578ACCD10D_zpsvxf2fesd.jpg 5CC478C9-7FB9-4357-9A57-6BFD27F99C76_zpscwskidhx.jpg

It's hard to picture it with it being black.

Thanks for the photos! Looks like it extends further to the back of the car than on a 991R (not surprisingly, as the 991 is longer). So based on my limited understanding of such things, I'm betting it will work pretty well. On the other hand, while it's hard to see how close it is to the ground from those angles, I'm guessing this is not something that would get along well with speed bumps. Then again, is your car lowered? Anyway, if you have a chance to post any side view pictures, that would be great. Thanks again!

I would say my car is very low. Backing out of my driveway I only hit my front lip. Doesn't matter what angle I hit it at that front lip scrapes. Diffuser doesn't hit. Pulling it my driveway nothing hits. I do have kwv3 coilovers. With how stiff they are there is very minimal strut/spring movement. If I were on something that moved easier I'd probably scrape. I've had the front splitter on long before the rear diffuser with no issues going over speed bumps. Although I try to avoid them at all cost bc I hate going over them whether I'm in a SUV or my pcars. My cayman on h&r springs hits its rear diffuser made by joe toth a lot easier than my 996. Again though I contribute that to the suspension setup. Also you can really only position the diffuser a certain way due to the exhaust manifold cut outs that are on the side of the diffuser. I also noticed suncoast removed the diffuser again after I ordered mine. My understanding is Porsche isn't really wanting to sell these to people unless you own a R. So when it was relisted on suncoast I jumped on it. Hopefully Porsche will lighten up and release these permantly for sale.

hmmm ... http://www.6speedonline.com/forums/a...-diffuser.html

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