[flash]

for those of you who were curious as to what it would take to go how fast, here is a spreadsheet for you that calculates it pretty darned accurately - i fiddled with a few things, and did some measuring to get some input figures, and it came out almost exactly matching 2 other programs i have



this does, of course, presume that you have enough corrected downforce for the increased lift factors at increasing speeds, which is always a problem, and requires a lot more sophisticated modeling software to accurately determine - we all know that at extremely high speeds much bigger wings and things are needed - what most people don't know is how poorly the rear of the underside of the 968 is designed, relative to even the 951, which had additional underbody items to help things - this factors in heavily in power requirements, as the drag there holds back the car, and also adds to rear lift - the wings to correct it also add drag, so the power requirements will go up beyond what the spreadsheet says at extremely high speeds



but it's fun to play with anyway, and gets you pretty darned close in a perfect world - you can see clearly that it takes a LOT of power to get up there

aero1-968.xls Size: 32.5 KB  Downloads: 37

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[bombfactory]

Another project to add to the list Flash: Bolt on rear end underbody air-smoothing tray. Surely there's a solution that looks better than the 951's. In fact, on our cars it should be completely out of sight.



-Austin

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[flash]

lol - already on it

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[moses]

Even some 924's had a rear undertray, despite not being about to go fast enough to much need one. I never learned why Porsche deleted it. I'm interested in aerodynamic improvements to increase highway gas mileage (don't bother commenting, it's an illness).

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[flash]

lol - not at all - i am working on doing the same thing right now for the denali



as for the 968, i have a couple of designs in mind, but i haven't made up my mind on which way to go yet - it sort of depends on what i end up doing with my exhaust

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[Lim Yong]

Flash,

Maybe a two piece tray so it can be customized for people with different mufflers. I think I can cut

to fit my exhaust which is a small straight through in the back.

...Luck

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[flash]

lol - i know just how i would do that too



i just have to do some "tunnel tests" to make sure i'm trimming air and not piling it up - i have the ability to do that right here too, so not a big deal

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[rxter]

Improved mileage in the Denali - seriously. You could try putting a block of wood under the accelerator...

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[DaveN]

I can already imagine you, a hair dryer, a large paper towel roll and a scale 968 - LA freeway exit "Bob's Experimental Wind Tunnel" next left <img src="/forum/images/smilies/968/laugh.gif" class="smilie" alt="" />

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[flash]

funny stuff



seriously though, it's not hard to create a device that allows you to see airflow pathways - all you need is a fan and a smoke generator



re denali: there are no underbody trays at all - it's a big uneven surface cavern under there that just kills fuel economy

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[ds968]

bah, the chart is useless; it only goes to 190 mph. <img src="/forum/images/smilies/968/tongue.gif" class="smilie" alt="" /> need to see what it would take to reach a respectable top speed <img src="/forum/images/smilies/968/wink.gif" class="smilie" alt="" />



kidding aside, I'm definitely a candidate for the underbody tray project.. could care less about mpg efficiency, my only interest is eeking another few additional mph at the top end and not compromise stability one bit

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[Nikoror]

If you want to do some modeling don't forget that things scale with Reynolds number, so if you are using 1/5 model you need 5 times the velocity, so for 120mph test you need 600mph surrounding air velocity (assuming all the experiments are done in air). Good luck with that <img src="/forum/images/smilies/968/tongue.gif" class="smilie" alt="" />



Oh, in addition, a very common mistake is to think that Cd (coefficient of drag) and frontal area are independent and reducing frontal area automatically reduces drag. WRONG! Frontal area is just a reference projection that is used (just like wing area is used for airplanes) and it has no particular physical significance. Changing frontal area can (and does) change Cd, so an increase in frontal area (adding a front air dam) can decrease drag (Cd goes down) and a decrease in frontal area (removing fender flares) CAN INCREASE drag (increases Cd). Of course, there are some modifications that seem more physically intuitive where adding or removing frontal area has the expected outcome, but don't think that Cd stays constant! The only thing that matters is the product ( Cd*Frontal area) for EACH particular test case <img src="/forum/images/smilies/968/smile.gif" class="smilie" alt="" />

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[craigawoodman]

I too would be interested in something that cleans up the airflow under the rear of the car - diffuser anyone?

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[tamathumper]

http://www.968forums.com/topic/10819-944..._p__106741

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[flash]

it is amazing to see how small changes in frontal area, profile, and contour change Cd - it can go up or go down, depending on what change is made - this is tricky stuff to be sure, and not something to be taken lightly - i've seen quite few guys on the track actually lose speed by installing the wrong front "aero" unit



also, increasing downforce usually means speed loss, but higher speeds mean need for more downforce, so that one is also a very tricky one



not sure if i will ever get to the rear diffuser or not - i have a design that should work well, but i may dump it in favor of dual exhaust

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[Lim Yong]

Can you show what your design looks like??

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[flash]

not yet - not sure until i make it and then test it - won't do that at all if i go dual exhaust



it would involve significant permanent modifications to the car though, including some welding and the need to repaint the rear bumper cover

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