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Posted by: swatteam2 May 17 2006, 12:44 AM

Hey Everybody,

My name is Mike and doing an aerodynamics research project at the University of California, San Diego on an advisor's 1972 Porsche 914, involving CFD flow analysis, 1/18 scale model testing in the water tunnel with various aero configurations, as well as full-size coastdown drag testing with those same configurations, etc. My question is - are there any published values for the drag coefficient of this car? I've done a forum search and seen some estimates of .45, although the number seems to have been contested by others. Any help on this would be much appreciated! Thanks,

-Mike

Posted by: Brad Roberts May 17 2006, 12:57 AM

COOL. Can I come and watch? I live 10min from the school.

Andy has the "Das Grosse" book with all the factory info in German. Let me call him real quick.

Posted by: So.Cal.914 May 17 2006, 12:58 AM

These are static tests? Reason I ask is my car will drop in the front about 1 1/4" to 1 1/2" at about 90 to 100 mph. I believe this would change those figures. Others have their cars even lower to start with than mine.

Observation.

Posted by: Brad Roberts May 17 2006, 12:59 AM

72 914: 0.37 73 is 0.38


B

Posted by: SirAndy May 17 2006, 01:00 AM

brad is correct ...


Andy


Attached image(s)

Posted by: Joe Ricard May 17 2006, 05:08 AM

And I imagine any of the other stuff that gets added will drastically effect Cd.
I know the cow catcher air dam and little flippy tail fin slowed the car down quite a bit on top end. Like 10 MPH
I normally run the early wrap under valance and slick trunk lid. windows up.

Posted by: type47 May 17 2006, 05:38 AM

i've seen a C chart with different values for lights up, top off, windows up, etc.

Posted by: jd66921 May 17 2006, 09:13 AM

 914_Cd.htm ( 762bytes ) Number of downloads: 181

Posted by: URY914 May 17 2006, 09:18 AM

Must be the tits on the front bumper? I wouldn't think they would make that much diff.

Posted by: ChrisNPDrider May 17 2006, 10:16 AM

Mike- cool project!
No doubt aero efficiency is a major player in gas milleage and top speed, 2 things we all want to improve. If you have any wiggle room in testing the areo efficiency of different 914 mods, I think we would all be interested in more info on the effects of GT flares, rear spoiler/ducktail, stock front air dam vs. LE air dam, lifting the back of the roof, and maybe smoothing out the area under the exhaust/muffler.

Try the "Search" feature in the upper right of the page - lots of 914 mods out there. One really good thread on aero improvements could probably be found by searching "lift and roof"...there are some vortex generator tabs that were attached tot he rear edge of the roof that MIGHT be making a signifcant improvement.

Posted by: MattR May 17 2006, 10:21 AM

Are you in FSAE down at UCSD?

Posted by: Joe Ricard May 17 2006, 10:47 AM

What ever happened to the guy with air tabs on his race car? Was there any benefit from it?

Posted by: SirAndy May 17 2006, 10:54 AM

well, even the 916, the brick with the trashcans hanging off the sides (GT Flares), is still better than the 911 Turbo ...

pretty good for a shoebox with wheels ...
Andy

Posted by: swatteam2 May 17 2006, 01:48 PM

Hey, thanks everybody for your responses! For our project, we'll be testing various combinations of the following modifications (both scale model and full-size) - top on/off, windows up/down, an air dam, and 2 sizes of rear spoilers. Unfortunately since our project timespan is only 10 weeks, we'll have to restrict our tests to these modifications and probably won't be able to look into other things like fender flares, etc.

Basically we'll be doing comparative drag testing in the water tunnel (already have previously collected data on lift-coefficient effects of these mods). The water tunnel has a flow velocity limited to about 2 m/s which means that for a 1/18 scale model, we'll only be able to achieve a scaled speed of a few miles per hour. The published values that you have given me will help a lot in seeing if testing at such scaled speeds will still yield reasonable estimates of Cd.

We'll also be doing flow-visualization in the water tunnel using dye lines to validate the CFD flow analysis results and observe any salient aero effects of the various mods.

As far as real car testing, we'll probably head over to the scripp's ranch area (some nice flat stretches of road) for roll down testing to see if the drag trends observed in CFD and water tunnel testing for the various configurations are reproduced in real life. We most likely won't be able to get accurate estimates of the true Cd in this method since it would be difficult to determine the rolling resistance of the car at the speeds tested, but we should at least be able to get comparative values. Any ideas on how we could accurately estimate the rolling resistance at around 70 or 80 mph would be welcome! Thanks again for all your help, I'll keep in touch!

-Mike

Posted by: Joe Ricard May 17 2006, 02:07 PM

Alot of variables in rolling resistance too.
Tire width and compound. Air pressure in tires
Bearing adjustment.
Brake rotor trueness (pads can drag on warped rotors). Speacially on the back.

Posted by: TonyAKAVW May 17 2006, 02:45 PM

Go Tritons!

-Tony (EE c/o 2000)

Posted by: newto914s May 17 2006, 04:05 PM

You ever watch "Baywatch." tits slow thinks down a lot
But I'm not complaning

Posted by: Al Meredith May 18 2006, 06:03 AM

I knew I had this old PANO took a while to find it. I have no way to scan the article , but if someone goes to Panorama May 1978 there is a great article about Weissach Engineering. There are pages of graphs showing everything from "lift at ther rear wheels at differing speeds with and without spoilers : to: CD of a 914 in all configurations. As stated before .363 closed up, .380 lamps up, .381 lamps down windows open, .389 lamps down windows up and roof off, .447 windows open and roof off lights down, .464 all dirtyed up (windows open roof off and lights up.

If someone has May 78 please scan ....good info Al

Posted by: ryap Oct 11 2006, 04:41 PM

All the drag coefficient info is nice to have, but it doesn't do you any good unless you know the reference area! Anybody know what that is? I assume it is the projected frontal area. Difficult to measure without a planometer. My rough guess is 23 sq. ft.

Posted by: Borderline Oct 11 2006, 05:27 PM

the guy who had the vortex generators was Greg (race914) Braun. I met him at Laguna Seca a few weeks ago. He told me that he really likes the vortex generators. He liked them so much he put a bunch on the back of his trailer to improve gas mileage!!

Posted by: SGB Oct 11 2006, 07:21 PM

Andy's hair?

Posted by: marks914 Oct 12 2006, 09:31 AM

I have been trying to get my car into our full size tunnel here at work but it has been busy.
Its been running 24/7 for a few years now, suprisingly, the race teams take up as much time as the production vehicles.

When I do get a chance, I will let everyone know. Probably won't be able to get too much data based on how many sensors we can use, but it would be cool!
I think our tunnel gets up to 175 MPH

Mark

Posted by: bd1308 Oct 12 2006, 02:01 PM

that should be good enough for David Willouby, who is planning a 200MPH 914 with a v8 in it. His 350 is FAST though