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Posted by: chris914 Jul 13 2006, 05:38 PM

I thought it would be fun to have the students were I work look at the Aerodynamics of my 914 and the 914 in general.

http://www.cassidy-online.com/porsche914/index.html

Chris.

Posted by: J P Stein Jul 13 2006, 05:52 PM

Jeeze, what was that, a Corvette club AX?
You didn't bring enuff HP to that thing.
I've never seen an AX course that open....but I'd like to

Posted by: nebreitling Jul 13 2006, 06:16 PM

THAT is interesting. those water-dye tests are remarkable! the water-dye test analagous to a 914 at what speed? do issues of scale change things? could you do some aero modelling with various spoilers and such available? so many questions...

Posted by: chris914 Jul 13 2006, 06:30 PM

Yes the PCASDR (San Diego) AX's are a good size. Most lap times are 1:20 - 1:50 seconds

Chris

Posted by: Brad Roberts Jul 13 2006, 06:33 PM

I knew several people who ran/run full composite belly pans front to rear under their 914's for AutoX. Specifically designed to work under low speed conditions. Absolutely amazing how much different the car handled. They won Parade one year with the setup in a SCCA GT2 914 (2.9 liter back then)

They now road race open wheeled cars in SCCA.

He knew his aero shizznit also!

Tell us how you can help us.


B

Posted by: Jeroen Jul 13 2006, 06:35 PM

excellent stuff!!!

the flow over the rear decklid looks even worse than I thought it would

any chance you can do another test with some kind of gurney flap at the end of the top?

Posted by: chris914 Jul 13 2006, 06:38 PM

Yes, scale is a big deal. That is why it was also done with computer molding using "Flow Works" as well. The Flow Works matched the Water Tunnel and Wind Tunnel at lower speeds. Flow Works was used for the higher speeds. Also the yarn tests showed the same results which was really nice.

They also did several rear spoilers that I bought from Ebay.

Chris.

Posted by: Dave_Darling Jul 13 2006, 07:15 PM

I was under the impression that the belly pans were intended for the Big Track, i.e. high speeds. Been a long time since I talked to him, though... I wonder whatever happened to his 914?

--DD

Posted by: URY914 Jul 13 2006, 07:42 PM

Damnit Brad
Now I gotta build a fuching belly pan. I'll NEVER be done with this car.

Paul

Posted by: drew365 Jul 13 2006, 08:04 PM

Your car will be like the Winchester House, you'll never die as long as you keep modifying it.

Posted by: URY914 Jul 13 2006, 08:06 PM

That's what I like about autocrossing.....so few rules to go by.

Posted by: Brett W Jul 13 2006, 08:38 PM

It would be interesting to see what happens to a 914 when you add GT flares.

You need to find the article published a couple of years ago in Racetech Magazine. They covered the FSAE car built by Missouri Rolla. They spent time in the Ford wind tunnel. According to the article they saw a 35% improvement in aero effectiveness below 45mph.

Posted by: Sammy Jul 13 2006, 08:38 PM

LOL, aero doan work iffn you don't get out of second gear. Kinda like putting a huge wing on the back of a front wheel drive honda civic that only goes 90 mph.
On a real track it matters, on AX it don't.

Posted by: turboman808 Jul 13 2006, 08:45 PM

Yeah I was curious about the same thing. Lots of guys in the padock are playing with vortex generators on the roof. They are pretty darn ugly though. Very curious what a gurney flap would do. And for that matter the vortex generators.

Gurney would be cool if it worked though.

Wish I knew more about aerodynamics

Posted by: MattR Jul 13 2006, 08:48 PM

Yes you can scale aerodynamics. You're looking at the Reynolds number similarity. You must minimize compressibility effects when choosing a new medium too. Its a simple calculation, but obviously much more difficult to effectively analyze.

Basically you assume the reynolds number (inertial over viscous forces) of the model equals the reynolds number of the actual. You'll also need to use ideal gas laws to include pressure.

Posted by: nebreitling Jul 13 2006, 10:22 PM

mostly true -- but the FSAE, Amod, and other guys have proven that there is some benefit, as Brett pointed out. given a light weight/purpose built AX machine that would compete nationally, it's worth investigating.

then again, i've driven 90mph AX courses....

Posted by: MattR Jul 13 2006, 10:32 PM

I went to this year's FSAE West event at Cal Speedway. There were a few teams competing with full aero and it just looked like it slowed them down. Whatever aerodynamic grip they got was negated by the speed they lost trying to manouver around the cones with 2 feet of wing sticking out on each side. But FSAE is a totally different beast.

Posted by: chris914 Jul 13 2006, 10:47 PM

There are still limits based on the tunnel that the model is in. That is why the students used "Flow Works" for all the different speeds. The water tunnel with the laser doppler and the wind tunnel with the pressure rake was used to back up the results for the computer modeling.

The dye and the yarn are more "old school" for the visual effects.

Chris.

Posted by: Air_Cooled_Nut Jul 13 2006, 11:29 PM

Very cool and, yes, so many questions!

What difference does the air intake for induction and cooling, as well as the exit of both said airflows, make towards the aerodynamics in testing (where those effects aren't being created) vs. real world?

And shouldn't the side/underbody flows be tested with a "road surface" since underbody dynamics will affect airflow?

Still, really neat stuff!

Posted by: ChrisNPDrider Jul 14 2006, 12:08 AM

Nice poster, they get an A. I was a little disappointed to read the Conclusion, because I know that if it is a nice sunny day I will drive with the top off and the windows down even if it is a 5-6 hp loss (estimate). Although, some have said that having the top removed can help because it saves weight, then roll down the windows and it WILL lower the center of gravity. Good for autocross!

Posted by: Brett W Jul 14 2006, 05:34 AM

According to Racecar Engineering, Missouri recorded an 800% improvement in aero downforce at 40mph with front and rear wings. They recorded a 1.6g skidpad at competetion. Their windtunnel test showed aero gains as low as 20 mph. They are using triple stage wings with angles of attack in the 30-40degree range.

Posted by: chris914 Jul 14 2006, 10:09 AM

Was that on a 914?

Posted by: Sammy Jul 14 2006, 10:37 AM

I could build all the downforce in the world into a car, but the drag and extra weight would make it a tank.

To get any appreciable downforce at 40 mph the drag from the wings would have to be huge, the benefit is outweighed by the cost.
This thread was about reducing drag at an AX, big wings don't accomplish that.

Posted by: URY914 Jul 14 2006, 11:14 AM

Downforce
Drag
Downforce
Drag

Aero balance

Posted by: davidginsberg Oct 29 2008, 06:31 PM

Does anyone have any info or more pics of this car? I imagine the hatch was a custom thing, but man... if there is one thing the 914 could really benifit from at high speed it would be this---- as indicated by the test results!