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Posted by: astronomerdave Nov 6 2009, 01:21 PM

Does anyone know where I can find this information (for '72 914/4) ? . . .

• drag coefficient
• vehicle height, width
• gear ratios

Thanks,
--Dave

Posted by: Elliot Cannon Nov 6 2009, 01:36 PM

A number of years ago, someone did some wind-tunnel testing on a 914 to determine drag coefficient. Anyone remember who that was?

Posted by: SirAndy Nov 6 2009, 01:41 PM

I don't have the data handy (i'm at work) but the 914 CD has been posted here before.

If i recall correctly, it's slightly better than a 911 turbo ...
Andy

Posted by: SirAndy Nov 6 2009, 01:44 PM

The search works!

"+drag +coefficient" found this old post:

http://www.914world.com/bbs2/index.php?showtopic=51962&hl=drag++coefficient

0.38 for the 914-4
0.39 for the 911 turbo

Andy

Posted by: flipb Nov 6 2009, 01:45 PM

From http://books.google.com/books?id=CZoH7ldJr-IC&lpg=PA28&ots=-TcpvAsctA&dq=Porsche%20914%20%22Wind%20tunnel%22&pg=PA29#v=onepage&q=Porsche%20914%20%22Wind%20tunnel%22&f=false:


"It is interesting to note that, aerodynamically, the 914 displayed 20 percent less drag than the contemporary 911."

Posted by: SirAndy Nov 6 2009, 01:54 PM

Btw. that book has some pretty substantial errors.

But the crash pics are cool!
Andy

Posted by: marks914 Nov 6 2009, 02:03 PM

Cd is only a number, what is more important is CdA
That is what really tells you how effective the aero is.
Mark

Posted by: johannes Nov 6 2009, 02:25 PM

Yes, and the 914 is not bad at this because of the small surface.

Posted by: astronomerdave Nov 6 2009, 02:33 PM

DOH! In my haste I forgot the "+" and searching on "drag coefficient" by itself gave me mostly nonsense.
thanks,
--Dave

Posted by: ericread Nov 6 2009, 04:16 PM

Here's two .pdf documents that should be of help...

 2007_Drag_Coefficient_Poster.pdf ( 179.71k ) Number of downloads: 439


 2008_Drag_Coefficient_Poster.pdf ( 276.51k ) Number of downloads: 335


Eric Read

Posted by: astronomerdave Nov 6 2009, 04:38 PM

Cool. That first poster is hard to read some of the numbers but I found the original source for it. Check it out here: http://www.cassidy-online.com/porsche914/aerodynamic_aids/index.html

Posted by: ericread Nov 6 2009, 04:40 PM

Thank you for posting the original source. I downloaded it but I couldn't remember from where I downloaded it.

I hope it helps!

Eric

Posted by: ewdysar Jun 20 2010, 11:41 AM

I hate to resurrect and old thread, but after more than an hour of searching, it looks like this info just doesn't exist.

Does anyone know the CdA of a 914, or alternatively the frontal area of a 914 so that I can do the math?

Thanks,
Eric

Posted by: Al Meredith Jun 20 2010, 12:09 PM

Porsche Panorama May 1978 page 27 figure 14. I can scan if you all want me to but, the numbers are as follows:
Lamps closed , windows closed ,roof on Cd= .363
lamps open, windows closed, roof on Cd= .380
lamps closed, windows open, roof on Cd= .381
lamps closed, windows closed, roof off Cd= .389
lamps closed, windows open, roof off Cd= .447
lamps open, windows open, foor off Cd= .464

Posted by: ewdysar Jun 20 2010, 02:43 PM

Thanks for the info, but I'm looking for the CdA, not the Cd. The various permutations of the Cd are easily found. As Marks914 aptly stated "Cd is only a number, what is more important is CdA. That is what really tells you how effective the aero is."

The CdA is merely the frontal area of a car (object) multiplied by its Cd. I have yet to find either the frontal area or the CdA.

The Cd of a 911 (0.32) is less than a 914, but the 914 is reported to be "slipperier" due to its lessened frontal area.

Thanks again.

Posted by: jmill Jun 20 2010, 03:58 PM

Attached image(s)

Posted by: jcb29 Jun 20 2010, 08:56 PM

Try this site:

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

Posted by: ewdysar Jun 20 2010, 09:44 PM

Yep, I've read that site and all of its attachments is great detail. There are many figures provided with a lot of relative differences between various configurations of a 914. However, while the formulas included reference frontal area, there are no values for area provided.

So I'm still looking... does anyone know the CdA or the frontal area of a 914?

Thanks, Eric

Posted by: type47 Jun 21 2010, 07:58 AM

according to the technical data in the workshop manual, the height of the 914 is 1.23 m and the width is 1.65 m so I would think you could get a pretty good estimate of the frontal area if you assume it's a rectangle so you would get 2.03 sq m as the frontal area

Posted by: ewdysar Jun 21 2010, 09:51 AM

Unfortunately, this looks like a significant overestimation. As an example, an '86 911 Carrerra is spec'ed at 1.821 sq m (19.6 sq ft). Even the 928's were right around 1.95 sq m (21 sq ft).

Still looking for the answer....

Thanks,
Eric

Posted by: sean_v8_914 Jun 21 2010, 10:02 AM

send chris an email

Posted by: SirAndy Oct 7 2010, 02:35 PM

Any update on this? I just downloaded a dyno app for my phone to get me some HP/Torque estimates and 0-60 times.

One of the inputs needed to set it up is the frontal area of the car ...

Posted by: jd74914 Oct 7 2010, 03:05 PM

Why don't you take a picture, print it on graph paper, count the squares, and then figure out the apsect ratio of the squares? I'm not sure how else you would calculate something like that without building a full CAD model and taking a crosssection.

Posted by: nathansnathan Oct 7 2010, 03:22 PM

I just google searched "porsche 914 cda" and found a thread on 914club where a guy , 'Hydra' says

"A stock 914 with the top up has a Cd of 0.36 and a frontal area of 1.6m2, giving us a CdA of 0.58"

This, he says is from the book "Aerodynamik des Automobils, by Wolf Heinrich-Hucho, c. 1986".

http://www.914club.com/bbs2/index.php?show...mp;#entry905152

hope that helps

Posted by: johannes Oct 8 2010, 06:02 AM

I think you can go with 1,6 square meter that seems to be very close to reality ... I made my own experiment with Sketchup and found a figure that is very close to 1,6 m2

Posted by: Richard Casto Oct 8 2010, 06:04 AM

I know very little about Cd CdA values, but I agree that you could do this via digital photo. Use a quality lens (to avoid distortions), get back a bit and zoom in to flatten the perspective, include a reference measurement (such as a yard stick, etc.) in front of the car. Take the photo and overlay a digital grid over the image that matches up to your reference measurement and count squares. The more granular the grid, the more accurate your results. If you want to go crazy, count pixels!

I think there are computer programs that will measure the area of an irregular shape in an image file. I think Adobe even has one. There might be cheaper or free software that does that.

If you want to do it manually, I can see getting a few large poster boards, marking them out with a grid, put them on the back of your garage, use a laser pointer attached to something to keep it perpendicular to the car and then trace the outline of the car. You would need a helper to trace the "shadow" you create with the laser pointer as you move it around the outline of the car on the poster boards. When done, count squares. If you park the car between to parallel walls, with the car perfectly perpendicular to the walls, you should be able to make this work. You could create a jig that holds the pointer perpendicular to a wall and then just slide the jig around on the wall as you trace the outline.

Posted by: johannes Oct 8 2010, 06:06 AM

Closeup

Posted by: Richard Casto Oct 8 2010, 06:06 AM

You beat me to the punch! I had no idea Sketchup did that and it's free!

Posted by: johannes Oct 8 2010, 06:08 AM

For your information a second mirror ads 0,014 m2 to the surface ...

...

Posted by: johannes Oct 8 2010, 06:19 AM

Yes Sketchup does this... This software is mostly focused on architecture and architects need surface figures.

Your method with the squares is way to complicated. Usualy, you decompose the shape in basic geometry like rectangles, triangles, disc and you can approach the surface very quikly.

Posted by: Richard Casto Oct 8 2010, 06:39 AM

Sorry, yes, I assumed whoever would do this would realize that you do not need to count every granular grid square individually (1, 2, 3...). Large areas can be resolved quickly by converting into shapes. As you say, boxes, triangles and portions of circles alone would get you pretty close. Clearly its the area around the edges that require more attention and how close your shapes matches defines your accuracy.

And clearly you have shown a cheap way to do this via computer. This assumes your image you are starting with is accurate. I have seen that image before and I assume it is sourced from a Porsche document of some sort. The question is if that is an accurate rendering, or an artists rendering that just happens to look right. If I was taking the time to do this, I would create an accurate image via photo just to be sure. That also lets you handle different car configurations (GT flares, wider tires, different air dams, etc.)

It should be relatively trivial to create an outline of the car via a photo. I have used Sketchup very little, so I can't say if it will, but I have used other tools to create vector diagrams by tracing an outline of a photo which could then be imported into Sketchup.

Posted by: johannes Oct 8 2010, 06:40 AM

Sorry for the hijack but speaking of Sketchup, I started a project that I won't finish for sure ...

Posted by: johannes Oct 8 2010, 06:45 AM

This comes from a Porsche document and should be almost accurate. Those were made using technical drawings.

Posted by: 1968Cayman Oct 8 2010, 10:07 PM

Never even heard about Sketchup. This is pretty neat- going to be a long night. I hate you.

Posted by: Elliot Cannon Oct 8 2010, 11:53 PM

I know for sure the car is a lot more aerodynamic with the roof on and the windows up. In that configuration I get 26 MPG. With the roof off and windows down, I get 23 MPG.
Cheers, Elliot

Posted by: jcb29 Oct 9 2010, 01:18 PM

"Johannes" - Would you mind sharing your Sketchup model of the 914? I'm an architect with way too much time on my hands and a good working knowledge of Sketchup. Would like to take a shot at finishing you model. Send me a PM for email address.