Study (model) of various aeromods affecting a Porsche 914
 

I found a web-page people may be interested in. Way at the bottom are the results of several CAD flow model tests and the corresponding effects to the vehicle's performance. I am personally more concerned with the Cd reduction and Horsepower Gained (or less horsepower used) than the down force, but interesting none the less.
http://www.cassidy-online.com/porsch...pare_chart.jpg

Hey Wyatt - good find!

Hope you don't mind I moved it into its own thread. Definitely a threadworthy post.

I don't understand the difference between "rear wheels blocked" and "rear wheels blocked flush with car", but there's a significant difference between the two.

yes I means what its says
 

I frequent this web site often, it means when you cover the wheels make it smooth, dont interfer the the vehicles lines, but the most interesting thing is when covering the rear wheels, notice the rear window he has two different shapes and notchback and a fastback shape, the 914 uses less HP with the notchback shape then with fastback.........

Thanks! Definitely don't mind, the reason I posted it was so it would get seen. I noticed that between the "notchback and fastback" shape says that one has rear wheels blocked, and the other says rear wheels blocked flush with car. Even though the pictures look like they have identical wheel blocks, I wonder if that's how it was run. Either way, we can get a good idea in CAD, but we have to prove in the real world.

Perhaps the non flush test was to simulate a flared situation comparable to a 914-6 body fender flare.

I take it you mean the "GT fender flare", as the 914-6 had the same fender lines as the four-cylinder 914s.

If you're curious about the details, there is an email link at the bottom of the page with the results. I'm sure he'd be happy to talk about them with you.

It would be prudent to write him anyway and ask if you can post the image here; he very explicitly states that you must have permission to use the data on the page...

-soD

Good score! !Muchas gracias!

Yes, thanks, the GT Flares. Hard to believe that a stock 914 weighed almost 2900 lbs.!

It didn't. The early four-cylinder cars weighed closer to 1900 lbs., the six-cylinders about 2100 lbs, and the later four-cylinders about 2200 lbs. Not sure where the 2900 lbs figure comes from--the current Boxster is right around there...

-soD

You know, being such a gearhead, I shouldn't have just accepted the stats below even if it is from a PCA page. I knew better. V

http://vista.pca.org/stl/index.htm
Specifications for the 1970-76 914-4

Engine:

Horizontally opposed flat-4 cylinder, mid-mounted 1.7, 1.8, or 2.0L engine
Bore and Stroke: 90x66mm; 93x66mm; 94x71mm
Displacement:1.7L (1679cc); 1.8L (1795cc); 2.0L (1971cc)
Horsepower: 80 (1.7L), 79 (1.8L), 95 (2.0L)*
Compression ratio: 8.2:1 ; 7.3:1 ; 7.6:1
Cd:
Fuel system:
Drivetrain:

5 speed manual transmission



Suspension:

Independent front with lower control arms, spring struts, anti-roll bar
Independent rear, with semi-trailing arms, transverse torsion bars, anti-roll bar



Dimensions:

Curb weight: 2,892 lbs
Wheelbase: 94.5"
Length: 170.9"
Width: 68.3"
Height: 50.2"
Track front/rear: 58.2"/57.1"
Ground clearance: 4.9"

Hi,

My name is Chris and the Aero stuff here is from my web page. It's interesting to see it has other interest besides my Autocross racing.

2,892 lbs is the gross vehicle weight. The curb weight is 1982 lbs.

I have two 914s.

My 1972 914 that is setup for Autocross use weights less then 1982 lbs.

Porsche the Ultimate Guide: The ... - Google Book Search

My stock 1974 EFI 2.0L gets +30 mpg.

Good luck with your pursuits, Chris

Great post!! I am surprised adding a front air dam(front spoiler) caused a loss in HP!!

It causes a loss in HP because it adds additional cross sectional area. I am not sure, but it looks like the model doesn't use a very highly contoured underside, and may not have used a moving road surface or spinning tires either. The reason people put the front air dams on their cars is to prevent air from traveling under the cars "aerodynamically dirty" underside. If you have a smooth underbelly, an air dam is just adding area, but for the those that don't, it actually helps.

That makes a lot of sense.

Hi All,

Remember the Porshe 914 is built kidna like the original VW Bettle chasis. So, its already flat along the underside, mostly. So, its not supprising that the air-dam caused a drag increase. One of my brother's friends had one, so, I had a look underneath.

914 racers give up HP using air dams and rear wings on larger tracks above 90MPH to gain stability.

"Mostly" is right. The underside is stamped with stiffening ribs, mostly across as I recall. The area under the front has extra projections, including a tow hook, or a huge hole cut for getting air to and from an air conditioning condensor.

Just aft of the cockpit, things get really dirty. There are, on most 914s, a couple of plastic flaps on the bottom that actually aid cooling. The engine bay is open to the bottom, and the transmission sits aft of that with nothing but the rear suspension and the drive axles next to it. The exhaust has one pipe on each side that stretches straight back to a muffler that goes across the whole width of the car at the back, and then there is the vertical panel.

Finally, the air that cools the engine is sucked in through the engine lid grille (just aft of the rear window), pushed through the engine's cooling system, and dumped out underneath the engine. The two flaps I mentioned help create a little bit of negative pressure under the engine, to help draw air more effectively through.


(PS--Hi, Chris! Good to see you checking this thread!)

-soD

Ressurection

So it's helping to increase under car pressure and draw some air from the wake which may help reduce the wake. Huh. Interesting.

I find it hard to believe that the drag is greater with the fastback but we don't know the angle so perhaps it is more than the ideal of ~12-15 degrees

Chris914, if you are still on the list, what was the angle of the plane from the roof to the decklid on the fastback model you tested? Most studies show ~12-15 degrees optimal with a big jump in CD starting at ~20+ on road vehicles.

I measured ~22 degrees from this blueprint which would make it too steep for good CD reduction. Perhaps a little less slope would have yielded better Cd reduction than the stock roofline.

http://www.puffofsmokeracing.com/Car..._blueprint.jpg

When I built the SedanKamm for my Suzuki Swift, I used a 2x6 standing on it's side to help bring the angle closer to the ideal 12-15 deg, and it worked really well, I am sure something like that could be done on the 914 and make a big improvement. The roofline looks like it already tapers pretty well, so a Kammback should really show improvements!

Found this chart on 914 world

914/6 also on chart.

http://www.914world.com/bbs2/uploads...1229465651.jpg

With a microscopic frontal area of 17.2, a Cd of .36 and CdA of 6.19, the 914 is a prime ecomodding target. WANT!!1!

Porsche 914 mileage Ad:

29 mpg claimed.

http://www.youtube.com/watch?feature...&v=muRuHA-MREs

Nice stache.

I don't know about the angle, but there are other considerations. For instance, at speed there is air flow going forward along the rear deck. I guess this is where the cooling and induction air come from? I have seen a number of photos and even a video or two of tuft testing done on 914s over the years that show this. It's pretty counterintuitive, but the tufts clearly showed rear-to-front air flow.

The water-cooled conversions don't have that problem, but they generally have air flow coming in through the front panel and exiting either under the front or out of the front wheel wells. None of which is that good for the car's aero.


EDIT:

Oh, and I averaged about 34 MPG two weekends ago. Over about 600 miles, at freeway speeds, including some mountain climbing. (I-80 from SF to SLC.)

-soD

forward deck flow
 

Bill Bernstruch,one of the videographers who has attended Bonneville and El Mirage events put a VHS together long ago which included a 914 at El Mirage.
As the car makes it's pass you can clearly see the dust from the lakebed coming up and forward over the rear deck,completely filling the void all the way to the roofline,and within it,you can see a tranverse vortex circulating above the cooling inlet slamming dirt through the hole.
A K&N wouldn't last 2-seconds inside there.
I suspect that this could be witnessed on any Beetle,Ghia,912,and 911,and it's an intentional design criteria for the air-cooled engines.

Resurrecting this thread as I am considering buying a slightly rough-around-the-edges 914 as a grocery-getter/middle age crisis project.

It's red, just the right colour for a crisis car. But that will have to change. I hate red cars. It's interior is quite good considering and it has remarkably little rust for a wet coast car but it's had some dodgy bodywork done. Original EFI is missing. It has a single carb.

If I can get it for ~$2000, I might just go for it. Can't go down in value and it's "moddable" and considering the condition, I won't feel like I'm committing vehicular blasphemy. HMMMM.

The three main things to look for in a 914:
- Rust
- Rust
- Rust

Go to the 914world website and do a search on "hell hole" to see some of what can happen.

-soD

I posted this a while ago, nesting it here for future reference.

Driving with the top down?
http://ecomodder.com/forum/showthrea...tml#post374311
Yep, those are numbers for a Porsche 914, lamps (headlights) up and down too.

They're also a prime candidate for an electric conversion. I went looking for the Cd/CDa of a Beetle (0.46/19.3) and I found this:
1947 VW Beetle-Based V2 Sagitta is More Aerodynamic than New Golf, Mercedes CLA!
I see what you did there. As for the colour, go to YouTube and search on "Plastidip whole car". What's the single carb, a side-draft Weber?