Study (model) of various aeromods affecting a Porsche 914

[chris914]

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

[rav]

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

[wyatt]

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.

[rav]

Originally Posted by wyatt

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.

[donee]

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.

[chris914]

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

[some_other_dave]

Originally Posted by donee

Remember the Porshe 914 is built kidna like the original VW Bettle chasis. So, its already flat along the underside, mostly.

"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

[orange4boy]

Ressurection

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.

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

[orange4boy]

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.

[wyatt]

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!