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The Karmann Ghia coupe and the 356 Porsche coupe, windows up, are usually listed as both having a drag coefficient of about .37.

The higher the number, the more drag you have, and the slower you go.

A .37 is not bad for a body from the 1950's. A VW Bug weighs in at .48. Modern Porsche 911 variants are down to .30-.32. You can slick up a Ghia with fairly little effort, using a front airdam and a Porsche RSR-type "ducktail."

At RetroRacing, aerodynamics became an issue when we needed 135-140mph top end to run the bigger tracks. What you need at high speed is stability.

An airdam to keep air from under the car is a great start on that, and the RSR ducktail cuts lift without adding downforce ... downforce being another type of drag.

Here's a photo of our sheet-metal ducktail.



And our airdam. for the track, it has to almost scrape the pavement. The brake cooling ducts have to be real, not typical street "styling" vents.



We made our airdam out of heavy sheet metal and bolted it to the back edge of our bumper with simple small "L" brackets. Here's the pattern.



More can be done, of course, but these basics really helped stabilize the car above 120mph. And now that we have the horses, and as soon as the 3.88 gets here, at Sebring we reasonably expect a stable 135 MPH and are hoping for 140.

How does it feel with the new ft spoiler and rear duck-bill?

Slick!
Consider adding a splitter to that front dam?

Standing the engine lid off on my sons SB oddly seemed to help stability at high speed, probably be much more effective on a Ghia.
Might even have less drag...

(probably acts much like a spoiler, while not looking quite as cool)

Probably some ram-air effect available with side plates for the stood-off engine lid.

Our aero aids really helped the car feel more secure, more stable, at high speed. We can't confirm that it has increased our top speed -- yet.

We had been getting nose lift above 100mph, which was easy to see in our post-game videos.

One of the 911 GT3 drivers did a double-take as he saw our car as we went through tech. "You don't see that everyday," he said.

The above photo was taken last year at Roebling Road at a NASA event.

FJC

Just curious where did you find the drag numbers? Anything that was done pre-internet tends to be really hard to find.

Hi Sideshow,

Being involved with the Porsche racing people had many benefits. They knew all the drag numbers on their cars and the VW's. I just kept a note on it. The front of the 356 is better than the Ghia, but the Ghia actually has better aerodynamics than the 356, as the back of the 356 drops away too sharply for a smooth airflow. That's why the 911's were longer and shaped like they were.

Both cars were calculated at about .37, the edge given to the Ghia. The proof of the drag numbers was how the 356 and Ghia, both equipped with similiar final drive gearing and horsepower, achieved similiar top cruisng speeds and have a little left.

A 1,6 60-horse Ghia and a 1,6 60-horse Porsche Normal both can easily cruise at 90mph. Even the 1,2 Ghias and the earlier 1,1 356's could almost hit 85-90 with no headwind.

The factories both cited 78mph or so, but they always sandbagged.

FJC

Thanks, you are a wealth of information. Any chance you know any drag numbers on the trucks (1962-1965 single cabs)?

sideshow wrote:Thanks, you are a wealth of information. Any chance you know any drag numbers on the trucks (1962-1965 single cabs)?

Approaching 1.0...

Hi Sideshow,

I wasn't aware that Busses had aerodynamics, or maybe it was they were aerodynamicaly unstable when parked?

Just a joke! (Like the big new wing on his car created a ground-loop at idle.)

Take a look at this article. It conforms to what I know, which was the busses were at .70+. I think at past .80 if you go too fast you start going backwards.

http://www.europeancarweb.com/tech/0610 ... ndex3.html

FJC

belive it or not, but in fact the VW bus shape was developed after wind tunnel studies!
I recall they got the cd down from 0,7 to 0,45 or something like that, of course the frontal area is still much larger than a bug, and the different roof line of 55 and later buses probably makes for much worse streamlining than the early ones.

Yup, the Cd of a Bus is actually better than that of a Bug (obviously the total drag is higher due to the much larger frontal area).
That's still not saying much (a Bug is pathetic at about .46) but they did a pretty fair job of streamlining a brick enough to give it a Cd of .42.
I've never seen figures on the single and double cab, presumably they'd be a little worse than a Bus. I remember being able to go about 3MPH faster in my `63 Singlecab with the sidegates down

FJC, I never hit the top speeds you do on the short tracks, but even at 80MPH there was a discernible handling advantage to getting the front dam as low as possible - here's an example, this one was made from an old conveyor belt:

Are you allowed to run side skirts? What about door windows?

SCCA and NASA (actually all organizations I know of) require that the side windows be down or removed from the race car. To partly help, we have 911-type drip rail fairings which are very effective.

We don't yet have NASCAR style vent-wing air deflectors for the first quarter of the window, but they are legal for us. These are just clear Lexan air deflectors. The NASCAR guys usually have air scoops in their deflectors. We probably don't need them. Nothing like sucking in a wasp at 130mph at Sebring.

The airdam is a Godsend. I don't think we need splitters, and it should be clear to everyone what we have a real airdam, not a spoiler.

FJC

This old (2003-4) image shows us on the back straight at Taladega Gran Prix Raceway. Not too long a run, plenty of opportunity to hit 120mph for us.

The still from a video shows what might look like a level car -- except that we have an adjustable front Puma axle, and normally set with about a two-inch rake, our first effort at aerodynamics.

The wind is lifting our nose back up to normal ride height. and look at all that daylight under the car! That's air trying to lift us right up and into the weeds.



Our airdam nixed the lift (and now we really need to raise the front ride height up an inch to make the rear suspension work harder). The key to rear stability is the ducktail. This little aerodynamic beauty is just made for the Ghia.

Porsche introduced the ducktail on the 1973 911 RSR. It killed rear lift without adding drag. It also helps force air into the cooling slots.

If you make your own, the key points are to keep it low enough to not obstruct the driver's rear view, and to provide rain drains in the design.

We have some bolt holes in ours to be able to add a clear Lexan lip above the ducktail if we need more area.

FJC

FJC,

Are the "bumps" just in back of the rear window air intakes? If so could you provide a detail picture of them?

Thanks!

I'm reminded of the pre-season practice day when we took the 25 car out for a shakedown sans front air dam. The entire 'glas nose was lifting up several inches by the end of each straightaway (it was held in place by rubber straps) and we had to tie it down with bailing wire to finish out the day. Air dams are a good thing!
We weren't allowed to run door windows back in my day either, but of late it's become standard practice - the tracks around here all allow them on the RH side so long as they're easily removable from the outside (wing-nut Dzus fittings are the norm). Your A-pillar fairings look like a good deal, you could probably fair the "B-pillars" in for a few inches too to reduce the "parachute" effect of the rear windows. Is your back window glass or Lexan? Ever considered putting some slots in it to bleed captive air out?
The effectiveness of the airdam would be dramatically improved if you could run side skirts to help keep the underside air pressure low - you'd probably gain more than enough downforce there to make that ducktail superfluous. I'm pretty certain that it is adding some drag.
I'm just theorizing here, our development of the 'Ghia body aerodynamics came to an abrupt halt when our rules were changed to allow plastic bodies:

Yep, that's still a VW-based rear-engine chassis
...at speed the front dam and side skirts would contact the track, it stuck like a Chaparral.