(Let me preface this with the following. I am not a Porsche Tech, and I'm not an EE. I am, however, a big fan of the Porsche 928. I am a PCA member, and I post regularly on Rennlist. I own two 928s, a US-spec 1985 928S, and a German Spec 1979 928, so I know the car pretty well for a layman.)

According to Adam's description of the reverse engineering myth, the myth is as follows: That when Porsche designed the 928 in the mid 70's, they discovered that the car was more aerodynamic going backwards than going forwards. The problem is, they never actually tested the car in question, and there are a number of issues with the testing done on the car:


1)The model, and the wind tunnel. The first myth they effectively confirm, but inadvertently, is using the wind tunnel test. Take a look at the rear of the car they use in the test. Notice how it looks less like this car than it does this car, at the bottom of the page. The first car is the car the myth is supposedly about. The second is the car they actually tested in the tunnel. The car they actually tested was a 928S4.

The problem is, while a 1979 928 and a 928s4 looks the same to an untrained eye there are vast differences in their aerodynamics. Here is the list of Cd and frontal area, actual and adjusted for Cd of all 928 models (source) :

• 928 MJ 1978 to 1982: 0.41 x 1.95 = 0.799
• 928 S MJ 1980 to 1986: 0.38 x 1.95 = 0.741
• 928 S4 MJ 1987 to 1991 (also CS. GT): 0.34 x 1.98 = 0.673
• 928 GTS MJ 1992 to 1995: 0.35 x 2.02 = 0.707

What does this mean? The 79 928 has 19% more drag than the car they tested in the wind tunnel. At 100 mph, that is a pretty big bump in drag. Just doing some back of the envelope calculations, that means that a 79 model to the scale of their S4 model would instead of being subject to .34 lbs of drag, the car would be subject to something .405 lbs of drag going forward. It actually might be more like .41 lbs of drag, because my calculations are based on the fact that the S4 has a wing(wing=drag), and they removed the wing from their model.

That calculation gives a number 9.5% HIGHER than the number Adam posted for the backward car in his own wind tunnel test. Of course, we don't have a wind tunnel test for a backwards 79, only a backwards S4, so we can reasonably posit that backwards S4 is more aerodynamic than a forwards 79.

So that's the problem with the scale model test. Now, just like on the show, we go full-size, which brings us to the next problem:


2)The full size car is also the wrong car.
Take a look at the picture of the car at the beginning of the segment. Notice that black thing under the nose of the car? That's a front air dam. It keeps the air from going under the car, and keeps it away from the decidedly non-aerodynamic bits like the suspension, exhaust, etc. What does it mean? Look at the front of this car. This is a 1978 928. What is missing from it? An air dam. The air dam was standard equipment on a 928S which was available from 80-86.

As we can see from the above list, a 928S has 8% less drag than a 1979 928. Like the S4, they deleted the rear wing, making the car MORE aerodynamic than stock, and MORE aerodynamic than a 79, and perversely, with the spoiler acting as a bucket when reversed, might make it LESS aerodynamic backwards than a 79.

Looking at the car, I know what model it was. It was a German market 928 imported via the gray market. The tell-tale rear foglight and h4 headlights give it away. Someone more knowledgeable than me might be able to tell you the exact year. I can't.


3)The timing is not trustworthy.
Looking at the timing results, something is wrong, specifically with the 0-60 times. The quarter mile times come in relatively consistently at 14 seconds. That is about right for a Euro-spec 928S(310hp) with an auto. The 0-60 times, though, are just wrong. Yes, the 928 was never designed for stoplight battles. It shines at speeds over 80mph. However, the slowest 928(1978 auto) did 0-60 in 8 seconds flat. A Euro S(your car) has 90 more HP, and no car in that HP class is doing 0-60 in a time more than 6.5 seconds. Next time you do a test that involves acceleration, get a g-tech as backup.


4)Forward test, windows up. Backward test, windows down.
Just like it says. In the forward test, the windows were up. In the backward test for the same car, the windows were down. According to this source, rolling the windows down has a negative effect on the aerodynamics of the vehicle. By leaving the windows up in the forward vehicle, and down(by necessity) in the backwards car, the test became skewed. Also, the backwards car has none of the wheel wells covered. Coupled with the lowered window, wouldn't that create a zone of moving air inside the car, which as we saw in the pickup truck myth, plays a serious role in the overall aerodynamics of the vehicle?

On a tangential note, The backwards exhaust is not the same as the forward one. What was done to normalize backpressure(which affects HP) between the two systems? How would that affect fuel economy and performance?

5)Ride height, weight balance, rolling resistance, etc. A lot of people seem to be agitated about this. I'm not quite so convinced. While weight balance will have an affect on handling, on aerodynamics test, factors, not quite so much. I think the difference in ride angle, though, is an issue. I can't quantify the issue, so I'll just leave it there. Rolling resistance is also an issue. A lot of the engineers here give some credence to it. I do as well, to an extent. However, correct me if I'm wrong, isn't rolling resistance, bearing friction, etc linear in its effect? So at 100mph, isn't rolling resistance much less as a % of total drag than at 10mph?