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Again, retest.

Look at the old GM station wagons that had problems with the rear window becoming filthy dirty. They made several attempts to keep the rear window clean due to the "Swirl" when the wind made when it reached the back of the car. I remember at least two designs on the top, some wings on the sides and a extended rear overhang in attempts to break up the vacuum created. One was a shortened version of the one shown in the last pix (this is a similar version but bigger and more abrupt) and the next one which changed the direct the flow to go down the rear of the wagon breaking up the swirl. (mixed in here you will see some of the attempts I was talking about. https://www.bing.com/images/search?q=GM ... ORM=IQFRBA)

This is why I recommend coming up with a design then testing it then making mods... if necessary before it becomes a permanent feature on your toy.

Lee

The red Ghia is Mike Fischer's. He started with some traditional aero mods, front air dam, etc. He actually spent time in the wind tunnel and found out that all of his mods didn't work and the Ghia was actually better without it. The second picture is how he is running it today. He went 190 mph last month at World of Speed at the Bonneville Salt Flats. I run the blue Ghia. I have been 174 mph and the car is very stable. The car has been a little loose in the 140-150 mph range before, but then over that the car gets stable again. I don't know if this is track conditions or something else? Once above 150 its like driving on the freeway.

Interesting input.

I wonder if the air dam in front could have been a big problem due to bulk, shape and placement.

If one does a search of air-flow patterns pictures it is very interesting especially as wind or object speeds increase and/decrease. Like stress loads, air does not like to be bossed around to change pattern of flow.
I have wondered if the stiffening beads on the floor of the tunnel and the floor of the pan might have any effect on the air-flow patterns such as lift or suck down... Hmmm! The shape of thing also can cause the wind patterns to change.

(I was sitting at my drafting table one day when my supervisor came up and handed me a piece of paper, it said I was being transferred to the SST wind tunnel to work as a model draftsman. I think my smile went clear around my head but... then he said I wouldn't be going and walked off. The next day Nixon cancelled the SST. That is one of several reasons I got interested in air-flow and it's patterns)

In the past he air-dam(n)s were there to break up the air-flow patterns underneath the vehicle but, again, it could also cause problems due to size or shape/pattern.

For what it is worth... Lee