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Fachsenfeld's tail
With respect to diffuser angle limits beyond a cars original rear bumper I have no data.
Hucho,in his section on racing cars shows some long-tail configurations along with lift/drag comparisons.
Hucho also has a full-boat-tail drag tabulation for the Daimler',M-B C-111 record car,and you'll notice that this car also has the upsweep.
Ditto,EV-1/Impact LSR car at Ft. Stockton,Tx.
The only thing that I would throw out,is that a 'track' car would require special considerations that a 'road' car would not.And I believe an upswept tail is a concession to cornering downforce ( say 257 mph around a curve in the Olds AEROTECH ).
The other 'clue',is from GM's display at EPCOT Center in the 1980s,where they displayed an 'active' diffuser which went 'flat' at highway speeds.No upsweep!
On the road,there would be no concern for approach/ramp/break-over clearance,so a 'flat',or 2.5-4.0 degree diffuser wouldn't pose an issue.
My streamlining template reflects a 'flat' diffuser as used by Jaray,Klemperer,Lay,Kamm,GM,etc. which would require the 'active' approach,something I don't feel to be a big deal.
Why Fachsenfeld did what he did died with the man.I would not,and will not design as he did,however,what he did remains a very high bar,better than anything I've seen to date.
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