There's been some interest in mods for notchback cars,especially rear spoilers.
I've gone back in time,looking over early wind tunnel investigations and will share the following.If you have any of Hucho's books you should already have this data.For those who don't yet have a book,here are some talking points for design.
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From SAE Paper# 760185,by Hucho,Janssen,and Emmelmann,they tabulated results from wind tunnel tests of eleven ( 11 ) different rear end configurations for the Golf/Rabbit-Jetta,at Volkswagens climatic wind tunnel.
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In every case,the drag was asymptotic as the roofline architecture met the 'Template'.( please see
'Aerodynamic Streamlining Template Part-C').
At 30% aft-body length ( Golf/Rabbit),as derived from the 'Template,' the following coefficients of aerodynamic drag were observed:
Backlight Angle-------------------------- Cd------------------------------
28-degrees----------------------------- 0.44
26-degrees----------------------------- 0.43
24-degrees----------------------------- 0.42 ( production angle )
23.5-degrees--------------------------- 0.40
23-degrees----------------------------- 0.39
22-degrees----------------------------- 0.38
21.5-degrees--------------------------- 0.37
20-degrees----------------------------- 0.36
18-degrees----------------------------- 0.35
12-degrees----------------------------- 0.34 ( 'Template')( 19.04% reduc.)
10-degrees----------------------------- 0.34 ( Kamm recommends c.1955 )
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The Jetta,at 42% aft-body could go no lower than Cd 0.365 even when the trunklid ( boot ) was raised 450mm ( 18-inches ) @ 'Template' height,netting only a 9.2% drag reduction.
The 'notch' costs us and cannot approach the low drag of the K-form 'Template' roof.
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The Scirroco,@ 31 % aft-body reached a drag minimum of Cd 0.377 with the best rear spoiler configuration.From backlight header-to-trailing edge,the spoiler relaxed the down-slope angle from 24.5-degrees,to 18-degrees,far below the 'Template.'
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With the 'Template' roofline,the shorter,frumpy Golf returns lower drag than the 'sporty',longer Scirroco.
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Had the Golf had the 'Template" roof,and windshield of,and plan body taper of either the Renault Vesta II or Daimler M-B Boxfish,it would have seen a drop to Cd 0.19.
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A few cars which have pushed their spoilers to the 'Template':
1988 Lotus Esprit Turbo
1994 Saleen S-351 Mustang
1995 Ferrari F355 Berlinetta
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The original CAR and DRIVER mods to the 1974 Ford Pinto altered the rear angle from 26-degrees,to 23-degrees( Hucho's maximum angle recommendation[22-degrees by W.A.Mair]),@ 37% aft-body,using the 30-degree up-swept rear spoiler,for a 7% mpg improvement ( 11.6% drag reduction )
Same spoiler,straight back = 1.36 % mpg improvement
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It appears that while the spoiler can help,it does not have the drag reduction potential offered by the fastback,K-form roof,which is what Hucho claims in his book.
And the 'Template' appears to hide in plain view,embedded within the data scatter-plot.
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If you must do a spoiler,try and get close to the 'Template',even if it takes a bi-wing or tri-wing arrangement.