Kammback vs spoiler-or both?

[aerohead]

Originally Posted by oldschooleco

hmmmm i like the idea of fender skirts, i can definatly make something work. what about sail panels? coming from the side edges of the roof?, or on the edge of the the body near the tail lights?

I found a blueprint for a 1968 Dart GTS Hemi online.I looks like your car.
With rough measure,it looks like the aft-body of the car constitutes about 34.6% of overall length and your existing backlight is at 30-degrees to the horizon.From backlight header-to-rear of trunklid you have about 14-degrees of slope.
From Buchheim et al research of fastback cars from 1981,at this length,your backlight would be at about 18-degrees for the lowest drag.
18-degrees occurs about two gas cap diameters behind the back of the gas cap,or about half the length of the trunklid.
Since you have a notchback,we might guess that you don't need any kind of rear spoiler.Your trunklid is already too long,aerodynamically.
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If you extended the C-pillars back halfway across the trunk you'd create the 18-degree slope in elevation.
If you filled in the void with a series of stepped, louvered 'spoilers',you'd capture a vortex inside there.
You'd have some privacy.It would take some load off the air conditioning.And the outer flow should skip off the vortex with a clean reattachment without recirculation into the new recessed area.
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For lowest drag,you'd have to chop your rear quarter panels to create an actual fastback.I don't think you want to go there.
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As to other mods:
*There might be a chance to soften some of the leading edges,especially around the headlight buckets,or fab some plex domes,as CAR and DRIVER did on their 'Crisis Fighter Pinto' back in '74.
*Your A-pillars,drip rails,and recessed glass are a problem but I don't know of an aesthetic solution for that.
*An invisible grille block if you don't have that
*Loose the engine-driven fan and go to electric if you haven't already,or flex-fan,or good thermo-clutch unit.
*Rocker panel extensions,a couple inches
*Extend quarter panel bottoms down even with new rocker extensions
*The rear skirts would be good
*Belly pan with diffuser,according to Carr would cut Cd 0.07.

[oldschooleco]

i extended the rocker panels down at the pinch welds, or is that to far in? but that sounds good i have about a week to get it together before the 2000 mile trip

[freebeard]

hmmmm i like the idea of fender skirts, i can definatly make something work. what about sail panels? coming from the side edges of the roof?, or on the edge of the the body near the tail lights?

I'll point back to that other thread, and aerohead's post

i extended the rocker panels down at the pinch welds, or is that to far in? but that sounds good i have about a week to get it together before the 2000 mile trip

Look for basjoos' Civic. He successfully used two rocker panel exensions, one each at the inner and outer tire faces. It extends behind the rear wheel and tapers into a spat. or skeg.

Make sure whatever you do won't fall off half way.

[aerohead]

Originally Posted by oldschooleco

i extended the rocker panels down at the pinch welds, or is that to far in? but that sounds good i have about a week to get it together before the 2000 mile trip

Going back to the 1982 Trans Am Firebird wind tunnel work at Lockheed Marietta,GM's guys got the rockers flush with the outside of the wheels,and down to where a belly pan would be.The air would have a straight path to the rear,losing no momentum by not crashing into the wheels/tires.
Carried behind the rear tires,the extensions,wrapping around into a rear valance panel preserve the momentum all the way to the rear.Subaru published a drag reduction for their 1986 XT by creating this 'parachute.'

[aerohead]

I looked through my rat's nest.
*I think the 'naked' Dart is around Cd 0.485.
*It's aft-body is about 42% of the A.S.'Template' II.
*An approx. 4.8-inch rear deck extension(4.8" beyond the trunklid) would bring it right to the 'Template',so I'm reversing myself when I say it would be okay to do a serving tray type spoiler.
*At 42% of the 'Template' the Dart has the potential for around Cd 0.192,with all the whistles and bells.
*The 1991 GM Ultralite is 42% 'Template',with Cd 0.19.
*The 1956 Dodge Dart concept,by Ghia,registered Cd 0.17,at 63% of the 'Template.'
*basjoos' 2005 AeroCivic is Cd 0.17,at 50% 'Template',with its beautiful wheel integration.
*A 'Template' car,at 63% would come in at around Cd 0.149.

[oldschooleco]

Thanks arrowhead, I've got some thinking to do. ill keep updating with pics as i go

[oldschooleco]

Did some mocking up/mounting. do you think just the sail panels would help out any? thank you

[aerohead]

Originally Posted by oldschooleco

Did some mocking up/mounting. do you think just the sail panels would help out any? thank you

I do think they could help.They would forcibly control the 'area' occupied by the turbulence,sculpting the wake behind the roof as it were.
Pickups like the Ridgeline,Escalade,and Avalanche are using them after a fashion.
Lotus has done it.Many of the supercars have used it.
I did it with my CRX and incorporated wings and spoilers into the buttresses to further modify the separated flow.
I would give them some gentle inward plan-taper,based upon the 2D-flow wing section of minimum drag.
Here is the MG EX 181 using it for it's body

[freebeard]

How wide is the fender to either side of the decklid? That will determine how much you can taper in plan. If you reach all the way forward to the edge of the quarter-window, it will make it easier to get a smooth transition contour and eliminate a visible seam.

With maybe a flat bar across the top of the backlight to tie the two pieces together, and a convex curve to the trailing edge.

aerohead's example may seem a long way from you car's body shape, but notice the flat underside and wheel spats. You could totally do that.

[oldschooleco]

ok, if thats the general consensus ill make some sail panels. are the wheel spats like those straight down chloroplast things in front of the rear tires? whats the best way to cut chloroplast