Learn me some about rear spoilers
 

I'm now driving my 66 Chevelle eco project and am getting over 30 mpg.
Im working on a front air dam now and will probably add a small splitter to the bottom of it.
is there any kind or a rear spoiler I could add to the rear of the trunk lid to help airflow off the back?
Either a small wickerbill or a flat extension. Which would be better for economy? I don't need downforce.
And how do I calculate the overall height or length and the fore-aft placement along the trunk lid?
Thanks,

Use the aerodynamic template. Make a spoiler that makes your car fit.

https://ecomodder.com/forum/attachme...1&d=1381330118

Very interesting tool. But will this help me with a spoiler solution? I can see how this would size a kamback but not a spoiler.
Do I add a spoiler up to meet the line? Like a wickerbill? At what angle?
Or do I go back to meet the line? And at what angle? The last of my trunk lid rolls down quite a bit.

I would try a lid extension that is similar to what the pro stock drag racers’ run. I don’t know if i will help your mileage but it would very cool on your Chevelle.

Build a spoiler to meet the line. Go up if you want a spoiler, go back if you want to build a boattail.

rear spoiler
 

CAR and DRIVER experimented with airdams and spoilers on a Ford Pinto,a Dodge Van, and a Datsun 240-Z.
A wrap-around airdam would help keep air from under the Chevelle,cutting drag and front lift (my '67 El Camino was terrifying at an indicated 120-mph!)
A rear spoiler,tilted up at no more than 30-degrees above horizontal,just touching the 'template's' imaginary contour might be a good first approximation.Theoretically,a straight- back foil, ala NHRA/Bonneville,would be lower drag,but it would have to be 'soft' like the rubber-edged Porsche 911 Carrera 'whale-tail' employed, to protect pedestrians.
Rocker panel extensions like on the Trans AM Firebird, even with the bottom of your airdam would help.
Forget the splitter.It will only create drag.
An electric cooling fan if you don't already have one. Keep the fan shroud.
Block all the grille except for the actual perimeter footprint of the radiator.
The headlight recesses could be covered with Plex/Lexan.
MOON covers are a known drag reducer.

On further examination, I can’t possibly reach the line with a spoiler. It would be like 8” tall @ 30 degrees or @ 12”+ if straight back.
Would a partial spoiler help at all?

I may add some side skirting but not to the bottom of the front air dam.
I do run an electric fan and the grill opening is already smaller than the face of the radiator. I may reduce this some because the face is designed for a big block and I’m running a 217 cubic inch V6.
Moon covers while efficient are not in the cards. I dont have a way to attach them to my wheels.

can't possibly
 

Run a simple straight line off the back of the roof,down at 23-degrees and see what that looks like.That's the absolute maximum slope the car can tolerate without flow separation of the central downwash core.If you can capture a locked-vortex between the top of the spoiler and top of the roof,the 'sound' flow (as Hucho calls it) will skip over that vortex kinda as if it were a solid structure.You'll pay for all the kinetic energy lost to feed the vortex,but it should kill the rear lift,and with it,some drag.Can't say how much.

https://i.pinimg.com/736x/8c/16/c0/8...c63e86979b.jpg
https://i.pinimg.com/736x/8c/16/c0/8...c63e86979b.jpg

I don't see a problem. Just don't back into pedestrians.

You might consider sail panels that roll vortexes down onto the deck lid to constrain the size of the wake.

IMO, a rear spoiler is supposed to "spoil" air flow, and therefore increasing the local air pressure upon a surface up stream, to intentionally increase DF. Increased drag is thought to be an acceptable trade off in most cases.

A flat added extension (trunk?) should, again IMO, maybe better be called a "smoother (or divider?)", which might reduce drag. If a "smoother", just by forming a hard boundary between a higher pressure area and a lower pressure area in space, will also increase potential DF, with little drag increase, if any.

Doesn't seem to me that a typical rear spoiler often reduces drag, leading to better mpg.