Aero questions. Options

[jmz]

Does this tail even do anything?

Decided to run the car with air dam and “tail” at next weeks race.

Been running in standard GT configuration most times. (Recently repainted back to original green)

[technicalninja]

I am a member of hybridz.org, a Datsun Z car forum.

Years ago, one of the members rented a NASCAR wind tunnel for a weekend and tested various aerodynamic devices on an early Z (pre-79).

Zs are worse aerodynamically than a 914. The side view of the car shows the entire body forms a high camber wing that creates a serious amount of lift above 60mph.
The faster you go the worse it gets!
A stock 240Z gets dangerous above 125 mph.

Spoilers attached to the rear of the car (similar to the OPs set up) helped to REDUCE this lift but did not create any down force at all. They helped but were not the solution.

A simple air dam up front created significant down force on the front of the car.
Enough down force to un-balance it.
Adding a 1" splitter doubled the down force and a 3" splitter tripled it!
Stuff on the front of the car worked bad-assed. Everything made down force!

What they had to do on the rear of the car to attain enough down force was a big assed wing at or above the top of the car.
By the time they achieved balance with the 3" splitter the wing was HUGE. Wider than the car, above the roof line, and looking like something out of the Japanses bosozoku world.

It would be ok on a trailered race car but not for a streetcar.
It was an "in your face" super wing!

Balancing any frontal device takes either a big obnoxious wing, properly designed rear diffuser or maybe both. The diffuser is the first place to start on a Z.
They did not have a diffuser for the Z when they tested in the wind tunnel.
Zs don't have much room for a diffuser. The rear suspension is in the way.
914s are far more adaptable for a rear diffuser than a Z IMO.

Z cars are not 914s and the overall shape is different but aero is aero and a functional rear downforce device is either going to be a well-designed diffuser or a BIG ASSED wing in smooth air in my book.

One thing to remember. If you're making 400lbs of down force the bracing for the wing has to transfer that to the chassis and needs to be MEATY. Two 200lb men should be able to jump up and down on the wing at the same time.

Same thing with the diffuser, it's going to be pulling on the bottom of the car with great force. You should be able to hang a couple of 200 lb. men off of it with the car in the air on a lift.

Diffusers usually beat wings in the downforce/drag department.

Almost ALL aerodynamic devises for downforce create drag. There is no free lunch.