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)

[brant]

Green looks great!
Love the tire source decal
That’s actually the shop I met Dave ferguson at
And they are closed now
But the absolute best tire shop there has ever been

[mgp4591]

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)

Good looking car! I would think that the speed would make a big difference. There was a study a few years back that indicated that the outer edges of the rear were the best spots for downforce while having the center of the trunk area "spoiled" as well will reduce the straightline speed because of too much drag.

[Brett W]

Do you have a good way to quickly adjust the angle of attack?

[jmz]

Do you have a good way to quickly adjust the angle of attack?

Negative. It’s fixed in place.

[jmz]

Green looks great!
Love the tire source decal
That’s actually the shop I met Dave ferguson at
And they are closed now
But the absolute best tire shop there has ever been

That’s Dave’s photo from
the SCCA log book I have.

I tried to get into the reunion and if it mattered would have used that livery. It’ was really hard to get in this year and they didn’t accept the car…

[BillJ]

I thought you had to have a car that was raced pro in period?

[jmz]

I thought you had to have a car that was raced pro in period?

Seems so now but I don’t think that was the case in previous years.

[Charles Freeborn]

Many of the "period correct" restrictions are being lifted as organizations are desperate for entries. Several of the mods to my car (crank fired ignition, big brakes, etc) are overlooked now. I don't know if that' a good thing, but for me I'm only interested in having fun so if someones car is better prepped than mine, c'est la vie.

[stownsen914]

Yes a spoiler like that will be effective. A more vertical stance would be more effective, which might be what Brett was getting at. Air over the rear deck of a 914 is pretty turbulent, made worse by the sharp dropoff and void behind the roof. A spoiler helps direct the air and give moderate downforce.

[yeahmag]

http://www.cassidy-online.com/porsche914/a...aids/index.html

[914sgofast2]

http://www.cassidy-online.com/porsche914/a...aids/index.html

From that study it looks like adding a rear spoiler without drastically lowering the car only costs you horsepower!

[sixaddict]

I have been told as much by several people……but here’s a couple of points.
When buying the front spoiler from Sheridan he asked me how will I compensate out back. To me that meant I needed ( and had ) a wing. Secondly, from experience the car was much more stable in high speed turns….. ex: turn 9 at Road Atlanta is flat out
Without wing it would “dance” across the track whereas with wing it held the line even as the track drops down….
To me the down force (stability ) was worth the potential loss of top end……purely anecdotal but I guess seat of the pants can lie.
FWIW

[stownsen914]

I'm finding that aero study a little hard to understand. The spoiler and wing configurations are listed as having negative values in the Downforce column. One way to interpret that is that they are providing "negative lift" which is what aerodynamicists call downforce. The wing and spoiler configurations both show negative values in the Downforce column. These should be higher downforce configurations, so I assume negative values mean negative lift i.e. downforce? Which would match conventional wisdom that these add-ons provide downforce.

[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.

[windforfun]

I would go with an inverted F35 design. I'm sorry, but I couldn't resist. I'll leave now.

(IMG:style_emoticons/default/biggrin.gif) (IMG:style_emoticons/default/biggrin.gif) (IMG:style_emoticons/default/biggrin.gif)