My car originally had the original grillwork + an A/C condenser. The a/c never worked really well. Once I added a ducktail the a/c improved radically and my engine temps decreased.
I am not an aerodynamicist, but my guess is the smooth x-factory grill had a laminar flow right over it. Air was probably sucking air OUT of the engine compartment and stalling the fan. Leave a gas cap off of a wing tank and the air traveling over it will suck fuel out.
Look at F1 cars and see how they use little bumps on surfaces to create low pressure areas.
It is really an art and the master is Adrian Newey.

Get grilled
 

When I first made my grille. it was for my own use on my 73 hotrod. The inspiration was the mesh grilles that were used on some of the early 911 race cars...like the 911R (see below) I make both diamond mesh as well as the round perf hole type......the intent being lightweight and retro-cool aesthetic...and function. My 3.2 in my 73 with Carrera cooler and Elephant finned cooler pipes never ran hot, no matter how hard I pushed, but all of my driving was "very spirited" street driving.......Jeff's driving is another thing...just try chasing him through the twisty bits

Mounting the grille below on the underside of the decklid may provide the required 'dimples" in the airflow.....

Jeff,
We are running out of warm days....but here is the next test, if you are up for it. Mount the grille on the underside and repeat....

regards,
al

http://forums.pelicanparts.com/uploa...1665849853.jpg
http://forums.pelicanparts.com/uploa...1665849853.jpg

Maybe some flow paint?
That would be really interesting.

That a great idea, Al! I love the more "R"-ish look to it since, as you know, my car is what I believe the 911 R would have been had they continued to produce them through 1972. I may or may not have time to conduct such a test before our weather turns, but it is certainly something to try when I can. I really do not like the stock grill on my car (much preferring the X-Factory unit) since, well, it looks kinda "stock"... And that's just not me..

So, The Higgins Equation got me to thinking about the grill obstruction calculus for our cars. To that end I worked out a rough idea of what the total open area is for the 3.2 Carrera and it's plastic grill.

As can be seen in the photo, it's not open all the way to the edges, and all the components that form the grill are a single piece.

I calculated that the potential open area measures 745 by 182.5 mm, or 1359.625 cm2. For Obstructions I calculated each finger as 172 by 5 mm for which there are 11, and for each cross of mesh I measured 20 by 20 mm with each being 1 mm in width, for which I counted 99 per opening between vertical rails. Each of the three rails measured 180 by 18 mm. Total obstructed area cames to 35020 mm2 or 350 cm2

Thus, the total open area is just over 1000 cm2, mas o menos. I hesitate to ask what the restriction factor of the condenser measures....

http://forums.pelicanparts.com/uploa...1665897670.jpg

Very interesting results. I wonder if adding a ~1-2" mini-duck-tail to the bottom of Al's grill would be helpful...

The grille on a classic aircooled 911 without the rear spoiler is located in an area of negative air pressure while the air flows almost undisturbed over the rear of the car. The whole body works as an airplane wing (compared to the wing profile). That's why the car gets lighter esp. on the rear axle on higher speeds (even with the engine weight behind the rear axle!). The air is sucked out of the engine compartment.
If a spoiler is mounted, the grille then lays in an area of positive air pressure because the air flow is forced to overcome over the spoiler. This creates a slightly positve pressure and one exit for the air is into the engine compartment. This enhances the air flow into the engine compartment and so the engine gets better cooled and the AC condenser if mounted will also be more cooled by air stream.
Taken from the book "The Porsche 911 story" by Paul Frère
Paul Frère - Wikipedia

Conclusion: Every classic 911 benefits from the rear spoiler. It's also proved that the cars go little higher top speed with a rear spoiler. Here in Germany - you have to mount both spoilers (front and rear) if you want to mount a spoiler. Spoilers must be approved by technical inspection (TÜV). Reason is that the aerodynamic balance is disturbed with only a single spoiler (either front or rear), no matter what personal experience tells...

Thomas

More stability in crosswinds too.
Thats one of my favorite Porsche books. Paul Frère was a legend.

Very true. I have borrowed a duck tail in the past, and it made an appreciable difference in stability once up into triple digit speeds (mph, not kph). Regardless, I'm not installing one on my car. It's not at all the theme I'm after.

The 911S was first available with a front spoiler in 1971. They had no rear spoiler. Many (most?) G body cars were delivered with a front spoiler with no rear spoiler.

Porsche even stated that it was acceptable to run a front spoiler only, but advised against running a rear spoiler only. I believe Frére covers this in his book. Lots of photos available of 911 ST's as well as narrow body 911's running a front spoiler without a rear spoiler back in the day.

http://forums.pelicanparts.com/uploa...1666020336.jpg

http://forums.pelicanparts.com/uploa...1666020336.jpg

Absolutely true. Porsche started during the development of the 2.7 RS with aerodynamic experiments on the rear spoiler:

http://forums.pelicanparts.com/uploa...1666032371.png
https://i.imgur.com/ruvaKNu.png

Then they continued the works on the first Turbo and later the Carrera 3.0. Because more and more 911 owners started to mount the spoilers on their own cars Porsche reacted and in Germany they made a TÜV certificate to approve retorfitting the spoilers on cars originally without spoilers. Porsche feared that unexperienced drivers could be overwhelmed by the rear spoiler like it happened in the 2000s with the Audi TT mark 1. They originally came in 1999 without the rear spoiler. Then a few but serious accidents especially when the car crosses a hilltop the rear axle got lighter and the car lost traction and the driver was overwhelmed. So Audi recalled all early cars to mount rear spoilers and an ESP on them.

https://www.nytimes.com/2000/02/20/automobiles/audi-offers-tt-fix-after-5-deaths.html

Thomas

I was actually looking for some wind tunnel testing of a classic 911 the other day, would be interesting to see the smoke forms.

Nothing found yet, also the Frère book does only contain the pic I posted with the threads glued on the body... would be quite interesting indeed.

The Frère book contains a chapter about the aerodynamics development on the 911 with some quite interesting numbers on how about the spoilers affect the cW value / drag coefficient.

Thomas

Yup, Frère's book is wonderful. Ref. page 177, caption of photos of a G body car in the wind tunnel on scales:

Total lift with no spoilers is 397 pounds (he does not indicate at which speed this lift is generated other than to say "at maximum speed"), with 276 pound of that at the rear and 121 pound at the front. This is aero lift generated by the body shape.

Addition of a front spoiler and the "tray" (we call it a "whale tail") rear spoiler reduced total lift to only 38 pounds. Rear lift was reduced to 29 pounds with the tray, 68 pounds with the duck tail. So, we can assume that the front spoiler reduces front lift from 121 pounds to only nine pounds.

Now imagine the affect of leaving that 121 pounds of lift at the front while reducing rear lift from 276 to 68 (duck tail) or 29 pounds (tray). Pretty easy to imagine what a handful that would be, hence Porsche's recommendation to never run just a rear spoiler of any kind.

I wonder about that (rear spoiler only). I've not driven a car with that kind of aero imbalance, but my thought would be that the car would tend towards understeer at speed and premature front lock under braking. Not ideal, but I'd rather have that than run just a front spoiler and no rear from the sound of it.

Is there more to it than that?

You can get an idea about rear spoiler only from reading the 1974 road test from Car and Driver. USA market cars did not come with a front spoiler, making for some interesting comments from testers. Widely available by a quick search….

I've driven plenty of early cars with no front spoiler. Most of them hot rodded beyond their original performance levels by one of my R Gruppe buddies. I'm here to tell you, once up approaching and then into triple digits, they can be terrifying to drive. The front end lift makes them feel like an old pickup truck with a load of gravel in the back. They go quite nose high and really start to wander, with a two lane road being barely wide enough to keep it between the lines. Braking is unchanged, since weight transfer happens so fast. I could not imagine even trying to corner, without a good stab at the brakes first, to get the front end back down. Just an all around lack of stability.

My car, with its "S" front spoiler, is night and day different. As long as we're going straight, it's pretty darn planted. No wandering, no lift, good stability. Cornering at high speed, however, gets kind of "interesting" I take turn one at my local track (Pacific Raceways near Seattle) in the low 130's if I don't chicken out and lift well ahead of it. The straight away is flat (the end of a drag strip), but the piece of pavement we turn onto for turn one connects to that drag strip at what must be a ten degree angle downward. It's like flying off a curb in a way. They very strongly admonish even the drivers of modern cars to "never lift through turn one!", and far too many have suffered the consequences of ignoring that advice. Lifting almost ensures a high speed spin. It's terrible.

My approach is to either keep the throttle pinned, or to "lift and coast" before I turn in. Even at that, my car "wags its tail" to a very disconcerting degree. I've had experienced track day drivers ride with me and beg me to never do "that" again. Yet my son's SC, with its whale tail, and friends' cars with duck tails are rock solid through there. As long as we don't lift.

So, I've driven all four configurations under discussion both on the street and on the track. The progression in stability from no spoilers, to front only, to front with duck tail, to front with "tray" or whale tail, is very noticeable. Porsche knew what they were doing. I'm sure that surprises no one.

Very interesting, thanks. I've only ever driven them with spoilers front and rear (first tea tray then duck), so I haven't experienced them without. Even with the spoilers the rear will moves a very healthy amount when cornering at speed--a few corners I've been through I wouldn't want to try without one for sure!

https://www.caranddriver.com/reviews/comparison-test/a15142825/1974-porsche-911-vs-911s-targa-911s-carrera-archived-comparison-test/

Interesting here that they noted the front unloading by 60lbs at speed with only a rear spoiler. I can see why that would be so uncomfortable.

On my 73 RS clone , I enjoy the benefits of the front and rear aero in handling and cooler oil temps.
At the moment Kermit has a happy HP number of 230 at the crank , 3.0 cis engine , small port.
The aero package helps with the oil temps immensely, I don't think I have ever seen temps over 225 * even at Chuckwalla Race track in the summer . He runs just a trombone oil cooler.
He is fantastically stable at high speeds as well 143- 146 mph many times , he just goes .
On narrow body cars i'm comfortable up to bout 128-132 mph with a S front spoiler and stock deck lid.
Alignment and koni red shocks are part of the fun, and 60 series tires .

There is a lot of good to be said for the aero package of the Carrera RS

Ian

Kermit is a 1980 911SC

You want to have some real fun, find someone who will loan you a standard deck lid, sans spoiler. It will remind you of of that scene in Days of Thunder where Harry Hogge screams "loose???!!! I'll show you loose!!" and pounds the rear spoiler flat on Cole Trickle's car. It makes the car a lot more "exciting" to drive. ;)

Like I said, my son's whale tail equipped SC is so, so much more stable than my car. It does "wiggle" its tail every now and then, but I've never caught myself looking out the wing window while traveling "straight" exiting turn one at Pacific.

Jeff, Don't think I've seen this mentioned here, but a weak ground to the gauge can cause it to read high. I found this out on my car by accident when tracking down a similar hot issue. Gauge was saying 250, but cooler lines and engine were more like 200. In fact the thermostat to the front cooler was opening and closing because it hadn't reached temp yet, but my gauge was high. Did all the regular stuff and then one day by accident I turned the headlights on and the gauge went from 250 to 260.. ah ha!