911 rear spoiler - anyone here know about aerodynamics?

911 rear spoiler - anyone here know about aerodynamics?

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Orangecurry

Original Poster:

7,396 posts

205 months

Sunday 7th February 2021
quotequote all
So I'm thinking about reducing weight on the 993 - as a kick-start to doing some serious driving in the future.

A good place to do this, I am told, is the bits hanging out the front and back. Front is quite easy. Rear is not.

The rear engine-lid, and spoiler motor plus electrics, inner grill, spoiler-wall etc etc must weigh a decent amount. Just the actual spoiler weighs 2488g

The ducktail lid is an easy way to lose weight, but not only does it looks poor on a 993 IMO, the aerodynamics look completely wrong.

The RS spoiler looks 'correct', but as it's a big old thing, you are adding another 5kg? back into the wrong place?

Why has nobody made a single piece engine lid plus spoiler in the shape of the standard deployed spoiler?

Is it because proper racing needs a bigger spoiler on the back, and so the addition of weight is tolerated, and nobody else gives a toss? hehe



so I've marked the spoiler-wall, and the direction of air I assume it is there to prevent? To stop the engine-bay filling wih crap.

Anyone know about aerodynamics enough to comment on what the deployed spoiler is achieving, and what the ducktail is doing in comparison?

The angle is completely different, the ducktail being steeper and enclosed at the sides....

Nobody else cares do they? hehe



Edited by Orangecurry on Sunday 7th February 17:36

g7jhp

6,958 posts

237 months

Sunday 7th February 2021
quotequote all
Well it was always suggested you remove the front lower spoiler and rear tea tray spoiler on 3.2 Carrera's as per non-Sport models.

On the 993 I'd have thought you have two options. Either add a lighter ducktail lid or a lighter flat lid.

Quick google brings up J-Tec Composites Ltd who do a carbon lid at 1.5kg rather than 5.7kg for original steel lid (all their numbers).

They seem to have a website as Porsche Projects



Sure there must be other options.

g7jhp

6,958 posts

237 months

Sunday 7th February 2021
quotequote all
This graph is often quoted regarding front and rear end lift. Link here



As for ducktails probably worth searching on Google/Pinterest.

Design 911 do a fibre glass ducktail for the 993. Link here


Orangecurry

Original Poster:

7,396 posts

205 months

Sunday 7th February 2021
quotequote all
thanks yes I'd seen that one, and many others hehe but what I guess I'm asking is do you need a spoiler on the back of a 993?

I'm thinking of the Audi mk1 TT which allegedly killed everyone on the autobahn until they stuck a three-inch spoiler on the bootlid.

I'm happy to do without a spoiler, but what does the OE shape achieve, and does the ducklid achieve anything positive, or is it just for looks, with owners dreaming about the old 2.7?

Orangecurry

Original Poster:

7,396 posts

205 months

Sunday 7th February 2021
quotequote all
g7jhp said:
This graph is often quoted regarding front and rear end lift.
Excellent graph thank you, illustrating the point, but on the early cars.

So - does the ducktail 'work' on a 993, or is it just looks and it simply messes up the airflow?

Steve Rance

5,435 posts

230 months

Sunday 7th February 2021
quotequote all
The rear spoiler on my 964 does not appreciably effect the cars balance in high speed corners. Early on in my ownership the motor failed and at spa there was little difference through Blancemont 1 and 2 with the spoiler not deployed. I ended up buying a new motor because I like to deploy the spoiler to allow heat to escape from the engine bay - and I couldn’t stand red lights all over my dashboard. My experience in racing Porsches has been only water cooled cup cars. Again in these cars, the rear spoiler has a marginal effect on the cars handling. I would be more concerned with lift under the front axle at speed if I were you. There is a lot of weight over the rear axle of a 911. I would not be particularly concerned with removing the rear spoiler on a 993. I would however ensure that there was a sufficient exit route for hot air within the engine bay.

Orangecurry

Original Poster:

7,396 posts

205 months

Sunday 7th February 2021
quotequote all
Great experience - thanks Steve.

Steve Rance

5,435 posts

230 months

Sunday 7th February 2021
quotequote all
My pleasure. Good luck with the project.

Please update the forum with your progress

Wozy68

5,387 posts

169 months

Monday 8th February 2021
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OC.

I've been thinking seriously about reducing weight in my 993 (For when I whip your butt around the twisties in the Pyrenees).

Its been a lot of sweat and tears and cost a Kings ransom ..... but I reckon I'm nearly there now.

Decided to keep the rear seats in to add a little weight over the back as I feel she's riding a little high.

The doors, wings and bonnet are (I'm told) a super lightweight transparent weave carbon fibre. As you can see from the photo the bonnet is up so you can see weight reduction in the boot.

However after all this expense I'm especially proud of the lightweight wheels .... They did cost an absolute fortune and are made of a special titanium by all accounts. biggrin


Orangecurry

Original Poster:

7,396 posts

205 months

Monday 8th February 2021
quotequote all
Your car has never looked so good.

Are those guns on the front scuttle? They won't help you.


Wozy68

5,387 posts

169 months

Monday 8th February 2021
quotequote all
[quote=Orangecurry]Your car has never looked so good.

Are those guns on the front scuttle? They won't help you.


OC, they're just the bonnet hinges. NLA in transparant carbon weave .... Sorry mate, I just thought you'd have realised what they were. wink

F6C

455 posts

37 months

Monday 8th February 2021
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Obvs the details can be fiendishly complex re aero, but broad brush in this context is pretty simple. It's about spoilers versus wings.

Neither the old 2.7 RS ducktail nor the active pop-out on the 993 is a wing sitting in airflow. Instead it's a relatively simple spoiler device designed to disturb an area of low pressure created by the sloping roofline of the 911.

In short, the 911 shape generates a large pocket of low air pressure behind and above the sloping rear deck of the car. This not only generates lift, but also drag - ie the location of the area of low air pressure generates pull on the car upwards and backwards. That's why by disturbing that area of low pressure, the ducktail not only reduces lift, it also reduces drag. Which is pretty clever despite the devices being pretty simple.

But contrast, a wing higher up in the airflow would also reduce lift or contribute to downforce depending on net lift, but would increase drag.

The 993 RS 'spoiler' is for once correctly termed. It, the 2.7 duck and the 993 C pop-out are all spoilers designed to disturb undesirable airflow and its consequences, whereas something like the high positioned wing on a 991 GT3 RS is intended to have air flowing across its own surfaces. Basic concept is quite different.

Long story short regarding the original question, the finer details won't matter too much so long as it's roughly the right size. Things like the angle of the duck versus the pop-out and the 'enclosed' or open sides of the duck / pop-out won't be hugely consequential. The reduction in lift isn't generated by air hitting the spoiler and pushing down on it / the car. You mainly want something sticking out into the area of low pressure. Something in that location on the rear deck of roughly the right size will do the job of disturbing the big pocket of low pressure.




Orangecurry

Original Poster:

7,396 posts

205 months

Monday 8th February 2021
quotequote all
Fantastic - thank you very much.

It's good to understand stuff - spoiling airflow is not size/shape critical. Perhaps I can stick the wife on the back?

TDT

4,910 posts

118 months

Monday 8th February 2021
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I thought it would be something like this?

Crudely - The pop-up spoiler achieves the following...



Raising of the spoiler creates a greater upper surface area to keep smooth fast airflow attached to the car, and also creates a low pressure area under it and cleaning up the trailing edge.
The air on top of the spoiler is flowing faster creating a higher pressure and because high pressure alway acts on low pressure the spoiler is pressed down.
The balance of this interaction determines how much negative lift/downforce you have.
Dependant on the desired outcomes..the size and placement of the wing also contributes as this has a lever type function so dependant on its placement, the front has to also have some treatment in order to stop the front lifting/lightening....

Happy to be corrected..


Edited by TDT on Monday 8th February 18:14

Orangecurry

Original Poster:

7,396 posts

205 months

Monday 8th February 2021
quotequote all
I think those two answers are complimentary?

So is it the bigger, specifically wider footprint of the RS M002 spoiler the reason why Porsche also added small spoilers on the front corners?

TDT

4,910 posts

118 months

Monday 8th February 2021
quotequote all
Orangecurry said:
So is it the bigger, specifically wider footprint of the RS M002 spoiler the reason why Porsche also added small spoilers on the front corners?
That would align imo.

Yellow491

2,911 posts

118 months

Monday 8th February 2021
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Leave it alone orange,2.5 kg makes no difference.
If you have to do something fit the standard street version rs tail.
Dont take wait out of the front,you have a huge variable in the fuel tank.
Best advice good tyres and set up.smile

braddo

10,399 posts

187 months

Monday 8th February 2021
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Steve Rance said:
The rear spoiler on my 964 does not appreciably effect the cars balance in high speed corners. Early on in my ownership the motor failed and at spa there was little difference through Blancemont 1 and 2 with the spoiler not deployed. I ended up buying a new motor because I like to deploy the spoiler to allow heat to escape from the engine bay - and I couldn’t stand red lights all over my dashboard. My experience in racing Porsches has been only water cooled cup cars. Again in these cars, the rear spoiler has a marginal effect on the cars handling. I would be more concerned with lift under the front axle at speed if I were you. There is a lot of weight over the rear axle of a 911. I would not be particularly concerned with removing the rear spoiler on a 993. I would however ensure that there was a sufficient exit route for hot air within the engine bay.
Interesting. I wonder if a difference would be more noticeable if the car were on skinny little 185 tyres for example.

Then again, my mind gets drawn to this video and a lack of splitters and spoilers doesn't stop a 60s 911 being guided around Spa at an astonishing rate. biggrin

https://www.youtube.com/watch?v=CvDD8V9Liq8

For a 993 maybe the difference is only going be apparent if you're doing 150+ mph on the autobahn on a gusty day. Oh, and by reducing drag (as per excellent post further up) it gives a higher top speed to brag about. hehe

Orangecurry

Original Poster:

7,396 posts

205 months

Monday 8th February 2021
quotequote all
Yellow491 said:
Leave it alone orange
hehe how many times have I heard that advice....

Anyway - no just the spoiler on its own weighs 2.5kgs.... I want to get rid of the metal engine-lid+spoiler motor+ inner grill+ wires and the spoiler, and replace with a lightweight engine-lid and something to spoil the air, or not bother.

I'm thinking 'not bother', and put a featherweight mesh over the lightweight engine-lid hole = better cooling as well. Same as the older 911.

Wozy68

5,387 posts

169 months

Tuesday 9th February 2021
quotequote all
Orangecurry said:
hehe how many times have I heard that advice....

Anyway - no just the spoiler on its own weighs 2.5kgs.... I want to get rid of the metal engine-lid+spoiler motor+ inner grill+ wires and the spoiler, and replace with a lightweight engine-lid and something to spoil the air, or not bother.

I'm thinking 'not bother', and put a featherweight mesh over the lightweight engine-lid hole = better cooling as well. Same as the older 911.
How much for the spoiler .... Remember I'm poor. CASH payment wink