Effective street legal underbody aero or marketing stunt?

[BobDole]

In place of the rear wing, Porsche's limited edition 911R features a rather crude looking rear diffuser:



Any guesses on how effective this might be on a street legal 911? Obviously you're limited by the engine layout, but I'm curious if this sort of design can be adapted to other 911s.

[giantfan10]

In place of the rear wing, Porsche's limited edition 911R features a rather crude looking rear diffuser:

http://rennlist.com/forums/attachments/ ... oto180.jpg

Any guesses on how effective this might be on a street legal 911? Obviously you're limited by the engine layout, but I'm curious if this sort of design can be adapted to other 911s.

per the Porsche head of Gt cars says it works ... watch this video

[J.A.W.]

Some road authority registration regulations mandate a minimum ground clearance..

That low-set under-tray presents as a venturi device, but perhaps Porsche 'officially'claim it to be a form of skid-plate..
..as 'safety' add-on protection for the grouped mechanicals there..

As for the aero.. I recall that even ~30 years ago on the W124 M-B - the considered attention to low drag aero included a
similar under-body flow path, with shaped covers on the suspension coordinated with the flow-profiled fuel tank/rear apron..

[gixxer_drew]

Functional is a relative term. For production car people 2kg of downforce and the marketing people will have a field day.

For a race car you make that downforce starting from a blank slate and taking an early coffee break would be pushing it.

[olefud]

The classical advantage of a diffuser is slowing the slipstream to increase the air pressure. This is particularly important at the vehicle rear to avoid a vacuum behind the vehicle. Not much downforce but still effective as discussed;

http://www.roadandtrack.com/motorsports ... upe-story/

[Tommy Cookers]

The classical advantage of a diffuser is slowing the slipstream to increase the air pressure. This is particularly important at the vehicle rear to avoid a vacuum behind the vehicle. Not much downforce but still effective as discussed;
http://www.roadandtrack.com/motorsports ... upe-story/

iirc the Daytona coupe featured a rising (rear lower body) tail with a flat end - also the story mentions Kamm's papers as the inspiration

mid 50s British cars won hundreds of races using a rising rear lower body line (D type Jaguar, Lotus 11 etc) mostly without a flat end
and hundreds using a rather non-rising rear lower body with a large flat end (Cooper T39 etc)
Costin (Lotus 11 body design) said the rising line etc cured stability problems by fixing CoP wrt Alpha and Beta (drift&crosswind) AoA
as a reflex aerofoil section fixes the CoP

does the rising line count as a diffuser ?
http://www.simoncars.co.uk/jaguar/dtype.html

the Lotus 11 and Cooper T39
http://www.lotuseleven.org/DarkAges4/headfairings.htm
http://www.racing70s.pwp.blueyonder.co. ... ttag12.htm

[J.A.W.]

To be fair, the marketing of needful race-aero on road cars goes back a ways..

The Aston-Martin DB 6 was a `60s Kamm tail job, & NASCAR raced fastbacks & shovel noses in the `60s too..

See the fairly radical Mopar approach here.. http://www.allpar.com/model/superbird.html

[olefud]

The classical advantage of a diffuser is slowing the slipstream to increase the air pressure. This is particularly important at the vehicle rear to avoid a vacuum behind the vehicle. Not much downforce but still effective as discussed;
http://www.roadandtrack.com/motorsports ... upe-story/

iirc the Daytona coupe featured a rising (rear lower body) tail with a flat end - also the story mentions Kamm's papers as the inspiration

mid 50s British cars won hundreds of races using a rising rear lower body line (D type Jaguar, Lotus 11 etc) mostly without a flat end
and hundreds using a rather non-rising rear lower body with a large flat end (Cooper T39 etc)
Costin (Lotus 11 body design) said the rising line etc cured stability problems by fixing CoP wrt Alpha and Beta (drift&crosswind) AoA
as a reflex aerofoil section fixes the CoP

does the rising line count as a diffuser ?
http://www.simoncars.co.uk/jaguar/dtype.html

the Lotus 11 and Cooper T39
http://www.lotuseleven.org/DarkAges4/headfairings.htm
http://www.racing70s.pwp.blueyonder.co. ... ttag12.htm

Tommy, I’m not sure going too far afield on my part is worth it to readers hereof. However, as you know aero importance increases exponentially with higher speeds. Your examples are from the small bore era. When Ferrari got the rules changed to include bigger engines the significance of aero also increased exponentially. The Daytona coupe bolted a well thought out body on the archaic AC chassis and cleaned house. The rear of the Daytona is all diffuser to increase pressure aft of the car’s rear. This was pretty much exclusively drag reduction –no apparent down force.

[J.A.W.]

Curiously enough, while the racing Alfa-Romeo TZ's also featured a quite similar rounded/chopped Cobra/Kamm style tail ..

..by `66, the 1st Ford to actually outright win a - 'Les Vingt-quatre Heures du Mans' - was not only a squat GT-40 of Lola design origins, & endowed with a mighty 7 litre NASCAR mill, it was also festooned with all manner of add-on aero-plates, sprouting these extra fins & spoilers.. to try & cope with the prodigious speed..

Even more curiously, the losing Ferrari LM from that year featured a 'flying buttress' inset rear window style..
.. as emulated by the classic `68-70 Dodge Charger, & this feature was also found wanting at NASCAR speeds..
.. so needed remedial drag reduction.. by the extending the window back flush.. with the sloping pillar edges..

Note: AFAIR, those be-winged Mopars were the 1st stock production bodied cars to exceed 200 mph, as recorded at Bonneville, regardless of claims by Ferrari for their 'omologato' 288 GTO, over a decade later..