leejo

Hi all. Quick 2-part question -

I was just reading about F1 having two DRS (Drag-reduction system) zones at the Hugarian GP this weekend, and wondered if the same principle works for the 911. So the question is:

1. If the spoiler is up and you push the button by the fuse box will that retract the spoiler even at >75 mph? (I'm 1500 miles from my car at the moment or I'd just drop everything and find out myself!)

2. If yes, then is that a terrible idea on an interstate? Increased gas mileage vs. liftoff...

Macster

Unless Porsche changed things after 2002 you can't lower the spoiler with the button once it has been raised by the car obtaining the deployment speed.

IOWs, once up from speed it stays up until the speed drops to the retracting speed.

(IIRC (it has been about 10 years since I last played with my car's spoiler button) even if one raises the spoiler manually one can't lower it once the deployment speed has been reached nor can one lower it even if the vehicle's speed is below the deployment speed but still above the retract speed.)

Given the big push and pressure from regulatory agencies in various regions to lower fuel consumption and reduce CO2 emissions that Porsche continues to raise the spoiler at approx. 75mph and lower it at 50mph (or 40mph in the case of my Turbo) says that Porsche feels very strongly the spoiler needs to be up at highway speeds. Whether it is for better vehicle stability or improved engine cooling or better overall vehicle aerodynamics or even if the spoiler up makes zilch difference in fuel economy, I can't say.

leejo

Thanks. I'd be amazed if the the spoiler is the only thing allowing a 911 to perform highway maneuvers that Honda Odysseys routinely accomplish - not flying of the road at speed on an interstate - but cooling, etc...who knows. I'd also be amazed if lowering the spoiler turns the car into a Prius, consumption-wise, but over the course of a long road-trip it might make a mea$urable difference, so I thought I'd ask.

996pp

Dude....if you're worried about mpg then you should have bought a prius

MiamiC70

Is it just me or is this forum taking an "odd" turn as of late?

leejo

It's just a question guys! But what is the spoiler doing on an interstate at speeds minivans routinely manage without flying into the woods? Besides looking awesome, of course. DRS on F1 cars reduces drag enough to gain about 15kmph on straights. Made me wonder if dropping the spoiler on a long straight-ish drive isn't a bad idea, really. Hard to believe that the spoiler is critical at 80mph but not required at 70mph...

Of course I don't want a Prius, but if I can push a button that saves me $20 on a weekend roadtrip by disabling a feature I don't need and am not really using, I'll take that free case of beer over throwing $20 out the window, sure.

leejo

I may have answered my own question from the physics end - academic apparently but whatever.

http://forums.pelicanparts.com/porsc...-spoilers.html

The link has a discussion and some reference material about the drag and downforce effect of various 911 spoilers vs. no spoiler. If I understand the charts correctly, at 90 mph the deployed spoiler is roughly analogous to a 50-lb bag of sand on the rear axle. Sadly, I doubt that's enough for my hypothetical free case of beer, even on a long haul, but neither is that going to make any performance difference at all on public roads unless one is driving VERY illegally. In the USA, anyway.

Macster

Well, neither my 02 Boxster or my 03 Turbo have instantaneous fuel consumption but my 06 GTO did and I can tell you the spoiler would have to be a frickin' air brake to affect mileage to any appreciable degree that my right foot affected gas mileage

Even steady speed cruising instantaneous fuel consumption would deteriorate from say 26mpg to the low '20's with any incline. The only way I found to help keep this somewhat in check was to disable cruise control and use my foot on the gas pedal and keep the throttle at the same position.

Now this meant the vehicle lost forward speed going up the incline -- sometimes (depending upon the grade) a lot of speed. Many the time the vehicle speed of 75mph (the legal limit) on level stretch would drop to below 55mph on a long incline. Towards the end or summit I was getting passed by big rigs, campers, even minivans.

We are talking about a 6.0l V8 with 400hp/400ftlbs of torque with a 6-speed manual transmission.

On the downgrade the vehicle speed would pick up, increase, and fuel economy would noticeably improve but often I would have to lift my foot off the gas to avoid going over the limit or to slow the vehicle down when encountering slower traffic. Often I would pass the same vehicles on the way down that had passed me on the way up. Overall I found this to be a tiring way to drive (and a bit more risky as it had me passing or being passed a lot more than I liked) and resorted to using the cruise control and watching fuel consumption drop sometimes from say 26mpg to under 20mpg to even on some particularly long steep inclines down into the mid-teen's.

At other times, on the other side of the mountain going down a downgrade, though the fuel consumption would improve some (and no coasting involved) so that over say 150 miles the average fuel consumption would 26mpg or thereabouts. (Which BTW is very close to what my 03 Turbo gets when driven over the same route the same way.)

Even on level ground just a dip of the right foot had the instantaneous fuel consumption going up. At low rpms -- say 1600 or so -- in 6th gear instantaneous fuel consumption might read 35mpg (who needs a hybrid?) -- but then with just a nudge on the pedal this drop by 10mpg or more.

BTW a semi-hard launch, with no tire spinning but leaving the line as quickly as I could and hard accelerating up through the gears would have the instantaneous fuel consumption reading the mid-single digits. While the acceleration was exhilarating it was dampened somewhat by the fact the whole time it was happening fuel consumption was 5mpg. I could feel the vacuum in my wallet.

Now some years ago (circa 1996 or thereabouts) a car magazine did a "spoiler" test on IIRC a Mustang Cobra. What it found was with the spoiler present or removed lap times, corner speeds/times and straightway speeds were all unaffected to any measurable degree.

Thus the opinion was the spoiler was not providing any noticeable lift reduction or extra down force. There's this side of the issue.

Then in coast down tests with the spoiler present and removed found the car slowed a bit faster, lost speed quicker with the spoiler present vs. with it removed and thus the opinion was the spoiler was in fact presenting a slight drag to the car. But enough to make any real difference to fuel economy? Not really.

And I believe the Porsche spoiler has the same negligible effect on fuel consumption, but I suspect (hope at any rate) it has a much greater effect on aerodynamics than the Cobra spoiler.

But you are welcome to run a test. Drive a distance at a steady speed below which the spoiler deploys so it is of course not deployed. Note the fuel consumption. Then drive this same distance at the same speed but with the spoiler manually deployed. If there's any noticeable difference in fuel consumption that can't be accounted for by other things like wind or climatic conditions, or traffic I'd be surprised.

I have to admit that even if it were found to affect gas mileage no way I'm driving either one of my cars at speed with the spoiler disabled so it would not deploy in order to save any gasoline.

Litesonic

wanting to know more about the physics of the car (and coming at it from seeing F1 benefit) is not something to be scolded surely.

Macster

Well, illegally to be sure but still one can encounter at say 75mph or 80mph (the legal limit on a few highways in TX) enough head wind that the car's effective wind speed could be in excess of 100mph. I've driven more than a few times with the winds blowing 25mph or higher. One can only wonder what the car might do at that speed with a good headwind.

Also, 50lbs of downforce isn't much but how much lift would there be without the downforce supplied by the spoiler?

In one case enough that cars would leave the road at speed. Which cars? The infamous early Audi TT.

leejo

Thanks for the feedback, folks. After poking around a bit it appears the spoiler reduces drag, which would indicate that mileage improves with the spoiler up rather than down. The F1 cars have wings that produce downforce (and lots of it!) whereas our spoiler instead reduces lift by disrupting the drag that otherwise would naturally occur in the area between where the air wants to flow - in a straight line parallel to the ground from the top of the car - and the lower part of the rear of the car (right where the spoiler is, duh), causing the rear to want to lift. So while the effect may be minimal at USA highway speeds, it appears to improve both handling and efficiency whereas the F1 wing improves handling at the cost of efficiency such that the DRS increases speed just by opening up a little gap on the wing. For those who don't follow the sport, those cars get over 1g of deceleration, more than any production vehicle including the Bugatti, just by lifting off the gas, so the drag they're dealing with is "non-negligible" as we used to say in physics classes.

I'll test when I get home but it appears that, without mods, the driver cannot override the spoiler's deployment at speed anyway. As I said, just curious...

Cheers!

Macster

Oh yeah. Lots of downforce. F1 cars top speed is drag limited. Very light cars with IIRC 800hp and with a top speed of IIRC 300kph to 350kph. Downforce is so high (up to 1.5 times the weight of the car at cornering speed of around 260+ kph) that drivers are (or were) experiencing up to 5 g's.

With so much downforce and drag any adjustment of the spoiler to reduce downforce would pay off big dividends in less drag and thus fuel economy would go up. Cornering speed would go down along with lap times going up though.

Craig Peebles

You can pull the fuse out ,but the warning keeps popping up that the spoiler is not deployed.

808Bill

Wow, 7 year old "dead thread" brought back to life!