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When's a tail necessary?

I understand that the tail on a 911 helps downforce on the rear end at high speeds. I also have read that having a duck or whale is even more important when a front chin or S spoiler is added, but I've never really known at what speed these aerodynamic aids become really necessary. 80 MPH, 100 MPH, 125? Street or on the track? Whats the scoop......?
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Old 01-12-2007, 02:10 PM
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My understanding is that you can run a front chin spoiler without a tail, but not visa versa. I used to run no tail and no chin on my '84 and it felt floaty above 100. My 964 had a tail and front spoiler of sorts and it felt more planted at speeds as low as 60 mph and definitely felt secure at speeds over 100 mph. I don't have experience with the chin only, but I would assume that it is more similar to running without either, than running both. For the speeds we "should" be running on the street, you likely do not need a tail with or without a chin spoiler.
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Old 01-12-2007, 02:42 PM
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Paul Frere talks about reducing lift with chin spoiler and rear trays at the 140mph mark, so we're talking a good clip. They can reduce the total lift effects of wind by hundreds of pounds at max speed. The effect, of course, is all relative to speed. I don't think the factory equipment quite reaches the point where they create downforce. Without them it's kinda like you're driving around a high speed bend while four guys are trying to lift your car off the ground with 300lbs of force. The tires have less weight on them, hence less cornering and braking ability.
So I think this doesn't become a tremendous factor until you're travelling at greater than 100mph. Maybe some of the racers (Jack O )will comment the differences they notice with their different spoilers.
I've also heard that a rear spoiler without the front dam will result in noticable understeer, which can happen if one were to jump a curb and knock the chin spoiler off. Personally, I have been very careful to avoid losing traction at 100+ MPH so I can't comment on what it feels like to slide a 911 around at speed. I have had the rear come out on me at 60 or so, but that was a throttle lift off situation. I know my car's default mode is power on understeer and it does have a Carrera spoiler.
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Old 01-12-2007, 02:45 PM
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YOu need a tail when you have no other way to cover the intercooler.
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Old 01-12-2007, 02:51 PM
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I had an 86 Carrera without a tail or front spoiler. I hit over 140 mph many times during DE's and the car never spontaneously spun out. Well it did once during braking for turn one at Summit Point, but one of those water pumpers blew a head gasket and put antifreeze on the track. I don't think a tail would have really helped much. Can you imagine hitting your brakes at 140 and your car immediately starts spinning? That one took several re-wipes.
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Old 01-12-2007, 03:11 PM
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_Very_ generally speaking, aerodynamic aids begin to kick in at about 80 mph, whether we're talking about feathers for the A pillars, eliminating the detached-flow step where the cabin ventilates onto the rear window or more brutal aeros. At 80 the effects are doubtless minimal, but if you tuft a 911, you'll begin to see them. Steve Weiner has done exactly that tufting, and knowing him probably laid on the roof to film them, so he's your go-to guy.
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Old 01-12-2007, 03:18 PM
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somewhere i heard tails on a 911 are mainly to disturb the vacume created above the rear end to prevent lift of the rear end rather than downforce. because of the shape of a 911 which is a rather like an airplane wing it naturally causes alot of lift
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Old 01-12-2007, 03:32 PM
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As pretty as the classic 911 profile is, it's an airfoil. A more aerodynamically correct profile would have been a, gawduh forbid, hatchback/stationwagon (Kamm effect).

I've only had my SC to an indicated 135 and the front felt la bit light to me. I percieved it beginning to get light at about 90 - I'm guessing that without the tea tray and chin spoiler I'd have felt a bit unsafe beyond that speed.
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Old 01-12-2007, 03:37 PM
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But did Steve videotape tufting while simultaneously blowing party glitter through a 3/8-inch hose with compressed air -- while driving through rush-hour traffic in Los Angeles?

I've done a lot of testing with aero, and I've run everything from a stock lid to a 964 lid to a ducktail to the big 3.8RS tail with a 60-inch wing on extensions. And I've taped tufts, monitored rider height changes and logged a lot of comparative lap times.

And in my opinion, the concensus here is pretty dead on. At speeds in the 70-90 range, the car will feel a little more floaty without aero than with it. If you have other suspension issues (aging shocks, failing bushings, bad alignment) the floatiness can feel more exaggerated than if everything else is in spec. A front spoiler moves air to either side of the car more efficiently, and reduces the tendency of the high-pressure air ahead of the car to get pushed down underneath the chassis -- which increases the lift already being cause by air adhering to the fastback rear of the car.

A front spoiler does not only reduce lift at the front axle, it works on both (althouth not in equal amounts). A rear spoiler will reduce lift at the rear axle (and have a very slight impact on lift at the front). A rear wing can produce enough downforce to counteract the lift created by the body shape and the air under the car, but it has to be a very effective wing and you need to be going very fast before the net lift is cancelled out.

At old-school autobahn or track speeds, both a front spoiler and a rear spoiler will make a noticeable difference. And a front splitter and rear wing combo will help even more. Interestingly, the big gains in lap times usually don't come from the points on the track where you have the greatest amount of negative lift being generated by your aero aids. Willow Springs is a good example of this, where the aero actually makes you a little slower on the very fast (120-140 mph) sweeper and also slows things down on the front straight (also 120-140 mph). It's rare that a track has 120+ mph corners where a car is going to be making really extreme lateral cornering demands. But the 80-120 mph parts of the track actually pay big dividends with aero. At Willow, those are primarily turns 1, 2 and 9.

For street driving, the aero aids are never actually necessary -- if they were, Porsche would be sued for allowing the cars to be sold without the spoilers. And I don't know if any of us are cornering at high enough speeds and using enough of the width of public roads to see a real difference from spoilers or wings. But they can make the car more stable, and as a result more relaxing to drive. In my opinion, a sensitive driver might feel a spoiler package at 70 mph, and really appreaciate it at around 85 and up. North of 120 or so, the difference is going to be more dramatic.
Old 01-12-2007, 03:42 PM
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As alluded to by others here,Paul Frere mentions that a tail creates a positive pressure zone over the rear, rather than the negative pressure zone of the tail-less 911. I have often wondered if this positive pressure zone also helps with engine cooling??? Any comments (I'm a relative newbie here)???

Eric
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Old 01-12-2007, 03:42 PM
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Yes, it does. But the spoiler doesn't eliminate the low pressure zone over the back, it just tosses a high pressure zone (ahead of the spoiler) into the mix, and reduces the net effect of the 911's aerodynamically clean, but lift-prone shape.
Old 01-12-2007, 03:52 PM
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I installed a RS style carrera tail on my SC several years ago without the front lip spoiler and it was scary over 100 mph. A couple of months later I installed the factory front lip spoiler and the difference was amazing. With both on the car I could feel a big improvement at high speeds. This was before I found this forum and know what I know now.
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wow imagine the looks that got going down the road haha
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Old 01-12-2007, 05:05 PM
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Another benefit to the spoilers is a reduction in sensitivity to cross winds at highway speeds. This effect is probably more important than the reduction in lift, at least to most drivers on the street.

JR
Old 01-12-2007, 05:34 PM
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so is there a benefit in having a carrera/turbo style tail over the old school duck tail? or vice versa?
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Old 01-12-2007, 05:54 PM
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General consensus is that the early turbo-style tail is better than the ducktail and the later turbo tail is better than the other two.

JR
Old 01-12-2007, 06:02 PM
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Quote:
Originally posted by Jack Olsen
Yes, it does. But the spoiler doesn't eliminate the low pressure zone over the back, it just tosses a high pressure zone (ahead of the spoiler) into the mix, and reduces the net effect of the 911's aerodynamically clean, but lift-prone shape.
Yes, based on your feedback, I understand. Thanks much for that.

I wonder... If one were contemplating, say, a duck tail or no tail at all... Does the help with airflow into the engine bay caused by the positive pressure zone factor into the decision (better engine cooling)??? I would assume that there can not be THAT much a difference as the factory offered with or without tail... But...

Thanks for any feedback, learning a lot here.

Eric
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Old 01-12-2007, 06:13 PM
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My mechanic (preps race cars, trained in Stuttgart, etc) was just talking about this. didn't Porsche get in trouble at one point for fitting a front spoiler without the rear?

I've been told repeatedly that if your going to run one, you need to run both. But, that's running the rear without the front is worse than running the front without the rear.

As was stated, it was explained to me that the tail doesn't create downforce. It eliminates lift. Again, IIRC, ducks reduce ~150 lbs of lift, whale tail ~300 lbs.

- Skip
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Old 01-12-2007, 06:23 PM
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An added benefit to at least a duck is better fuel economy. When running mine, I get 2 MPG better on the highway.
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Old 01-12-2007, 07:45 PM
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Yes the ducktail is the only tail that both reduces lift and also reduces overall drag for the car. The Carrera and Turbo and later tales reduce more lift than the duck -- but they also slow the car down with increased drag.
Old 01-12-2007, 08:24 PM
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