(Not sure if this is the right place, mods please move accordingly if necessary.) Alright, inspired by a recent thread I posted in, I decided to create a new thread on wings and spoilers, the difference and what they do. There will be a lot of words here, so if you don't like reading, sorry Also, following is my understanding of "How Things Work" based on my reading, research and studying. I may be wrong. First thing to understand: Wings and Spoilers are two different things, totally different in concept and how they work. Unfortunately, a lot of people talk about them like they are the same thing, which makes things really confusing. So just always keep in mind: Wings are not Spoilers, and Spoilers are not Wings. If someone talks about them like they are, they are about to give you incorrect information. Now, with that in mind: Wings Wings, or airfoils, are used with the express purpose of generating downforce. They are essentially upside down airplane wings. Where an airplane wing is designed to generate lift (so they can fly), a race car wing is designed to generate negative lift. Wings (which may be a single airfoil or consist of multiple airfoils) force fast moving laminar air flow upwards, which pushes the wing (and whatever it is attached to) downwards. So, the good: they generate downforce which pushes our cars to the road. This is great, especially for fast cornering. The bad: they also generate drag. This relationship is proportional: typically the more downforce the wing provides, the more drag it must therefore also create. There is no free lunch - with more downforce comes more drag. In racing, this trade-off is acceptable, as the cars typically are traction limited and NOT power limited (see end of post for a description of this). So, even though the cars give up some top-end speed (because of drag) and some acceleration (because of drag), the speed they are able to carry through the corners due to the greater downforce more than outweigh the negative of the drag. Wings need to be mounted high enough so they encounter the faster laminar air flow - on a production car this is typically a bit below the roof line. On a purpose-built car like an F1, the wing can be lower as the body is designed to channel air to it - however, one will see that if the engine air intake is thought to be the "roof line", the wings are usually just a bit below this, or thereabouts. For production cars, wings that extend above/beyond the roof line tend to have diminishing returns, because at this point the wing is drastically altering the shape/slipstream of the car, and producing A TON of drag/downforce. If the car is severely traction limited and has an overabundance of power, this may be necessary. However it is still extreme. A real-world example of this is F1 cars. They have a wing on both the front of the car and the rear. This creates drag, however the cars have more than enough power and they can go faster through corners. This is due to them running on (essentially) flat tracks. The downforce keeps them on the track through high-speed cornering. However, the cars have to be traveling fast enough for the wings to do their job. Going through a corner slowly is actually more dangerous/harder than going through the corner at a much higher speed. Because: downforce. There isn't enough at the slower speed, thus the car has no grip. At higher speeds, there is downforce, and magically there is grip when you thought there were none. Spoilers Spoilers are a different animal than wings - they are placed in the same spots (in the front, also called an "air dam" and in the rear). The job of a spoiler is different than a wing. To understand, one needs to realize that on a production car, the worst flow area is the back of the car/trunk area. The air, as it leaves the trunk, does a bad thing. It angles to the ground as it flows off. At slow, legal speeds it doesn't really matter. At higher speeds, this air flow can start to generate lift on the rear end. It could make your rear-end start to feel loose. Porsche had this problem, thus the iconic "whale tail" on their 911's. Ideally, the air coming off the trunk needs to be encouraged to go elsewhere - just not down. Enter the spoiler. The spoiler creates a pocket of stagnant air between the rear window and the spoiler itself. This dead zone encourages the faster flowing laminar air to travel over it. Air, like fluid, goes to where ever it's easiest to go. If it's too hard for air to travel into a spot (like the dead zone created by the spoiler), it will instead go around (or over) it. In doing this, the spoiler actually encourages air to slide off the rear car easier than normal, and results in a car that cuts a "slicker" path through the air. It actually reduces drag - unlike a wing. Of course, it doesn't specifically generate downforce either. However, what it does do is eliminate the rear end lift cars can experience. So it doesn't really create downforce, but rather cancels out the lift produced off the rear of production car rear ends. Nascar uses spoilers - that big wall looking thing on the rear. Initially, one might think that it creates a lot of drag. However, it does not. It does the opposite. If you think about it, Nascar's have those huge banked turns - the car riding up on this turn generates the downforce for the car - the car itself does not need to. So, Nascar runs a spoiler up front and a spoiler in the rear, to make the car slicker through the air stream. (Note there is still downforce generated via the smooth underside of the car). I suspect if a Nascar were to stay on flat tracks like F1, they would be using wings rather than spoilers. Also, most production cars use spoilers - though a lot are incorrectly referenced as "wings." Our stock SN95's fall into this category. The stock wing on the Mustang is actually a spoiler, not a wing. Most production cars have spoilers back there - even if there are gaps under the "wing" and trunk lid, the air won't travel under there - it's too hard and remember air doesn't like to go where it's too hard. Instead, these act like spoilers. Which manufacture's like - reduced drag typically results in better MPG, the potential cancellation of lift on the rear results in better handling (think sports models of production cars), and they don't really care about downforce (thus no wing) because one would need to exceed the legal speed limit to benefit from them. There are few exceptions, like the STI and EVO that have big wings - but the manufacturers position these cars for track use (whether or not the respective owners actually use them on the track). Summary, and Which Do I Want, If Either? So, quick recap: wings make downforce (good) and drag (bad). Spoilers negate rear-end lift (good), make the car smoother through the air (good) and don't make downforce (meh). It would seem that spoilers here are better, as there aren't really any negatives (maybe other than styling?). However, it all depends on what you do with your car. If you have a production car or drive on the street exclusively, then spoiler all the way (even little stock "wings"). If you race the car, and you need more downforce in the rear, then you pretty much have to go with a wing. How aggressive of wing (referred to as "angle of attack") depends on how much downforce you need and how much resulting drag you are willing to live with. If your race car is low on power, you would be better served by a spoiler. It won't hurt your run up to top speed on the straights. You should be able to carry enough speed through the corners - your car will be able to generate only so much speed. Think Spec Miata's. The vast majority do *not* have wings. They are power limited. If, however, your car makes gobs of power, the straights won't be a problem. Carrying that fast speed through the corner will be. You will be traction limited. If you find your rear end getting loose a lot on the corners, you may want to run a wing to provide some downforce. Depending on how aggressive you go with the wing, your power will offset the generated drag. Cars that tend to be very, very quick (like maximum attack/time attack type cars) - which also tend to have an overabundance of power - will run larger wings as they can use the greater downforce on the corners. And the power of the engine will overcome the drag. They will ultimately lose some top-end and acceleration, but with that much power, they have much more to gain in the corners then in the straights. So - in terms of our SN95's: If you are at stock power (like me, at 185 rwhp), run a spoiler. Having a spoiler is better than having nothing at all - because of the rear-end lift that can be generated. Though, it depends on how fast you get up to, you may not even notice it getting squirrely - in this case no spoiler would be fine. However, the benefits are there, so better off running one regardless. If you have decent power - you have a decision to make: are you carrying the speed through the corners that you want? If not, is it because your front gets loose or your rear? If you find that you are over-steering through the corner, you could tinker with your anti-roll bars, OR maybe you need some downforce in the rear. If so, a fairly non-aggressive wing might give you the bite you need without too much drag. If you have gobs of power, you probably want a wing. Possibly with an aggressive attack. The amounts of drag created will be compensated for by your power, and the added downforce will help you keep your speed up through the corners. This is just a guide - aero has to be tuned in conjunction with your suspension, anti-sway stiffness, tire pressures, etc. Further Reading: Traction Limited vs Power Limited On a race track, your car will accelerate, and go through corners. What the tires do here dictates whether your car is Traction Limited or Power Limited. Which one your car is dictates how you drive it. If you smooth into the throttle and get on it, if the rear tires spin - you are traction limited. In other words, you have enough power to break the tires loose and more power will have no benefit. The speed at which the car accelerates is based on the grip, or traction, of the tires. If, instead the car accelerates with no complaints or slipping from the tires, you are probably power limited. In other words, the car doesn't have enough power to break the tires loose. In a corner, this has an effect on how early one get on the throttle. Cars with lots of power have to smooth in the gas pedal or else the rear tires will spin and you find yourself maybe going sideways. On lower power cars, like Miata's, they can pretty much stand on the gas pedal almost right away (and in some cases, depending on the corner, they don't even lift) - there isn't enough power to break the tires loose. Keep in mind that traction/power limited changes with suspension and tires. For example, a lower power car with really, really crappy tires can seem traction limited (think of stock Mustang going around a track on spare tire donuts). Totally traction limited. However, put on a grippy set of R-compound tires and now all of a sudden that same car is power limited. Is the Rear End Design Really That Important? In a word, yes. The design/shape dictates how the air flows off and what behaviors are exhibited by the car. For production cars, it almost always never matters because 99% of them are driven at or below legal speeds. Race cars are different. As a point of example, take a look at a 1964/65 Shelby Daytona. Look at the rear, and that crazy slant it has going on. Peter Brock, when designing the car, specifically made it that shape so it would cut through the air better - the angle helped to separate air off the rear of the car and reduce drag. This was necessary to hit the higher top speeds to compete against (and beat) Ferrari. Believe it or not, if that rear end was vertical, or slanted in the other direction, it would have disastrous effects on the handling/slipstream of the car. Since, on our production cars, we can't realistically change the shape of the rear end, we instead use wings and spoilers to accomplish the effects we are looking for. Splitters and Diffusers Works similarly as wings and spoilers, but with a different focus. While spoilers and wings deal with air that travels over the top of the car, splitters and diffuesers deal with the air that travels *under* the car (although splitters can also generate downforce on the front of the car - and do; the amount of downforce is related to the size of the splitter). For them to be really effective, the bottom of the car should be smooth. Our Mustangs (and most older generation cars) are anything but smooth. Look under your car. See all that suspension stuff, engine and transmission bits, exhaust pipes, frames and stuff under there? Very bumpy. Not smooth at all. So splitters and diffusers on our cars are not super effective. They help, I'm sure, but the full benefit is not seen. One would have to install a full under-body tray to smooth it out. If one were to look at newer cars underneath (like an Audi A4), you will start to see a lot of plastic trays under there - it will look a lot smoother. On these types of cars, it's done to increase the efficiency of the car moving through the air, in the quest of the ever increasing MPG. They tend to not use splitters and diffusers though, or at least not aggressive ones like race cars. The splitter's job is to encourage air to travel under the car and speed it along. The under-body tray's job is to encourage this fast moving air to keep moving fast (a bumpy, not-smooth underside slows the air down, and starts to create lift, which is bad). The diffuser at the rear's job is to help the faster moving air re-integrate smoothly with the slower, low pressure air zone at the back of the car. These things working in concert can generate considerable downforce - in fact, roughly half of the downforce on a F1 car is made via this method (the other roughly half is made by the combination of the front and rear wings). If the air makes it through faster under the car then the air traveling over it, the whole car essentially turns into an upside-down airplane wing, thus downforce (and drag!!). So if you really want to get the full effect of a splitter and diffuser, you would do well to try to smooth out the underside of car. The danger, here, is to encourage the air to move quickly under the car, just to have it slowed down by the bumpy underside, and now all this air the splitter is sending under there starts to stack up and stagnate, creating a low pressure zone - and if the air over the car travels faster, you have now turned the whole car into an airplane wing - you will get lift. And this is worse than if you didn't run a splitter at all.