Aerodynamics

[thebigwaldo]

Is there anything I can do to my car to help out it's aerodynamics? I'm hoping to get better gas mileage when going home from college, so that means I kinda need to keep everything legal.

Thanks in advance

-Jeremy

[CARdiac]

Everything you do for performance will more than likely decrease you fuel economy. Things that can be done...the faster you drive over 55 the lower your mpg, and it is exponential. Find a happy medium, mine is 70 mph (I still get 28 mpg vs the 24 at 75). If you have the AC on mpg drops...to get the best mileage, and I used to do this w/my SUV when I was on a long trip, I would draft trucks (you can stay 20 ft behind and still get benefits).

[BillyCobin]

Now I thought this 55mph = best gas millage was a myth. Think about how many things go into determining your gas milage. Tranny gear, rear gear, cam, exhaust, intake, RPM, T/C efficiency, drive train loss, width of tires. Then, of course, there is the most important variable, the driver.

Here is my theory/understanding of the way to get the most milage out of what you have. If you want the best milage, touch the gas as little as you can. If you set the cruise for 55mph one day, and 70mph the next day, as long as the weather conditions are the same, and you never have to go off of cruise, I bet your milage will be almost the same.

Now what happens is that when you have the cruise set for 70, as opposed to 55, you will have to slow down for slower people more often. Then to get back up to 70, you have to accelerate harder then you would to get back up to 55. So this constant slowing down and speeding up to try to maintain a higher speed is what causes the lower milage.

At least that's my theory. I have tried it out in real life and as long as you can maintain the speed, then just about any speed isn't much worse.

So, to increase milage, I think your best bets are:
Skinny tires = less roling resistence
Intake/Exhaust = tend to lower your RPMs
Air dams = dam up the front with a body kit, less wind resistence
like CARdiac said, turn off the A/C, and sitting in the sweet zone behind a semi will help too
Most important, try to maintain a constant speed, don't go WOT, and try to keep it in 4th with an automatic, and 5th manual.

[vtv6firebird]

Just a little fact I thought I would throw in. Increases in performance, especially with intakte and exhaust will ususally lead to increased MPG, not decreased. This is just due to the engine being more efficent. It all depends on the mod, some will cause increased fuel consumption, like a blower or nitrous. It just all depends on your mod.

To best in crease fuel milage, just keep your tires properly inflated and be easy on the gas pedal. The amount of fuel delivered to the car relies heavily on the TPS, throttle position sensor. There are other factors involved but the biggest contributer is still the TPS, so the farther you push down that gas pedal the more gas you are gonna use, so keep the acceleration to a minimum. Lowering your car can also help in fuel milage. Keep windows up and t-tops on. Having the windows open makes a huge difference in milage actually.

Hope this helps. I have made the 100 mile trip from school back home many many times in the last 4-years so I know how you feel about the gas. Good luck

[OZ]

Watch out! Drafting trucks is not a great idea. Our hoods eat gravel chips big time!
I found out the hard way.

[img]graemlins/crybaby.gif[/img]

[magic1872]

WEIGHT REDUCTION, INTAKE AND EXHAUST

[Arctc Wolf]

weight reduction, intake, exhaust, and some body kit stuff. A body kit allows less air to go underneath the car, reducing lift. gears would also help fuel economy, but hurt performance.

[thebigwaldo]

Well as far as economy goes, I can get 30mpg on the highway w/ windows cracked and no a/c. With the a/c on, I get 24~26mpg. I'm just trying to tweak the most out of those #'s

As for speed being a factor... Last year on my way to a summer camp, a friend and I drove 110mph the whole way, only slowing down when the radar detector went off (two or three times) or when we went through a town. We did all this with the a/c on and windows up. Averaged 25mpg...

btw, speed limit here 70.

Thanks for all the replys

-Jeremy

[CARdiac]

Ok, let me throw this out here. Increase air in = increase power...why? Because our cars register the amount of air going in and adjust the fuel to maintain an optimal ratio....thus more air=more fuel, more fuel=less mpg.

Now for the aerodynamics. The 55 mph was calculated based on a drag coefficient curve. It is a steep curve meaning that as you travel at slower speeds the drag you create (your car must over come this drag to maintain forward momentum)...as you increase the speed the force your car has to overcome increases, thus less fuel economy. How to reduce the drag. There was an article recently that showed how a ford F150 got something like 30+ mpg on the highway. how did they do it? They blocked every inlet of air on the front of the car. The air was then forced around the sides (all inlets create turbulance which equates to drag...that is why the firebird gets worse mpg at night, because the lights are up causing turbulance). They also blocked off the bottom front to force air around the bottom of the car...in addition they covered the bed and drove at 50 mph. What does it all mean? My 94 gets crap mpg b/c i have a lot of nice turbulance creators in the front...

Just out or curiosity. Have you ever been driving at 70 mph and it starts to rain lightly...whe you turn your wipers on (intermittanly) you can actully feel it in the stearing and acceleration (however slightly).

Now why does a Corvette get 29 MPG on the highway? It is geared so that it stays in low rpms at highway speed and burns less fuel...but it also has very good aerodynamics and an engine w/enough torque to not feel slugish at low RPMs. Yes gearing does matter......that is why the 6th gear on a corvette is so high...better economy.

Now it has been a while since I took physics in college...but I think I am pretty close.

Turbulance is drag which increases exponentially with speed (ie wind speed).

[magic1872]

hey artic, i have a correction for you. You said "A body kit allows less air to go underneath the car, reducing lift." If our car produces lift then it would make it lighter. Thus increasing our MPG.

[Backfire]

Turbulance does not always equate to drag. Golf balls have a dimples surface to promote turbulant flow. It reduces drag when compared to a smooth surface, and the ball flies further. The taping of the F150 probably reduced the coefficient of drag (Cd) and thus reduced the drag force.

Whatever the reason, I don't see how there could be a magic speed that all vehicles achieve the best mgp. Every engine is going to have different efficiencies for a given RPM. And every vehicle has different drag properties, gearing, tires, etc. Even if different vehicles used the same motor (Ex: LS1 in F-body and Vette), the other parameters would change the best operating point. I have only been able to find Cd and frontal area data on the Corvette. If someone can find that data for an F-body, you can estimate the power needed to travel at a given speed. But without efficiency and fuel comsumption data for an engine, it would be tough to calculate the point of best mpg.

[ August 19, 2002: Message edited by: Backfire ]</p>

[CARdiac]

Yes, sometimes turbulance helps. The dimples actually creat little vortex's that allow air to flow over the vortex smoothing the airflow....but actually the shape of the dimple counts imensely...but they are more for straight flight that for distance. VW added rings to the valve seats to help air flow (smoothen it out more to help speed it up) in the same manner.

[2KV6fbody]

There is not much you are going to do for free that will help mpg except easy on the gas and on the brake. Anything else you buy will subtract from what you save in gas money. Get too close to those 18 wheelers and you will find out how those brakes work. I don't know how much effect it has but a car runs better in cooler air thus evening runs may be more efficient and you don't need a/c.

[nova]

<blockquote>quote:</font><hr>Originally posted by Backfire:

Whatever the reason, I don't see how there could be a magic speed that all vehicles achieve the best mgp.
[ August 19, 2002: Message edited by: Backfire ]<hr></blockquote>

The only thing you can say for certian is that the slower you go the lest aerodynamic drag you will create.

Aerodynamic drag increases proportionaly to to the square of the velocity.

Just to clear some things up though. Turbulent flow does not equate to a vortex. Vortex's are horrible little drag demons that do nothing but slow you down. They are created wherever two velocities and hence two different pressures meet. Vortices on aircraft cause enough drag that Boeing and other companies spend billions on research to eliminate/reduce them.

Turbulent flow is sometimes good and sometimes bad. Turbulent flow across a golf ball causes the air to "stick" to the surface of the ball for a longer period of time which increases drive length due to the fact the majority of the drag on a sphere is due to flow seperation at the rear of the ball.

Depending on the object turbulent flow can cause more drag than laminar flow. Airfoils designed for laminar flow experience huge increases in drag and huge decreases in lift when exposed to turbulent flow. Just looking at an object, though, you can't tell if it will perform better in turbulent or laminar flow.

Just a few points from an aerospace engineer in training. [img]smile.gif[/img]

[Backfire]

<blockquote>quote:</font><hr>Originally posted by nova:

Turbulent flow is sometimes good and sometimes bad. Turbulent flow across a golf ball causes the air to "stick" to the surface of the ball for a longer period of time which increases drive length due to the fact the majority of the drag on a sphere is due to flow seperation at the rear of the ball.

Depending on the object turbulent flow can cause more drag than laminar flow. Airfoils designed for laminar flow experience huge increases in drag and huge decreases in lift when exposed to turbulent flow. Just looking at an object, though, you can't tell if it will perform better in turbulent or laminar flow.

Just a few points from an aerospace engineer in training. [img]smile.gif[/img] <hr></blockquote>

Yeah, on a sphere such as a golf ball, simply going by boundary layer theory, laminar flow would be better. But in actual experiments, boundary layer theory fails and separation occurs. With a smooth sphere, separation occurs at 82* from the leading edge, while it occurs at 120* in turbulent flow. That results in greater pressures behind the sphere during turbulent conditions, which reduces the Cd and pressure drag.

I only went through a few fluids courses before I graduated, but it seemed like streamlined objects saw better results from laminar flow, while blunt objects, like the golf ball, fared better with turbulent conditions. Is there any basis for this that you are aware of? Seems like cars would fall more towards blunt objects( have you looked at the rear of a corvette from the side ;) ).