Do Front Air Dams On Cars Improve Gas Mileage?

I like it, but with this caveat: it should be a 5x8 sheet of plywood, unless his rig is unusually tall and narrow. But that is 40 sf, which actually reinforces your point!

Hey Laughing Jaguar, long time no see. If you go to some of the modder forums like this, there are many many examples of people experimenting with all sorts of aerodynamic ideas that some have found to really work. There are plenty of engineers on some of these forums who employ good testing methods to validate some of the gains or losses. Increasing the Cd is usually not a good idea but there are methods that actually do work that seem counter intuitive. There are many economy modifications and methods of driving that work well and once entrenched as habit have long term benefits.

GA, even though you beat me to the ecomodder link! As an ecomodder member, I can tell you that it is a good forum that thoroughly believes in A-B-A-B-A testing to prove efficiency. They usually use coast down testing on some back road for aerodynamic improvements and it takes a lot of time to do it properly.

Remember, the greatest improvement made on any vehicle is the nut behind the wheel!

I could be wrong, but I believe the purpose of the air dam is to control airflow underneath the car (by directing more air up or to the sides), to reduce high pressure that would cause lift, thus effectively aiding downforce. As Tobugrynbak points out, it is only useful above a certain speed, and as lyn points out, it certainly will not aid gas mileage.

Also see #11. This is what I have heard--to shunt the air around the vehicle rather than under it. This reduces the lift (increases the down-force) so that there is better traction for cornering and braking.

Hi All!

One option suggested was to replace a chip in the computer. Apparently GM tailors spark advance and mixture to suite the chassis and application for that V6 engine.

I went looking. Sure enough there are aftermarket chips available that purport to improve mileage.

Friends who still tinker with cars on weekends suggest I forget changing the chip and use the money to buy gas.

Fortunately, I don't use the Van much. Occasionally for buying large sheets of lumber but mostly on Astronomy jaunts to haul large telescopes to star parties; maybe six times a year.

Thanks all who responded. I wasn't optimistic but thought I'd ask anyway.

Clear skies!

Laughing Jaguar

Well then, that's it. Beers all around.

Something I forgot to mention. It doesn't change anything but does add more weight to the arguments against increasing frontal area:

This van has a Hi Top Conversion that raises the roof line at least another foot. Already it has to push aside a substantial column of air. I agree: adding to the frontal area would hardly make things better.

The drag of the air spilling off behind the van is my next inquiry. I'll attach a bunch of knitting yarn and film the turbulence which I expect will be substantial. Given the abrupt transition, I doubt there's much influence to be made but I'll look.

Thanks again for all who responded.

L.J.

Take a couple sheets of that plywood we talked about earlier, make a pyramid out of it and attach that to the back of the Hi Top. It'll break up the turbulence, and save just enough gas to pay for the added weight.

I think I'm really clever today.

Good to see you back again LJ.

The air dam on the 3500 Express I am working on is fairly flay across the front. If yours is like mine, it might be possible to give it a bit more of a point in the center, to push the air to the sides. Thy should b easy to find in junk yards by now. Just get another one to screw with so the van can go back to original if results don't follow the theory.

Have you got room for a few inches on top of the back? I thought the way to reduce air drag on vans was to try to turn the air coming over the top to flow down the back doors of the van. Good luck trying to find a 70s van accessory catalog.

The primary purpose for the air dam was to increase the flow of air through the radiator to assist in engine cooling. The air dams on race cars is to reduce the air flow under the vehicle to create a low pressure area under the vehicle and therefore increasing the down force and therefore the vehicle has a greater tractive force allowing it to be able to increase its speed and give them a better chance to win. The best way to increase the mileage would be to break the vacuum area behind the van which actually holds the van back requiring it to produce more power to maintain its speed. This was helped by the placement of a wing on the rear top of the vans to reduce this drag by redirecting the air flow reducing the vacuum on the back of the van. These were found more on the older SUVs such as the Suburban. an indicator of how much vacuum that is at the back of your van is take a look how much dirt will collect on the back doors. A better solution might be to take the windshield out and leave the back doors open just make sure that you tie everything inside down are we will be able to track you by following the trail of trash.

Drag is proportional to air density, frontal area, velocity², and shape. You can't really change the first 2, but you can slow down and possibly alter the shape. As others have stated, increasing frontal area by adding an air dam will not help gas mileage.

Since many vans are "brick" shaped, the drag coefficient c_d is close to 1.0

If you want better gas mileage at a given speed, one good option is to make the drag coefficient as small as possible (c_d<0.4) by streamlining the vehicle shape.

It is usually impractical to make these changes on an existing vehicle, but it would make a significant difference at highway speeds. See link below for c_d of some general shapes...

http://en.wikipedia.org/wiki/File:14ilf1l.svg

Cheapest way to get better fuel mileage is to change the differential gearing from 3.76 to 2.76.

Probably pick up a set of spider gears from Yukon for less than $150 and it takes only about 4 hours do change them.

Most of that is jacking up the van, taking the tires and brakes off and pulling the axles, 15 minutes to change the gears, slide the axles in, then put the brakes and tires back on, fill it up with hypo and go.

25% better mileage is best you can hope for tho.

I don't think you have sufficient experience in the rear differentials used on the current automotive vehicles. Spider gears are the common term used for the small intermeshed gears that are in mesh with the axle gears that are splined to the axle shafts. These gears have no effect on gas mileage.

The ring and pinion gear set will cause changes in gas mileage and pulling power. BUT, these gears are one of the most precision components used in automotive field. In this world on interchangeability, the ring and pinion are always sold as a set. Never exchanged singularly. The gear set is set on a machine at the factory, and compared to an ideal set. From that point, it is measured, and engraved with an exact variance setting. This is then compared to the set coming out, and new spacers are selected to bring the ring and pinion set to an exact setting. If the old set was a "-2" and the new set was "+1", the pinion would need to be .003 thicker than the last shim.

Special tools are required to set a ring and pinion set. Definitely not for an amateur.

And another thing. The OP stated he had a V-6, and an added high top on the van. How well do you think that V-6 will be able to pull that brick with 2.76 gears?

If the thought is that the gearing will help, try a set of taller tires. That will allow lowering the engine speed just as well.

Some of the benefit of the air dam is supposed to allow the air to spill out around the sides of the vehicle, instead of hitting obstacles all along the way under the van.

I tried the air dam parts (different combinations of sides and front) from a later model on my Vanagon and it slowed down on a down hill coast test.

Changing the air filter on a computer controlled vehicle won't help the mileage, the biggest restriction it the throttle, and don't forget jackrabbit starts. In the 70's Mopar found that chirping the tires gave better mileage.

As noted already, at speed most drag will be because of the shape of the van's back end. A tear-drop shaped moulding attached to the rear would improve consumption, but only above about 80kph (50mph). Otherwise go for the gearing option ($150 and a few hours will give a quick return). This means you will do a little more gear changing on hills, but should be worth it in the long run.

Forget the frontal dam.

If you look at the front of any new era (1980's on) automobile and then look at anything built in the 1960's, you will immediately notice the apparent lack of grill opening, or at least significant reduction of such, on the newer cars. They are now "bottom breathers" and the air dam serves as much as a deflection of air into the radiator as it does as an aerodynamic aid. Don't know about your van, but I suspect it is similar.

Air dams may reduce viscous drag under the vehicle slightly, however they may also increase induced drag at highway speeds. I would suspect it would be a net negative on fuel economy, however it also depends on the form of the vehicle. Mercedes has significantly improved the fuel economy of their transport fleets using subtle changes in form.

I have successfully applied a narrow strip of sandpaper to the surface in the airstream just ahead every substantial curve toward the rear, and have thereby mitigated parasite drag of equipment racks on construction vehicles to the point that their presence on the vehicle does not measurably decrease the vehicle's fuel economy.

Fluids guys can explain the Reynolds effect that makes this possible. Roughly speaking, it drives the local region flow fully into the turbulent regime at a low point on the Moody diagram. My foray into the calculations proved them to be entertaining, but impractical for me.

I use 1'' wide coarse sanding loops made for sanding tables, cut them at the joint, apply them with PL Premium to a surface prepared with acetone, and paint over them with the color of the vehicle. They are visible but not outrageously ugly. Not to me anyway. To me they are beautiful.

Those large, non-metallic pieces are freqently more than just a "dam". While guiding air around the vehicle the shape also creates high pressure zones along it's surface. Engineers often take advantage of this higher pressure to provide improved cooling for such things brake rotors, A/C condensers, oil and transmission coolers, etc.

Dear Jaguar: Are you "laughing" because otherwise, you'd cry? --Just kidding. Jags are cool, but their engines (XK-6) are heavy(600lbs WITH an aluminum head). Do you know any engineers that could turn your V-8(--I'm guessing here that your van has one) into this new Chevy system that goes to a V-4? Cadillac had one that was mechanical, and Chevy HAS one that is electronic. It was touted to me by a neighbor, but I notice he now has a new Toyota, so maybe the electronic version has "bugs" in it. Maybe a retrofit of new parts might be able to be done for your van(I'm also assuming it's a Chevy). Of course, this would be thousands, but you'd "invest" in the technology to move it to your next van, so it could amortize further than what you have now. That might add a several mpg more. I would think a dam extension would reduce drag/lift, adding another to a half mpg more. How about over-inflating your tires to 10% more, as long as you don't exceed 80mph, put nitrogen( instead of air) in your tires, and don't overload the van. How about cutting weight? Can you make your van a pick-up, instead of a van? There, cutting off the roof may affect the structure, such that you'd use steel tubing to reduce flex in the floor/sides. You could then put a carbon-fiber "cap" where your roof/sides were. You probably don't have a dual exhaust. I bet you could add duals (exhaust) yourself, with headers, putting a cat on each pipe to restore some of the lost back-pressure. If you're into "hot-rodding", you could "rod" for Economy. get aluminum heads, port them for smoothing of the flow, not hogging it out for volume. Ceramicize the exhaust ports, and exhaust valve face, so they are more heat resistant. Then increase the compression of the heads, to squeeze more power out of your fuels. Add a methanol-water injection unit to cool the charge in your now higher compression heads. If you have 8.4 compression, mill the heads to get 10 (11 if aluminum heads). Get a used overdrive of 50%, so on the highway, you can drop a 1,000 rpms or so at highway speed. These are all do-able ideas, except maybe cutting your roof. Do you NEED the space you have? You could take a section out of the middle, which might drop 400-600lbs. The van would be shorter, easier to manuver, steering would become more sensitive, and the frame, body, drive-line & exhaust would have to be shortened. Can you lower the van, putting air-bags in, so you are not so high? You could lower the bags to the lowest setting, decreasing the height 6-8", reducing drag(-use only on highways). If the "mods" were done by yourself/friends, the cost of them is drastically reduced to the parts needed and the celebratory beers afterward. Hope this helps.