A lot of posts are on the subject of MPG imrovement or HP improvement or both. Alot is discussed about the mechanical end of improving engine performance.

I have been wondering about reducing drag to some extent, in lieu of trying to fiddle with the engine (though I have - 4.6l Ex, AF1 and TP Xcal2).

One of the biggest losses at highway speed is form drag, of course. No matter how "aero" the front may be with curved lines, air dams etc, that big blunt rear end just creates way too much turbulence and negative pressure.

I was wondering if anyone has tried/experimented with installing a "wing" or rear spoiler (in particular on the end of an SUV) to push the air stream beyond the rear end and reduce the Cd.???

I'm not talking about the wind deflectors that supposedly keep the rear window clean, but that is getting at the heart of the issue.

I am looking at a couple of rear spoilers that stand about 5" high, have about a 7" surface and run the width of the vehicle, say about 48" to 50".

I cannot find any data on the net (yet) to allow me to evaluate the size of a wing relative to the size of the vehicle body. If anyone has any leads on aero spoiler /wing design, I'd like to know.

I've thought about making my own that might attatch to the rear roof rack cross bar. Possibly have one fabricated from aluminum but I would need to design it.

Anyhow, just looking at the HP/MPG issue from a different angle.

I have actually ordered a 1/18 scale diecast Expedition model (in my color too, how cool is that!) and I am thinking about making a small wind tunnel and see if I can observe the air flow. My boys would sure love it, as well as my oldest son's physics teacher

Any thought or ideas would be welcome.

 

Best way to reduce your drag is to go slower.....

Honestly though I think any type of wind deflector will increase your drag. I see where you're going with this but I think you will lose more with the deflector on top than you'll gain in reducing turbulence behind your truck. Although not practical the best way to do that would be to have some sort of coned shape cap that would taper off the back of your truck. This way you maintain a clean and smooth profile while allowing the air to move smoothly off the back of the truck and avoiding the turbulence you othewise would get.

 

I got a new idea to reduce drag lower the tailgate. That alone helps improve gas mileage.

 

shakeydays I got a new idea to reduce drag lower the tailgate. That alone helps improve gas mileage.

did you miss the mythbuster episode that busted the tail gate thing
also it's a expedition running with the hatch open (not)

my opinion is that the 4.6l is to small a engine for the heavy expedition
has to work to hard to get it and keep it rolling
try max air in tires helps some
mitch

 

Well, my Ex doesn't have a tailgait, so that won't work If I did have a tailgait, I wouldn't lower it anyhow, as it does not reduce drag (as mentioned above) and its a little dangerous - folks behind you may not notice it until its too late.

They are experimenting w/ pannels on the rear end of semi trailers that extend the airstream away from the big flat end. The suv's have the same sort of situation.

Adding a wing or spoiler can increase drag if the angle isn't right (they do this on purpose on the formula cars to help keep the rears glued to the track at high speed.

As for the engine, yeah it is a little underpowered, but with the AF1 intake and a 93 performance Xcal tune, it isn't bad at all. No problem excellerating and passing on the highway. Plenty of low end torque too.

Just need to do something about the blimp-o drag coefficient!

 

These trucks have a huge frontal area so there's only so much you can do (within reason).

One thing that may help sightly, both for drag and high-speed handling, is to clean up the undertray. This reduces air trapping/high-pressure areas (keeps it from slowing down/building up) as it passes under your truck, which also will help reduce lift a little bit.

But you may run into brake-cooling, rear-diff cooling and other issues.

 

It probably helps to add a wing, not one of those tuner deals but like on a deck lid on a NASCAR. Even still, you're probably splitting hairs with improvements. Think of those goofy wings they had on the Superbirds, damn things weren't any good till you hit 160 plus mph. At 80 mph that many cruise at max, not much will help. Hey man, be creative and try it, arm chair engineers proove nothing. The world has lost good old imagination.

 

The largest problem is the amount of air it displaces due to its frontal area. Sure, there is some loss due to neg pressure behind the cab, but relatively speaking, it's a small amount in contrast to the huge frontal area.

 

Keep in mind that wings on racecars are typically to generate downforce, which actually increase drag.

For minimizing airflow, you need to look at a land-speed car, not a racecar that must negotiate turns.

For a rear wing, you need something that "elongates" the rear so that it doesn't end abruptly. A NASCAR wing that is somewhat vertical will only increase drag; rather, it needs to be horizontal to smooth the airflow coming off the car.

A teardrop shape provides very low drag...rounded front, small frontal area and a long-tapered rear minimize the negative pressure/vacuum behind as noted. And smooth surfaces all around.

 

yes it does have a large frontal area. However, even a trim leading edged shape, such as a bridge pier, will have tons of drag if the rear end is blunt. Thats why they boat-tail bullets. Extending the end via a wing or spoiler is the idea - to allow the top and bottom air streams to rejoin smoothly and minimize the turbulence.

The trouble with a wing (not like the formula car's wings which are upside down) is design and testing. Even an aftermarket item that can be bolted on will be tough to test unless the affects are dramatic, like a 10% increase in mpg.

I hate to spend the money without some kind of computation to guide me.

I like the comment about armchair engineering! exactly right

I actually did find an abstract on the net from a study in University of Michigan-Deerborn which claims to have designed a wing specifically for Suv's and adresses all the things we have been discussing. I've emailed them to ask for more info or some design specs.

 

Well with all my mods, and 33x12.50x20 all terrain tires, i can get 15mpg, and still smoke a hemi