Rear Diffuser, Thoughts.?

[FormulaTwo]

I plan to make a belly plan for my civic. And cover as much of the underbody as i can.
I am also interested in trimming up the rear bumper to clear up airflow.

I was wondering if you guys had any thoughts on rear diffusers? The website varisusa.com makes some. They are expensive, But i would like to try and make my own if it would even be worth it.

I would like to trim out the rear bumper and install something like this
Center Rear Diffuser


The company also makes Vortex generators that bolt onto the sides of the rear bumper cover.



Both parts installed



Just curious to everyones thoughts.
If you feel it would be a good thing, help me come up with an idea on how to make something like this.

[SVOboy]

Jared on here chopped part of his rear bumper out.

Anyway, I see no issue with a rear diffuser as long as it was not designed to increase downforce or anything such. A chopped bumper with a gradual upslope on the belly pan is the good way to go though.

[krousdb]

The VX has what is referred to as a rear diffuser on the drivers side. It is more of a filler to keep air from getting hung up in the read bumper cover. On the passenger side there is nothing like that due to the muffller in the way.

I'm thinking that it might be cheaper to do what Jared did, essentially cutting out areas of the lower bumper cover to let the air out. My thought is that the Cyber Evo rear diffuser would be good for high speeds, but if you are going to be driving the speed limit it wouldn't have much benefit over a belly pan.

[FormulaTwo]

Cutting out the bumper would definitely be cheaper.
Just trying to get some ideas on it. I just havent seen any cut bumpers that look good.

Im thinking, maybe if i cut the bumper higher, and then added something along those lines, with a belly pan making its way back to a splitter of some sort.
Installing a home made splitter at an angle up towards the bumper.









Im just wondering if the diffuser itself will reduce drag or increase downforce?

[onegammyleg]

Rear aero aids can make some difference , but I feel that just adding an under bumper thing may be for more of pose value (as are most spoiler kits) than anything else.
Ide think that going for a full underbody tray would be needed and a smoothing of the airflow and combining it with the flow off the tail of the car would be needed to reduce drag and improve FE.
I remember reading about when they were designing one of my old Fiat cars that the designers initialy wanted it to have a larger boot area and a more streamlined roof section leading to the boot (sort of hathcback style- but with less style) , but the shape just wouldnt work in the wind tunnel.
It had excessive drag . I think it was over .43 (maybe - old brain cells shorting out here)
So they shortened the boot length by 15cm and raised the top of the roof at the rear glass.
This gave the airflow a seperation point at the rear of the roof instead of trying to get it flowing over the boot lid.
This dropped the drag to .39.
Not exciting for a modern sports or econo car , but for a sedan that coudl seat 4 adults comfortable in 1969 its not too shaby.

I dont think a lot can be gained by underbody work , but it probably is worth experimentation.
I'de perhaps start with just flat sheets full length and then if that is positive then go for difuser design variations on the rear ., and see what that does.

[thisisntjared]

the "vanes" in the rear diffuser provide stability with the air which allows a more consistant low pressure area under the rear of the car, thus more downforce.

that kit is more for down force that smoother aerodynamics however just that center peice without the vanes could help a lot in smoothing out the air flow if it never excedes 11 degrees i believe. i dont remember, but the rear panels should not be steeper than a certain angle... someone want to chime in with that number?

[thisisntjared]

Originally Posted by krousdb

The VX has what is referred to as a rear diffuser on the drivers side. It is more of a filler to keep air from getting hung up in the read bumper cover. On the passenger side there is nothing like that due to the muffller in the way.

here is some food for thought. with a stock muffler you might want to do 2 things:
1. throw aluminum sheeting on the front of the muffler(like 3-4ft long connecting to the body and covering the trailing arm. this removes the turbulence from the trailing arm and the front surface of the muffler. big aerodynamics saver, especially the factory muffler
2. cut that side of the bumper anyway in hopes of making a larger low pressure area directly behind the muffler to aid in exhaust flow. i know on motorcycles there is a clear cut advantage in racing if the exhaust tip is pointed directly at the low pressure system behind the bike. the low pressure system helps draw out the exhaust gases. its all speculation but i know that the rear bumper doesnt help much for areo anyway and all the air around the muffler has to go somewhere...

just food for thought, no hard core testing done...

[Bman83GL]

Indeed, reading SAE studies you will find that much drag-reducing research over the last five years has focused on the underbody. This can make a substantial reduction on drag.
If you are wondering if diffusers decrease drag (as well as increase downforce), take a look under a Toyota Prius, Honda Insight, or new Civic. They all have variations on the partial or full diffuser theme. My suspicion here, given that few people peer and the undercladding before making a buying decision, is that these do indeed make some contribution to reduced fuel consumption.

As mentioned, if you proceed with the idea, keep the angle of the diffuser at 10 degrees or below. The diffuser above looks much too steep to be effective, unless it is paired with a really radical wing above. The vanes help keep the air straight, improving velocity and discouraging turbulence. Look at the diffusers on the current ALMS GT1 C6R Corvette or DBR9 Aston Martin. LOTS of vanes on a shallow-angle diffuser.

Bman

[FormulaTwo]

Bman, good stuff. I agree that the vanes, do seem like they would be helpful in breaking up turbulence.

My only concern, I am still trying to fully understand the relationship with downforce and drag.

From what i understand, air travels faster over the car, than under it.
So, to me it seems like a completely flat underbody panel would help in reducing drag, BUT would cause more downforce.

Since air travels over a curved object faster than a flat one. Then it will create low pressure under the car.

Which got me thinking the other day at work. If you wanted to make air travel faster Under the car, the best way would be with some sort of fan.
And i know everyone will immediately bash this by saying that the fan would have to be driven by something, which in turn would cause more work on the engine resulting in less FE.

So I started thinking about the exhaust. Its just wasted energy anyways, if you could rig up fans in the rear bumper, propelled by exhaust gas. With vanes splitting them.

Lets say four 6" fans, with vans splitting them. Spinning from left to right, clockwise, counter, clockwise, counter.

To let the air spin off the back of the car in perfect vortices.
Then this would cause the air to travel faster UNDER the car, and reduce drag. While also clearing up Eddies behind the car.

So this got me googling around.
i found this site.
http://8w.forix.com/fancar.html
About Murray vs chapman in racing.


I also feel it would be important to remember that just because it is on a race car does not mean that it is for increased downforce. They are also very concerned with drag and reducing it.


Aston Martin


Fan car

[JanGeo]

Air doesn't move under the car - the car moves over the air on the ground and tries to drag some of it with it. By directing the air over the car you avoid dragging air under the bumpy bottom of the car.

[Silveredwings]

Oh. I thought a rear diffuser was when you ruffle the sheets after breaking wind. Nevermind.

[Bman83GL]

Silveredwings: You crack me up!

Formula2: As I was reading your post, I was going to direct you toward the Chapman "fan car," but I see that you have found this on your own. To JanGeo's point, a diffuser is only effective if you have a flat bottomed car. Even with a flat bottom, the surface friction is such that the air loses momentum as it heads from nose to tail. A gently expanding diffuser creates lower pressure at the rear, drawing the air that wants to slow under the car. While I imagine that most of this is for downforce, if you have efficient airflow under the car, it can reduce the amount of air going over the body. Depending on the Cd of the body and the smoothness of the underside, increased air under the car could actually reduce drag. If the velocity of this air stays high, it should not add ridiculous lift, either.
Some F1 teams have experimented with using the exhaust exits to stimulate the flow through the diffuser (per Simon McBeath). Due to the upredictable nature of the flow (goes away when you lift throttle) and the small positive effect, I think this was largely abandoned. I think that you'll find that you would need to spin the fans REALLY fast to have any beneficial effect.

All that said, I agree that for drag-reduction-only purposes, flat paneling the bottom will show greater effect than the diffuser. I think that adding the diffuser just makes the belly pan a bit more effective.

Thanks for posting the pic of the DBR9, by the way. Sorry for being lazy!
Bman

[omgwtfbyobbq]

Depending on how little pressure there is at the back of the car, directing air up into that area my reduce drag in the same way VGs supposedly reduce it. Which isn't to say that either works, but it's an interesting concept.

Diffused rear fascia
“We wanted to enhance the effect of the skin of the car being drawn down more tightly to the body and also reduce the visual weight of the rear end,” said Peters. “So we added a diffuser to the bottom of the rear fascia to enhance air flow and to add visual interest to the rear of 2005 Corvette.”

Four circular exhaust tips are integrated into the rear diffuser. The tips exit from the center of the diffuser and pick up the circular theme established by the four round taillamps. Framing the rear fascia with the black CHMSL, functional spoiler at the top and the black diffuser at the bottom produces a narrower cross section. In this way, the rear of 2005 Corvette is reduced both dimensionally and visually.

My guess is, the large the low pressure, the better a proper diffuser will work. Comparitively pointless for an Insight since it already terminates flow such that detachment is minimized, but probably decent for most hatches with flat backs/lots of area.

[omgwtfbyobbq]

Here's some more info, apparently Chrystler is using it on the Dodge Avenger concept. So it seems a properly designed diffuser has it's place on conventional high drag sedan designs, and probably any car made before 2000.

[omgwtfbyobbq]

If it works like VGs supposedly do, then alone I'm guessing it may reduce drag by ~5-20% depending on the older vehicle and mods that have already been done.

[mexellent]

Originally Posted by FormulaTwo

Im just wondering if the diffuser itself will reduce drag or increase downforce?

Undertrays rock anyway you cut it. Thats the beauty of it. Even if you were to put an aggressive undertray on your stock car, it would still help reduce drag of the car (while increasing downforce). Hell, just putting a flat panel underneath your car will still increase downforce and reduce drag at the same time(if angled correctly)!

We build cars with EXTREME aero packages. We know our shizzle.

[omgwtfbyobbq]

Oh sweetness! I was thinkin' about diffusers and was wonderin' if there could be a two stage kind of deal? Could we take something like this

with the air being directed upwards at a more extreme angle in order to fill in that huge low pressure area in the rear... combined with something like this

to smooth out the transition in the rear. For boxy cars, something that's kind of a two stage effect, with the ducting of the diffuser directing air to fill in that low pressure area while the fins/angle underneath it just direct the rest of the air so the flow seperates much farther downstream?

[omgwtfbyobbq]

As long as the area in front isn't increased, I don't think there'd be much to loose given how crappy most auto aero is. In fact, there's plenty of hot air already moving through the radiator. So heat would be added to it, as well as routing air flow that may normally be bouncing around the engine bay. Little fuzzy things can be kept out with mesh, which might hurt flow a bit, but would be better for all parties...

Thanks for the idea btw, keep 'em comin!

[thisisntjared]

Originally Posted by theclencher

Couple things about that diffuser: the air moving through the diffuser will have drag too unless enough heat energy is added to it. Also the height of the diffuser duct will displace the driver and other stuff upwards, thus increasing frontal area. Also rodents, raccoon, cats, chipmunks, and other various creatures might think that duct would be a great place to call home. So once again there ain't no free ride.

the ducts would not be enclosed.....

[omgwtfbyobbq]

That's the beauty of having a two layer system, I hope.
One takes the hot air that's flowed through the rad (or seperate areas) and dumps it out in the low pressure area behind the car, it's just there to even the pressure behind the car out. The other channeled section just directs the airflow such that it seperates as far back as possible, the advantage over just the channels being more air/higher pressure behind the car, so if done properly the redirected airflow from under the car seperates even farther back.

[omgwtfbyobbq]

From what I've been Googling a flat underbody with the appropriate angling is a diffuser, not sure what the fins do, but it's probably related. I wonder about routing hot exhaust behind the car.