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Old 09-03-2006, 05:05 PM   #1
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Rear Diffuser, Thoughts.?

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.
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Old 09-03-2006, 05:09 PM   #2
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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.
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Old 09-03-2006, 05:14 PM   #3
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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.
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Old 09-03-2006, 09:22 PM   #4
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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?
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Old 09-04-2006, 04:59 AM   #5
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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.
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Old 09-04-2006, 08:14 AM   #6
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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?
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Old 09-04-2006, 08:23 AM   #7
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Quote:
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...
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Old 09-13-2006, 09:50 AM   #8
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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.

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Old 09-13-2006, 02:40 PM   #9
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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
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Old 09-13-2006, 03:35 PM   #10
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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.
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