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Old 06-08-2020, 04:09 PM   #11 (permalink)
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Great answers thank you. I'm just thinking of ways that I can lower the CD on my Camry. Supposedly it is already 0.28, so I don't think there's much I can do other than adding a belly pan.
I think a full undertray (belly pan) is the single best modificaion to reduce drag on most cars. But to do a good job, it is a lot of work. This may be relevant:



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For the rear diffuser. Is there an optimal angle that it should slope to the bumper?
No there isn't. You can look at a diversity of tech papers and get a diversity of 'optimal angles'. It also depends on the length (ie as a proportion of the car length) formed by the diffuser. For example, in the tech paper quoted above, the length of the diffuser is either 33 per cent, or 18 per cent, of the length of the car. Very hard to install a diffuser that is a third the length of the car!

In addition, the shape of the car (eg wagon or sedan) and the design of the undertray ahead of the diffuser will have influence.

All that said, too steep an angle will cause the flow to separate - and you don't want that. For downforce, something like 8-10 degrees , and for drag, something like 5-8 degrees (and both with smooth floor ahead of the diffuser).

But it's pretty hard to get accurate angles working under the car, so spend your energy on making a good full-length undertray and just be sure to incorporate a gently sloping upwards bit at the back.

Edit addition: also keep in mind that aero lift that occurs differentially (ie more at the back than the front, or vice versa) will be yet another factor that influences the actual angle the diffuser forms on the road at speed. As with all car aero, the more you think about it, the more complex (and car-specific) it all gets.

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Old 06-08-2020, 06:14 PM   #12 (permalink)
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Something not yet mentioned here, I believe there are some benefits to be gained from allowing/intending some (waste?)air flow also on the top side of a diffuser, especially if any DF is sought, effectively making it a turning vane with negative lift, if it has uniform cross sectional thickness, meaning, a sealed diffuser panel is not always 100% necessary or best.
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Old 06-08-2020, 08:40 PM   #13 (permalink)
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I generally have not seen strakes on the difusers of high fuel economy cars. At least I don't recall them. Some interesting photos of Priuses.

Very small strakes in a sort of airfoil shape are visible in some of these, but farther forward, between the axles.





See link for a couple more:
https://priuschat.com/threads/photos...y-prius.68549/
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Old 06-08-2020, 08:45 PM   #14 (permalink)
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...there are some benefits to be gained from allowing/intending some (waste?)air flow also on the top side of a diffuser...
[citation needed]
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Old 06-08-2020, 08:45 PM   #15 (permalink)
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Good find.

They look like vortex generators to me.

(Hopefully that beam axle is well above the lower level of the fuel tank when the suspension is at normal compression.)
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Old 06-08-2020, 11:20 PM   #16 (permalink)
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"I believe there are some benefits to be gained from allowing/intending some (waste?)air flow also on the top side of a diffuser"

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[citation needed]
This suffice?

Here is my max DF purposed CF diffuser effort within my personal parameters. "Mini" spoiler is primarily to enhance the trunk mounted oil cooler exhaust vents and diffuser efficiency. In this pic it is unfinished with the yet to be cutout two exhaust exits thru the pair of the still foamed diffuser segments.



https://board.moparts.org/ubbthreads...ml#Post2783050
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Old 06-08-2020, 11:34 PM   #17 (permalink)
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This suffice?
Totally.

If I understand what you're doing, on completion, you can verify the effectiveness by temporarily closing off the cooler vents?
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Old 06-09-2020, 12:01 AM   #18 (permalink)
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Originally Posted by j-c-c View Post
"I believe there are some benefits to be gained from allowing/intending some (waste?)air flow also on the top side of a diffuser"



This suffice?

Here is my max DF purposed CF diffuser effort within my personal parameters. "Mini" spoiler is primarily to enhance the trunk mounted oil cooler exhaust vents and diffuser efficiency. In this pic it is unfinished with the yet to be cutout two exhaust exits thru the pair of the still foamed diffuser segments.



https://board.moparts.org/ubbthreads...ml#Post2783050
What does the floor look like ahead of the diffuser?

This?

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Old 06-09-2020, 12:09 AM   #19 (permalink)
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I also prefer the "b" diffuser in that diagram. I also like that the slope starts at the wheel. I really want to take my time to make a nice undertray, probably using sheet aluminum. I may also be replacing my 07 Camry with a 09 hybrid version, so I really would like to get the best mpg possible.
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Old 06-09-2020, 12:23 AM   #20 (permalink)
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Originally Posted by Taylor95 View Post
I also prefer the "b" diffuser in that diagram. I also like that the slope starts at the wheel. I really want to take my time to make a nice undertray, probably using sheet aluminum. I may also be replacing my 07 Camry with a 09 hybrid version, so I really would like to get the best mpg possible.
Have a look under your car first! In many cars, getting a good diffuser angle and length will mean chopping up the trunk floor / spare wheel well.

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