What is a diffuser?
 

I've seen plenty of references here of diffusers, but no explanations of what it actually is. What does it do? How do you make your own?

These should answer your questions
https://www.youtube.com/watch?v=iQDBkojzWIw
https://www.youtube.com/watch?v=L9WCVlWPcys

basically its just a flat panel (strakes optional) that slopes upward toward the rear of the car.

What do those strakes do?

So my Camry sort of has a 6" undertray extending from the front bumper. Could I do a rear diffuser only, or would that add drag?

Approximately how much would this lower Cd by?

I don't what the purspose of the strakes are. In a different video (and on here somewhere) Mr. Edgar used them to help with turbulence on the outsides (I think attributed to the wheels) that caused seperation on his insights undertray.

Unless the air is able to be reattached to the rear section then I don't think having the diffuser would help much. The stock middle section (no stock covers? Maybe from a hybrid model?) might be too rough to allow the rear diffuser to help any. Hard to say. Its best to just try something.

I would probably start at the front and work my way back. Even if you just skip under the engine bay. Given enough space (or length of the rest of the car) the air might reattach underneath the car and allow the diffuser to work at full capacity.

Think of the diffuser as the last part of an undertray that starts from the front. ie it's not a separate 'add-on'.

Strakes are vestigial fences. Fences have two functions, preventing intrusion from the sides and channelize or straighten the flow.

formula1-dictionary.net: Diffuser

More good stuff there, I learned about diffuser stall when you stand on the brakes. :eek:

Strakes on racing cars on diffusers are reported to control vortexes for maximum DF, I suspect the consumer versions are more for cosmetics. I disagree with the blanket statement on this thread of what are NOT diffusers. I think the critique should be more directed as to their effectiveness (likely not much in the examples), and OP should also set a goal, DF, Drag reduction, looks or some combination, for his needs.

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.

For the rear diffuser. Is there an optimal angle that it should slope to the bumper?

https://ecomodder.com/forum/member-f...venturism8.jpg

The angle is only one aspect. Generally you want a diverging duct, details subject to testing and verification.

This is the chart I used for my decisions. I made the "b" type diffuser, no strakes, and 4° for zero lift and the best drag according to the model. Of course, I do not know for sure that my car gets the benefits described because I have no way to test and confirm. But given that my previous arrangement was a fully open back end in which the inside of the bumper cover was practically a parachute in the flow, I suspect I am ok. My FE on the freeway is pretty great (60s and 70s steady cruising at 60 mph on the flats). Someday I might get to test it, but that ain't now.

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:

https://www.youtube.com/watch?v=L9WCVlWPcys

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.

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.

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.

https://ecomodder.com/forum/attachme...1&d=1591663116

https://ecomodder.com/forum/attachme...1&d=1591663131

See link for a couple more:
https://priuschat.com/threads/photos...y-prius.68549/

[citation needed]

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.)

"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.

http://https://board.moparts.org/ubb...ml#Post2783050

https://board.moparts.org/ubbthreads...ml#Post2783050

Totally.

If I understand what you're doing, on completion, you can verify the effectiveness by temporarily closing off the cooler vents?

What does the floor look like ahead of the diffuser?

This?

https://board.moparts.org/ubbthreads...-RightRear.JPG

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.

Not sure how close you are following this, but the one pic you posted here shows the temporary exhaust system, in front of the rear axle, for this segment of the aero treatment, the routing has been "cleaned" up, and the new mufflers are now vertical alongside the rear center fuel cell, and will exit as I mentioned thru the pair of symmetrical narrow diffuser channels. Its a and will remain a live axle leaf spring car , which really limits my solutions. I am very concerned as to the live axle movement aero interaction with the rear diffuser that I will see. I plan to add a decent sized axle mounted fairing to reduce its effects hopefully, thru its normal travel range. Specifically to your question, OEM mid floor is very "messy", I will incorporate a full between the axles underbody pan with side skirting, with an opening slit near the center for the twin exhaust tubes and driveshaft. Everything I am doing is on best guess, and it will see a low speed wind tunnel when completed in various configurations to see what is doing what, until I run out of time or money or both.

I'm not sure right now if I could want to do that! My car has been in the shop for over 2 weeks now though so I just need to get it back. I suspect that i probably won't need to do that though. Do most diffusers just go to the rear bumper?

The issue is that (and I haven't looked under the Camry) the spare wheel well / trunk floor is usually the lowest part of the rear of the car - and it is usually fairly flat. Therefore, a panel that slopes upwards at the rear, and extends far enough forward*, is likely to be too low at its leading edge, reducing ground clearance.

* = maybe look at the video again? It's only 60 seconds long. Yes, diffusers go to the rear bumper but to work, they need to extend well forwards, and preferably join to a proper undertray.

http://www.youtube.com/watch?v=iQDBkojzWIw

I found with my Insight I had to work within quite narrow constraints in terms of diffuser length and angle (I didn't want to cut out the spare wheel well).

Those pictures you have are good.

How much of the exhaust is supposed to be left exposed when making an undertray? I noticed in the prius picture that the whole thing is left open.

That is a good question! On the Insight I chose to leave the forward two-thirds (approx) of the exhaust exposed for cooling.

https://i.postimg.cc/Jngqnnc8/Figure-7-18.jpg

That's with the undertray made from ABS plastic sheet. I have been considering going to a new design of undertray, made from aluminium, and covering more of the exhaust, but I haven't quite made up my mind yet.

Just be thankful you don't have a boxer four-cylinder engine where your gas tank is. :)

Ending at the rear bumper is only a constraint if there are racing class rules.

https://ecomodder.com/forum/member-f...11-silver4.jpg

I think if this one back into a post the diffuser and spoiler would touch at the same time. But it shows the diverging side fences that I think are important.

I was scrolling through some pictures and remembered I had taken this photo a couple months ago. Even with the suspension hanging, the torsion beam tucks up behind the fuel tank:

https://ecomodder.com/forum/member-v...-millercat.jpg

diffuser
 

A diffuser will only function if it's behind a viable full belly pan.
The pan, on the worst-case vehicle ( pickup truck/ SUV) is measured at delta- Cd 0.045 improvement.
A 'long' 2.8-degree up-swept diffuser will improve the drag reduction to delta-Cd 0.070.
A 'fast' 4-degree diffuser cuts drag a little less than the 'long' unit.
Any diffuser with an up-sweep steeper than 4-degrees is for high-speed sports cars, or racing.
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According to a research paper on the SAAB 9-3, the diffuser strake/guide vanes help mitigate effects of lateral wind gust, plus decrease turbulence effects due to rear wheels.