What is a diffuser?

[Taylor95]

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?

[M_a_t_t]

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.

[Taylor95]

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?

[M_a_t_t]

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.

[JulianEdgar]

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

[freebeard]

What do those strakes do?

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

Its design includes vertical fences, some of which are curved, some stepped, and some angled, but all are developed through constant tweaking and evolution in the wind tunnel. The basic job of these fences is to keep apart the many different types of air flows found at the rear end of an racing car - areas of low pressure air due to the rear wheels, and the rear wing, and the air coming under the floor. All these different air flows have different energy levels and different speeds, and their separation makes them easier to deal with.

formula1-dictionary.net: Diffuser

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

[j-c-c]

Originally Posted by Taylor95

What do those strakes do?

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.

[Taylor95]

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?

[freebeard]

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

[California98Civic]

Originally Posted by freebeard

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.