What is a diffuser?

[j-c-c]

Originally Posted by JulianEdgar

What does the floor look like ahead of the diffuser?

This?

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.

[Taylor95]

Originally Posted by JulianEdgar

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.

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?

[JulianEdgar]

Originally Posted by Taylor95

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).

[Taylor95]

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.

[JulianEdgar]

Originally Posted by Taylor95

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.



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.

[freebeard]

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.



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.

[Vman455]

Originally Posted by JulianEdgar

(Hopefully that beam axle is well above the lower level of the fuel tank when the suspension is at normal compression.)

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:

[aerohead]

Originally Posted by 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?

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.