Diffuser Alignment & Positioning Vertical Pieces

[Brando]

27700

In positioning nose to tail:
Should the pieces be flush with nose, tail or centered.

Parallel or angled:
Should the inside 2 pieces be parallel or slightly angled. Should the outside be angled even further or even run parallel with the outside rail of the CF?

Mounting Lip:
Should the mounting lip of aluminum point towards inside or outside?

[Santiago]

My $0.02...




In positioning nose to tail:
Should the pieces be flush with nose, tail or centered.

Centered. Ideally you would want full-length vanes or near full-length vanes on a diffuser integrated with the belly pan. Since this one isn't, I think of it as trying to take more turbulent air ahead of the entry and cleaning up the flow a bit before letting it exit. So the plan is: get it in, clean it up, let it exit. If I was going to mount them off-center, I'd favor the forward entry side.



Parallel or angled:
Should the inside 2 pieces be parallel or slightly angled. Should the outside be angled even further or even run parallel with the outside rail of the CF?

Both (i.e. radial). Outside vanes run close to parallel with outside rails and inside are slightly angled at opposite angles. So think of a venturi throat. You want the air flowing through evenly and expanding. So having all of them parallel would mean only the outer elements expand laterally (they will all expand vertically if it is set up nose down as all diffuser's should be). Set them up so they all offer room for expansion in both planes.



Mounting Lip:
Should the mounting lip of aluminum point towards inside or outside?

Don't think it matters much. I think it's an aesthetic call.

FWIW, I'm not convinced you need to really sweat this, so maybe the answer to all Q's is: do what you like. Vane position/length & diffuser roof pitch (keep it 7-9 deg) are functional concerns in a working diffuser. However, without a flat floor ahead of the diffuser you're not really "diffusing" the fast moving air ahead of it. In this configuration, once the air leaves the flat floor of the main cabin, it gets chopped up and slows down under the engine. So I suspect the real aim of this rendition is to tidy up that situation before exiting the car - perhaps the hope is that it diminishes the negative pressure in the wake? What I mean to say is that I think this device's aim is really drag reduction rather than aiding in the downforce production ahead of it (which is what a true diffuser would do). I don't know for sure, but I don't think the "clean-up job" I'm speculating requires really optimizing vane position.

Come to think of it, maybe drag reduction would mitigate in favor of off-setting the vanes towards the exit...dunno. So centered. :)

Best,
-j

[Mechie3]

IMy diffuser shipped with a big piece of aluminum that I think is meant to close off under the engine. Not sure how well it'll fit as the exhaust hangs down a little below the frame.

[Santiago]

By hook or crook, find a way to make that aluminum panel fit.

Ideally, you would want that panel to come off of the main frame, which as I understand it is about 1" below the frame of the engine bay. Mechie, are you saying your exhaust hangs below the main frame or just below the engine bay frame? If it's just the engine bay frame, consider just hanging it 1" lower (use strong stand-offs, since this is the area that will actually have the greatest low pressure - and if the aluminum panel isn't rigid in its spans, consider bracing it).

Best,
-j

[Mechie3]

Below the main frame. Oil pan sits low too.

[RM1SepEx]

IMy diffuser shipped with a big piece of aluminum that I think is meant to close off under the engine. Not sure how well it'll fit as the exhaust hangs down a little below the frame.

big piece of aluminum?? picture please...

[Frank818]

I don't understand something. If there are parts of the engine sitting lower than the lowest floor point of the car from nose to back, it doesn't make it a flat floor, so I guess it will make the ground effect less efficient.

Is the point of fitting that diffuser in a way that it will flow air from the lowest point and up exiting just after the rear bumper?

[Jaime]

Even if the floor isn't flat, a diffuser will still create a low pressure zone under itself. Worst case, if there was a big air leak in the rear floor, it would make high pressure above itself and still be useful. Just think of it as a bottom mounted spoiler.

It also helps close the flow behind the vehicle to minimize drag.

[Santiago]

Even if the floor isn't flat, a diffuser will still create a low pressure zone under itself. Worst case, if there was a big air leak in the rear floor, it would make high pressure above itself and still be useful. Just think of it as a bottom mounted spoiler.

It also helps close the flow behind the vehicle to minimize drag.

Agreed, but only a small amount of low pressure will be created in the diffuser itself, and the bigger portion of that mostly at the point closest to what would be called it's throat. As it expands, flow is slowing and pressure is rising towards ambient. So you'll get something there (esp. on a well executed diffuser), but my guess is very little in this configuration. I think if you look closely at the published wind tunnel tests FFR did, you'll see reason to suspect this isn't adding much downforce. They added it along with increasing the rear wing AOA 5 deg and got gains largely within range of the increased AOA alone (according to APR's published data). My guess is this is mostly for minimizing drag.

Mechie, seems like some folks have their oil pans lower than others...or am I seeing things? Are you using the stock pan?
At any rate, having a bow in the aluminum panel under the oil pan wouldn't be a bad thing from an aero standpoint (provided ground clearance wasn't getting so small as to choke air flow, which it shouldn't in the S-car). Some folks might not like what that does to their ground clearance though...each will make a call on what's more important to them, ground clearance or a proper tray.

Best,
-j