Modifying the Malibu

[Cd]

Thanks for the information. I'm finding it fascinating.

[2000neon]

Hey good luck with the Malibu, I'm from just outside of waterloo and commute to mississauga and back and spend a ton of time on the 401, just figured I'd say hey to another local guy.

[kach22i]

I've never read of a rear spoiler lowering drag, only increasing drag and down force.

Interesting results.

I would think that vertical is drag and down force, horizontal (-10 degrees) is lower drag and uplift.

[ERTW]

I will try that george. Have you seen the hot rod bonneville 1980 camaro with a Cd of 0.21 as tested at A2 wind tunnel? The last gen infinity G35 aero kit used a spoiler Cd 0.27.

13° kamm back
Cx 0.329
Fz 377 N

15° kamm
Cx 0.328
Fz 412 N

Elliptical spoiler 2" tall x 56" wide
Cx 0.304
Fz 231 N

Elliptical spoiler 4" tall and 69" wide
Cx 0.331
Fz 98 N

The last configuration has virtually the same Cd and only 22 lb of lift...
I interpret the spoiler's domination over the kamm tail is this: the kamm tail has low pressure over a large area - hence the high lift. the spoiler creates a high pressure over a large area - hence low lift. imo, the kamm needs to taper the sides to be effective.

We've seen the "virtual tail". I propose the concept of the "virtual body" (it's all mine, never been thunk up before). The streamlines skim from the roof to the top of the spoiler. I'm investigating spoiler width to manage side flow (and vortices).

[kach22i]

Which car has an "Elliptical spoiler"?

What does it look like?

[ERTW]

George, instead of a rectangular spoiler, it looks like half of an ellipse. My thinking was that by tapering the edges, I could keep most of the downforce, and reduce drag. It turns out that the rectangular spoiler tests better in both respects.

Here's the kicker. I ran several tests using different spoiler widths, expecting to find a minima, and an obvious winner. Boy was I surprised. All of these tests ran spoilers that were 2" above the trunk deck, and varying widths:

60" wide
Cd .305
Fz 203 N

56" wide
Cd .304
Fz 231 N

50" wide
Cd .304
Fz 187 N

45" wide
Cd .302
Fz 208 N

40" wide
Cd .304
Fz 223 N

The 45" wide spoiler doesn't run away with it, and it has the lowest Cd and low lift. The vortices spilling off the rear are much less energetic than stock. Next I will vary the height, and curvature of the spoiler to see if there's any improvement to be had. I wonder if raking the spoiler benefits anything.

Note that the increase in downforce is all on the rear, and that there's only 208 N of lift on the entire car. Yes, the pitching moment is skewed, but it's only 45 lb, and I'm not racing.

Aero is so senstive to small changes. 45" ISN'T the magic number for the real car...2" likely isn't the ideal height. I'll have to do tuft testing to locate the edges of the spoiler. This CFD is just giving me (hopefully) a better sense of cause and effect. It seems that I want to keep it really close to the streamlines.

[ERTW]

After a short hiatus (because surfing on my iphone sucks) I've racked up 40 hours of CFD time in less than three days.

As always, the first 27 N came off pretty easily (it only took a single test) and the last 6 N came painstakingly (11 tests).

I raked the spoiler at 45°:

2.5" vertical height
Cd 0.303
Fz 205 N

1.5" vertical height
Cd 0.303
Fz 242 N

Again, 2" height seems to be the best compromise - and I don't expect a "magic" number that significantly improves on that.

My last test, I radiused the edges to 3".

Cd 0.300
Fz 188 N

I reduced the Cd by a full 27 counts over baseline This is an 8% reduction, which should reduce fuel consumption by 4%. It also reduced lift by 56 lb - 57%. I can't predict the drag and lift improvements for the real car.

I don't know if it's significant, but the spoiler width is almost 61.8% the car's width (golden ratio).

Until I buy a faster, multicore computer, which will allow me to improve the resolution of my results, this is where I'm stopping, as far as the spoiler.

[kach22i]

Any updates on this project?

[ERTW]

I wish l could say yes. I have black duct tape in the front grille - much nicer than the silver tape over top it.

Because my cfd model is so crude, l need to do tuft testing to determine spoiler width. My fam is too lazy to follow me with a camera :/. Im thinking oil drops will work just as well.

The "free" head porting will actually cost me hundreds just for parts and labour :/ some day...

I designed a canted valve cylinder head for the gm 3800 V6