Modifying the Malibu

[ERTW]

My 2008 Chevrolet Malibu is worse, aerodynamically, than any of the "old" W-bodies, and 20% worse than the 1990 Oldsmobile Cutlass Supreme. It quickly coasts down from 100 kph to 80 kph (it doesn't like to coast). I believe I can bring the Cd down into the 0.28 range, without any blatant, or gaudy, aero add-ons. I *may* have access to a full sized wind tunnel.

I have a goal of raising the highway mileage from 30 mpg to 45 mpg. With mild p&g, and staying below 65 mph, I've already seen 37 mpg and I believe it's a reasonable goal.

I plan on doing some internal engine work, and expect 10-15% improvement there. I'm working with a gentleman who specializes in high performance engines. He's seen huge improvements with engine mods. I need to get a Pontiac G6 ECU to access parameters with my HPTuners.

Down the road I'd like to swap in an F40 6 speed transmission. Aside from my pet peeve of the 4 speed auto of shifting in turns, it also has a 12.6% lower final drive. A manual is simpler to implement than the 6 speed auto, easier to control, and integrate with the ECU.

I have 225 series Goodyear Comfortred on stock steel rims, now inflated to 41 psi, and I'd like to use 205 series LRR tires on lightweight aluminum rims.

[ERTW]

I'm using the 2013 Malibu as a template, and to quantify changes.

It snowed the other day, and I took advantage by driving it on the highway. High energy air clears away snow, while draggy low energy air leaves snow where it is. You can see there are several areas of concern. Air separates from the leading edge of the hood due to the grille. Separation THAT early on is NOT a good thing! Most of the C pillar has separated flow. It appears that air separates mid way along the roof (and leaves the back window dirty/snowy). And predictably, the dead area at the bottom of the rear windshield.

I know snow is sticky, and doesn't show exact pressures, and I'll follow up with tuft testing when the weather warms up.

My first aero mod is duct taping the front end. I know, it's gaudy, and it's more proof of concept as this stage. I'd like to fiberglass the front fascia and paint it - it's worth around 14 counts. The 2013 Malibu saw a 39 count drop in Cd just from smoothing the front end (2/3 of the total improvement).

Next, I'd like to add a full underbody. I did a quick search for coroplast, and coincidentally, the distributor is a stone's throw from my work! I also plan rear wheel fairings. I like the look - it reminds me of the '30s and '40s cars. That should easily be another 20 counts.

If I can get air to wrap around the C pillar, the Cd will drop further. I'll look into air tabs and spoilers for a clean break away.

[ERTW]

this is purely anecdotal, and the weather conditions are all over the place...so take this with a grain of salt.

33 psi, open grille...it took about 4 seconds to coast down from 100 kph to 90 kph
41 psi, open grille...about 6-7 seconds
41 psi, taped grille...10-11 seconds.

i made a 95.8 km trim yesterday, about 80% highway, and returned 7.3 l/100kph.

it surprised me how much sooner the car reaches operating temp. about a km versus 1.5+ km. it also rises much quicker. when the car was a little warm, it reached op temp in 400m.

[Frank Lee]

this is purely anecdotal, and the weather conditions are all over the place...so take this with a grain of salt.

33 psi, open grille...it took about 4 seconds to coast down from 100 kph to 90 kph
41 psi, open grille...about 6-7 seconds
41 psi, taped grille...10-11 seconds.

I'll take that with a highway dept truck full of salt. No way.

[ERTW]

way dude.

on hills where i used to slow down, i now maintain speed or gain a few kph. i can now coast 1.3-1.5 km down a decline on hwy 400 just south of king rd. i have video for those doubting thomas'.

on the 400, which is very hilly, my previous best (about 20km stretch) was 37 mpg, i've since gotten 6L/100k @ 95 kph, and reached 5.8 following (1.5 seconds) a tractor trailer.

i'm definitely more mindful of my driving now, thanks to ecomodder. i sometimes shut the engine off at long lights. however, it dawned on me last night while i was eoc that the steering column will lock, and i can't use it on turns. it would have been a nice long coast.

[ERTW]

I made a rough 'wind tunnel' with some plasticine, a cutting board, and running water. I just printed out a malibu side profile, cut it out, set it on 1/4" thick plasticine, and carved it out. Running the water was a little tricky. I tried to keep it far enough up stream and straight ahead of the model.

I got a baseline, in which flow detaches from the roof. It's several inches ahead of the rear windshield, which is what I expected.

I followed up with a 'vortex generator' in clean 'air'. I was elated to see that the flow then followed the back window and stayed attached until the trailing edge of the trunk lid

Of course I have to do tuft testing to properly locate airtabs (or some such junk) and this is very promising. Ideally I would take my car to a full sized wind tunnel to confirm.

[ERTW]

So my kitchen tap "wind tunnel" may not have been so accurate

I've been doing CFD on the Malibu's basic shape. It's just a prism, as opposed the the tapered shape of the real car, and my model has a perfectly flat bottom. GM's published Cd is 0.34. I got 0.327 - within a few %. It's not super accurate, and I think it can show very good trends. I think I can say that a full underbody is worth at least 13 counts.

I've been studying various approaches to lowering drag: spoilers, vortex generators, kamm backs etc. here are my results.

[ERTW]

This was my first modified base shape. I put a nascar type spoiler on the back, about 2" above the trunk level. I got lucky, and I'm sure there's more to be had through more analysis.

Cd 0.305 -22 counts
Fz 203 N -234 N !!

I saw a huge drop in drag AND lift. Not to spoil the surprise...this is the best result I've had in 8 tests.

[ERTW]

I tried an L bracket across the entire roof, just below the peak of the roof, so that it doesn't contribute to frontal area. The idea was to trip the air into following the rear windscreen better.

Cd 0.382 +55 counts
Fz 391 N -46 N

It did reduce lift but at the expense of a huge increase in drag.

Vortex generators: V shaped pieces arrayed across the width of the roof.

Cd 0.326 -1 count
Fz 420 N -17 N

Any benefit in lift and drag is OK by me. It doesn't hurt, and it may keep the rear windscreen clean. Definitely worth trying. it may work well in conjunction with a trunk spoiler.

[ERTW]

A partial Kamm back - to the leading edge of the trunk - 11° rake.

Cd 0.343 +16 counts
Fz 428 N -9 N

Not useful imo.

A full Kamm back, 11° rake:

Cd 0.333 +6 counts
Fz 353 N -84 N

It reduces vortex drag (shown by lower lift), and increases form drag. The dominant vortex in the car's wake is very interesting.

Certainly following the real car's taper, I would expect a better result than 0.333. It doesn't look promising atm, and I'd lose the use of my trunk. I will try another few variations just for curiosity's sake.