Mirage aero mods for better fuel economy: 4.5% improvement @ 90 km/h / 56 mph

[MetroMPG]

To get ready for the 2014 SMCC Economy Run event in Montreal, I slapped together some temporary coroplast aero modifications on the 2014 Mitsubishi Mirage 5-speed that Mitsu loaned me.

See: Review notes: base 2014 Mirage 5-speed manual (Mitsubishi Canada loaner)

I actually ran out of time before the economy run to do a few more things that I would have liked to. But the truth is, the relatively low average speeds of the rally meant that these mods probably didn't contribute a whole lot. Mostly I did this because I wanted to see what someone with a bit of DIY skills could do to squeeze more efficiency from a Mirage in highway use.


The Mirage's good factory aerodynamics aren't easy to improve on ...

Mitsu hasn't left as much low-hanging fruit on the car as other companies do on theirs. The Cd. is 0.28 in North America, and comes with multiple factory aero features like:

- drag-reducing spoiler/roof extension
- partial lower grille block off (and the size of blocked area varies between the CVT and manual due to different cooling needs)
- relatively smooth wheel covers (base model)
- narrow section LRR tires (165 mm)
- "optimized" rocker panel design
- sharp trailing edge/corner on the rear bumper cover
- plastic underbody smoothing panels
- front wheel/suspension deflectors

Here's what I added to that...

1) Grille blocks



Cooling system drag is a significant part of the total aero drag of most vehicles, which is why Mitsu already "blocked" a good portion of the factory grille opening. For max efficiency, you want to permit only the right amount of air flow into the engine compartment that's needed. Any more than that is wasteful.

This is why we're seeing more and more automatic grille shutters on cars.

This mod is less extensive than it looks because Mitsu blocked out about half of the "apparent" lower grille openings in the 5-speed Mirage. I closed the upper slot plus a bit more than half of the lower openings.

If you're going to do this kind of mod, you have to be able to monitor your coolant temps - you don't want your radiator fan running all the time or the added electrical load will cancel out your efficiency gains (not to mention put extra heat stress on the engine & cooling system). I have an OBD-II ScanGauge that has a digital coolant temp gauge with "max value" recorded per trip.


2) Fog light inserts

I'll be the first to say there's probably no measurable effect from covering these up, but they looked lumpy and I had the plastic & tape out already, so why not? I liked the look, too!


3) Air dam



Added an ~8 cm / 3 inch dam as far forward as I could, wrapping smoothly back to the front wheel arch on the sides. It left about 3.5 inches of ground clearance (and it scraped 3 or 4 times during the rally, but probably wouldn't scrape at all in my normal driving).

This mod is hard to do when you can't drill the borrowed car to attach things!

I actually fastened this to the car by making a wooden 1x2 inch frame. I sandwiched the flat portion of the bottom of the OEM bumper cover between pieces of wood joined by screws through existing holes in the plastic.

Lots of tape (and later Goo-gone) used to seal the coroplast against the factory bumper.


3) Passenger side mirror delete, driver's folded back

I replaced the passenger side mirror with a small (approx 2.5 x 3 inch convex mirror) in a coroplast housing; I folded the driver's mirror back and attached a convex mirror to its trailing end.

These mods reduce both frontal area and drag coefficient. Doubly whammy.


4) Passenger wiper delete



It parks unusually high and at an odd angle on this car. Although it might also be angled so the local flow goes more from end than face-on. A quick tuft test would answer that.


5) Kammback/spoiler extension



The biggest opportunity to reduce drag on production road vehicles is at the rear - thus Mitsu's neat looking "backward baseball cap" factory drag-reducing spoiler.

The further you taper the roof down and the sides in toward the center, the lower your overall drag will be, by reducing the size of the trailing wake/low pressure behind the car. Of course, a complete "boat tail" add-on would improve fuel consumption by 10-15% at moderate highway speeds. Hardcore efficiency nuts put these on their cars (I've tried them on my 1st gen. Insight and my Firefly/Metro). But a full tail just isn't practical for use on a bumpy rally route, and not something you could throw together in the couple of hours anyway.

So what I did was extend the factory spoiler (following the existing roofline) by about 25 cm / 10 inches, and up to 38 cm / 15 inches on the sides. It includes a notch to show the LED center brake light, and as you can see, it didn't obstruct the inside mirror's rear view out of the car -- you could still see the horizon behind on a flat road.


Testing...

14 runs, for 7 bi-directional averages.

I used my usual flat, straight test route. Conditions were perfect: 25C and dry, no wind at all. There was never any traffic ahead of me in my lane (to throw off results with an inadvertant drafting effect), but sometimes oncoming traffic went past on the other side.

I would have liked to do an A-B-A style comparison, but didn't have time. Also, this car doesn't have cruise control, so I had to keep a laser focus on that speedometer needle. It's pretty easy to hold the Mirage at a dead steady speed on a flat road, but it did wander half a km/h or so up or down. So take all these numbers with an appropriately sized grain of salt.

I did A-B-C, where...

- A was all mods in place;
- B was all mods in place except the air dam;
- C was all mods removed


Results ...



I removed the air dam separately because I was most curious about it. It was pretty much a given that all the other mods would help (or have little effect), but the air dam had the potential to actually make things worse, and I wanted to check that.

The fuel economy improvement between unmodified (C) to modified (A) was 4.5 % @ 90 km/h / 56 mph based on these numbers.

Not bad.

There's even more aero improvement available to intrepid Mirage aeromodders, if you want to go further:

• more extensive underbody smoothing/panels
• rear wheel skirts
• smooth wheel discs
• lowering
• boat-tailing or rear box cavity
• etc.