by Alan LaFrance @Texas1911, find him on Google or read more articles at www.subaruwrxsti.org
http://www.subaruwrxsti.org/subaru-wrx-sti-gas-mileage/
My 2011 STI hatchback get’s a stellar 17 MPG average and I couldn’t care less! If I baby it on the highway I might eek out 24 MPG. The fact of the matter is that no one buys these cars for the fuel economy, they buy them for the boy racer looks and the driving experience that a mountain of turbo torque and AWD grip can only provide. However, why DO these cars suck so much gas?! There’s really 5 main reasons; aerodynamic drag, engine dynamics, the drivetrain, weight, and lastly a unique PEBWAC error.
Aerodynamic Drag
The Subaru WRX STI is about as aerodynamic as a British Ship of the Line at full sail. All of the things that set it apart from the traditional Impreza also add to the aerodynamic drag generated by the body and components. There are two components to estimating total aerodynamic drag; the Cd (Coefficient of Drag) and the frontal area. The Cd of the GRB is 0.34 – 0.35 depending on options and trim. Compare that to the Cd of the base Impreza which is 0.31 and suddenly it makes more sense. Compare that to the BRZ at 0.29 and yeah, ahoy matey! Add that to the increase in frontal area from those broad shoulders as well.
So why the big increase in drag coefficient? It’s nearly the same car isn’t it?! Aerodynamically the STI is a complex beast akin to a race car. The STI’s turbocharged engine requires significantly more cooling and the addition of an intercooler, both of which increase the size of the openings for fresh air. These openings are a major contributor to drag in more ways than one. As the air enters the hood scoop it is slammed into the top mounted intercooler. This builds pressure which then pushes the air through the intercooler and down under the car. This airflow now gets sandwiched between the car and road, and it does it’s best to bang into the drivetrain and exhaust components along the way. All of this increases drag and increases lift (reverse downforce). The same happens to the bigger radiator inlets. Notice the STI has a much more open grille than the normal Impreza as well, to improve flow to the radiator. In fact, according to my aerodynamic modelling the hood scoop on the GD / GR model is worth 0.15 to 0.20 alone!
Other additions in drag come from the larger wake generated by the wider car. The wake is a large region of low pressure that trails behind the car until airflow has time to close in and increase the local pressure. Part of the reason for the top roof line spoilers on the STI is to reduce this wake, particularly on the hatchback model. The large wing in the rear adds to the total downforce generated but downforce does not come without costs. Here’s a great CFD (Computational Fluid Dynamics) pressure plot of the WRC GR. Notice the region of high pressure (red / orange) at the front radiator opening and the large trailing lower pressure (blue / green) region behind the wing and rear hatch. (Thanks to Mathieu Horsky‘s Good Work!)
As if that’s not bad enough, in testing the wide wheels and tires common to sports cars can cost up to 2 MPG alone in aerodynamic drag. Automakers work to get around this by adding gurney flaps in front of the wheels under the front valence. They also extend the bumper work further down on the corners. With the increasing chassis weight and size of the wheels on most modern cars the section width of the tires has to increase considerably. Ergo, the 265mm tires on my GR might look beefy, but they are as aerodynamic as a stone wall.
Engine Dynamics & Gearing
It’s well known that when you mash down the accelerator the engine guzzles fuel, but the STI has a far greater thirst for fuel than even a 5.0L V8. This is because of the high amounts of boost pressure provided by the turbocharger. Combining all of that warmed air into the chamber requires significant amounts of extra fuel to be injected just to keep combustion temps, and knock, down. However, the benefit to the turbo engine is that it consumes far less fuel than the 5.0L V8 when it’s cruising down the highway. This is because at cruising conditions both engines will run roughly 14:1 AFRs; meaning 14 parts air to 1 part fuel. This means the 2.5L H4 will consume considerably less fuel at the same RPM as the 5.0L V8. The turbo in a way becomes “displacement on demand.”
However, the comparison is very limited in one way; the 5.0L V8 produces considerably more torque at low throttle and low RPM situations thus negating it’s need for high ratio gearing. An example of this is the Chevrolet Corvette ZO6. In late C5 trim the Corvette produces a full 100 HP more than the STI, and displaces 5.7L compared to the 2.5L STI. The kicker? They are well known for getting 30+ MPG down the highway. How the hell is this possible? The gearing.
As you can see the extremely overdriven 6th gear ratio on the Corvette paired with the considerably lower final drive figure allows the LS6 engine to almost idle down the highway. This combined with excellent aerodynamics and lightweight construction allows the Corvette ZO6 to get excellent highway fuel economy.
Just how much does RPM affect fuel economy? Here’s some data on internal engine friction versus RPM.
As you can see, nearly everything increases in drag as RPM increases. This is especially important when you consider that the STI is turning 2 more differentials and an extra set of driveshafts compared to the Corvette. These drivetrain components consume even more power and add additional weight to accelerate. In fact, the STI comes in at 3384 lbs. which isn’t bad compared to the Dreadnaught’s being produced by domestic manufacturers, but it’s a far cry from the original 2734 lbs. in the early GC days. ( 1995 STI )
The problem with extra weight is it’s a self-fulfilling prophecy. The more the car weighs the bigger the brakes need to be, the more reinforcement the chassis needs, the more load on the engine, the bigger the stress on the gearbox, it goes on and on and on. You add 10 lbs. here and suddenly you need to add 2 lbs. there. It takes a company with an entire focus on lightweight engineering to actually build a sports car under 3000 lbs. these days.
The Unique PEBWAC Failure
PEBWAC – “Problem Exists Between Wheel And Chair”
The real reason why my car get’s 17 MPG? … I love to mash on the go faster pedal. I paid good money for the STI because it has amazing acceleration and is such a well rounded vehicle to drive. The little money I would save driving a lesser car would not replace the smiles per mile that a fun sports car can provide. This is why I wouldn’t be caught dead behind the wheel of a Prius or some other soul sucking car. It’s not worth it … and it never will be worth it. I will put the last drop of oil on Earth in my turbocharged machine and smile amongst the carcinogenic fumes as I burn T-Rex’s family tree just so I can pass you, sideways in the middle of a blizzard.
Drive on.
