Subaru STI: Is Diesel the Intersection of Power and Fuel Economy?
We all know that fast cars are fun and fuel-sipping cars are environmentally responsible, but is there a middle ground?
Short of expensive electric sports cars like the Tesla Roadster, there may be a solution to be found in diesel. Not only can diesel cars be fueled with waste vegetable oil, biodiesel, or some mixture of these fuels, but diesel engines produce a lot of torque and get better fuel economy than their gasoline-powered brethren.
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Autoexpress reports that the Impreza lineup will soon feature a 2.0L diesel engine sporting 148 bhp - but that engine could easily be tuned up to 180bhp for use in a sportier WRX model. This model could go 0-60 in under 7 seconds and wouldn’t top out until a respectable 140 mph.
Certainly impressive, but what we really care about is the fact that this engine could achieve up to 45mpg and reduce CO2 emissions 40% compared to the gasoline-powered STI.
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Source: Autoexpress
A WRX != an STi — the STi is a MUCH higher performance car than your base WRX — in its current incarnation, it’s a 305 HP beast. So, a 180 HP engine is, in fact ~ 41% lower powered than the 305 HP STi which this compares it to… so if you’re looking at power vs. CO2 emissions, it sounds like the diesel option doesn’t do much for you, now does it?
Are you seriously comparing the 180hp diesel engine to the 300hp 2.5L turbo STI engine? Well then of course the diesel wins on CO2 emissions and MPG. Can you instead compare the 180hp diesel to the 2.0L turbo that’s in a regular WRX? Let’s not compare apples and oranges here.
bgd73:
First you claim in (1) and (2) that horsepower is irrelevant and torque is king, and then argue in (3) that diesels don’t have the correct gearing…
All that really matters is:
A) Horsepower
B) Gear ratio (ideally a CVT, but if not, your torque must spread over at least and octave to be useful for a limited number of gears)
A better reference car to see that Diesel is an intersection of efficiency and power is the BMW 1-series diesel.
For example, the 120d with 177hp, 350Nm (258 ft lbs) torque while getting 39mpg city, 57 mpg rural, 49mpg combined.
To be fair, the 1-series employs some engineering tricks beyond using diesel fuel to increase efficiency: regenerative braking, an oversized starter motor allows them to automatically switch off the diesel engine at traffic lights and fire it back up in the time it takes to depress the clutch, etc.
Nonetheless, the petrol (gasoline) engine employs the same tricks and gets significantly lower efficiency for similar performance.
bgd73: horsepower is not just a retarded number and i’ll show you why.
Imagine i’m trying to change a flat tire, and the guy at the shop when a little crazy with the impact wrench and i can’t get the lugs off. Thankfully, i happen to have a piece of pipe that i can put over my tire iron to give me more leverage. Lets go crazy and say it’s a good 4′ long, and to make things easier for myself, i put it horizontal and just stand on it, to try and break the lug free. Now, i weigh a good 200lbs, so with a 4′ bar i’m generating 800ft-lbs of torque, more than a F-350 Superduty!
My point, in all this, is that one way of view horsepower is the speed at which you can apply torque. In my example above i’m generating 800ft-lbs, but no horsepower, because the bar isn’t moving. Torque *become* horsepower when it’s used to rotate something. I guess what i’m saying is that torque is great, and all things being equal a 200hp car with 200ft-lbs will be faster than a 200hp car with 100ft-lbs, but a 300hp car with 150ft-lb of torque will blow away a 150hp car with 300ft-lb of torque.
The advantage of diesel is and always will be the relatively higher fuel efficiency, and the extreme durability of the engines.
What does this have to do with the STI? A base 140bhp engine hotted up to “not quite gasoline WRX standards” is a far cry from the STI model.
How about, instead, they do a direct-injection flex-fuel vehicle? By the time it makes it to market, efficient (read: NOT CORN) ethanol production should be ramping up (2010 or so).
And yeah, what bgd73 said. Gearing. Torque. Area under the powerband. Peak HP (or torque for that matter) figures are lame.
it’s not true that horsepower is worthless. horsepower is representative of the amount of torque available at a given rpm. super high torque at lower rpm is the equivalent of lower torque at higher rpm. essentially, it’s rpm * torque = horsepower. good acceleration requires both. period. if you don’t understand this, take for instance, a 200 hp engine with 200 lb/ft of torque, and add a 2:1 gearbox in front of the tranny. you will effectively create an engine with 400 lb/feet of torque max, but can only turn at half the rpm. therefore, the horsepower rating will remain the same. will this improve the overall acceleration of the vehicle? no…
I love my 07 STI, but I’d also love to improve the fuel efficiency. Sounds like the engine in this article is a lot weaker than the STI engine/tune, perhaps they’ll address that after establishing it on the other Imprezas. Hopefully there will be an upgrade path (Subaru sanctioned or otherwise) to good Diesel engines on existing STI cars when things are working well.
dan there are some seriously fast diesel sports cars out there already made by Audi.Google up the Audi R8 =)
What matters is the area under the torque curve.
I can give you a two-stroke engine tuned to produce a very high horsepower in a very narrow, stratospherically high RPM band, and there is no way that you can use that to effectively power a conventional car.
After seeing Audi’s dominance of the LeMans series with their diesel powered race cars, how can anyone doubt that the diesel is a viable engine for a performance car?