Quote:
Originally Posted by bradleyland
Eh, not necessarily. If you go on boost, you're going to suck down some fuel. When on boost, forced induction engines consume far larger volumes of air than NA engines, and thus more fuel. This is true even at lower RPMs.
A turbocharged engine is only a net positive when compared to a larger displacement engine that must move large volumes of air all the time.
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That's somewhat true, but there is more to it.
A turbo design attempts to give you the fuel economy of it's smaller displacement, when driven normally as one would a comparable size NA engine. But then, when the driver does want the power of a larger sized engine, he can still have it by giving it more throttle and inducing the added efficiency of forced air into that same combustion space.
A turbo engine doesn't have a net positive only against a power comparable larger sized NA engine, it also gives the benefit of the better fuel economy of the smaller displacement, when driven in the same manner. When MPG is compared to power production, the same size 3.0 turbo is actually MORE fuel efficient.
How is that!?
Consider that both engines have a 3.0 liter capacity.
The NA version achieves 230hp and 200lb ft of torque, and gets, in manual form, 18mpg city 28mpg highway.
The TT version achieves 300hp and 300lb ft of torque, and gets, in manual form, 18mpg city 25mpg highway.
The TT puts out 70hp more and 100lb ft more torque than the same sized NA engine, yet, it gets the SAME city MPG.
It is more fuel efficient, as it can produce 30% more HP, and uses the same amount of fuel. So, for typical city driving either engine uses about the same fuel. Remember, current EPA MPG tests use more realistic driving conditions than before. Achieving the same city MPG demonstrates the efficiency of the turbo design.
Along with that same city MPG, the TT also gives you more power.
Granted, if you get on it MPG falls, but it also falls for the NA engine once you smack the loud pedal as well.
Also, the TT gives you a whopping 50% more torque.
That is significant, and demonstrates how turbo charging can improve efficiency even compared to same size NA engines.
30% more HP, 50% more torque, same MPG.
On the highway, the NA engine has a slight advantage as it gets 3mpg better fuel economy. Part of this is due to the NA cars 120lb lighter weight as it has less standard content, and doesn't have the additional intake plumbing of the turbo system.
So, the TT takes a hit due to carrying more inherent weight, but also has more standard content, to bring the total weight up.
Overall, the TT has 3mpg less, or 12% less fuel economy, but remember, it also gives 30% more HP, and 50% more torque. Not a bad trade for that much more power output.
Yes, the turbo will use more fuel when you call up the boost. But, the NA engine will also use more fuel as revs climb to achieve more power.
But, given the very high percentage difference in overall power, I don't see an equal percentage loss in MPG.
Also, in a turbo you're not in big boost very often at all. It's not you drive it at 100% boost capacity all the time.
I drive my 135i manual fairly aggressively, with most of my driving at the same rate as any other car on the road. I still manage 22-23mpg overall.
That's pretty darn good for acceleration fun I get.
Some TT drivers get in the high teens as they are on boost quite often, and/or drive in hilly areas. But, a NA driver driving in a similar fashion is not going to get 28mpg average either. It's gonna drop big too.
The NA engine is a sweet piece that loves to rev, and needs to rev if you want more power. Those revs cost in friction and more fuel consumption.
There is no free lunch. With a turbo engine, you can achieve same acceleration as the NA engine with less throttle and less revs.
If the NA version were getting something like 24mpg city and 30mpg highway, then we could say there is a substantial difference.
But, that's not the case.
Overall, the TT engine can be considered more fuel efficient when you factor in the added power gained from the same engine size.
That's the "magic" of turbocharging.
