Article: adding a turbo to improve efficiency
 

Yet another good AutoSpeed article: Turbo'd for Fuel Economy - The way of the near future

A good overview of the issues around turbocharging.

The writer compares two 2.0 VW motors, one normally aspirated, the other turbo, and concludes:
Of course, to gain the most from the lower end torque, you really need better gearing.

A turbo is definitely high on my wish list.

Not exactly. You'll still be taking advantage of the increased low end grunt while accelerating, which is when most of your high fuel consumption rates are. You can shift at 2k and get good acceleration.

Of course, having higher gears would add even more benefit.

good article, i noticed that they never did actually tell you that when your going 0 to 60 in 7 seconds, that you are getting terrible mileage.

I think most of the gains they got in mileage is from the gearing. The turbo just let the higher geared car drive more like a normal car. If they can find identical cars one with a turbo and one without there probably wont be any difference.

Adding a turbo to a car and not changing gearing or going to a smaller motor is not going to help anything but acceleration really. Kind of a similar problem of doing aero mods and not changing gearing. The engine load decreases but the rpms are high enough you are still using a lot more gas than you really need to.

"or going to a smaller motor"
that i would like to see, two engines same power, one NA, and one turbo'd.
that would be interesting, OK who wants to do it?

PS. power and economy are usually inverse.

Oh, and let's not forget Autospeed's series on "The Story of Turbo'ing a Hybrid Prius" (1st gen. model.)

Where the author concludes:

He points out that, not surprisingly, the car's CVT contributes to the gains because it permits the revs to stay lower for a given road speed.

Parts 1 through 5:

http://www.autospeed.com/cms/A_2658/article.html
http://www.autospeed.com/cms/A_2659/article.html
http://www.autospeed.com/cms/A_2660/article.html
http://www.autospeed.com/cms/A_2661/article.html
http://www.autospeed.com/cms/A_2664/article.html

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I'm vaguely aware of a couple of Honda Insights that have also been turbocharged.

The turbocharger itself modifies the thermodynamic cycle of the engine and increases efficiency.

You can change mileage without changing gearing. The whole point of the article is to point out that a turbo will increase low end torque therefore the driver can upshift much earlier while maintaining the same acceleration. The 97 Volvo 850 is a good example of this. The non turbo 850 produces 168lb/ft at 4700rpm while the low pressure turbo produces 190lb/ft between 1800-5000rpm. While the fuel economy between the automatics is identical (LPT was only offered with auto in US) a manual trans car would give you the ability to upshift at much lower rpm.

I'll go out on a limb and say that while the ability to upshift sooner while accelerating is good, it's going to have a very small impact on overall fuel consumption, simply because we spend so little time accelerating, vs. time spent cruising (assuming you're not pulsing & gliding).

So unless you're driving a garbage truck or mail delivery route, better gearing matched to the turbo's good low end torque is what will return the largest fuel economy improvement.

I'm going to second Metro's comments and agree with Coyote X. Turboing increases power density. That will allow small gains if you can take advantage of increased low rpm power. However, the main gains are from regearing and using a smaller engine.