Quote:
Originally Posted by fielderLS3
What we basically disagree on most from a technical point of view is that with two engines that have the same power output, there won't be much if any difference efficiency wise from a NA engine and a "smaller" displacement turbo engine.
However, many here think a smaller turbo engine can make the same power as a N/A engine while using less fuel. Perhaps it could add more to the discussion if you explained scientifically why you think that is the case. Maybe I'll see the light and learn something, and maybe I'll rebut you. (And don't cite EPA ratings, because those are not an explanation of why).
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My understanding of forced induction is that it needs to be calibrated a little richer than NA when under boost, so at the same power output as a larger NA engine it will use more fuel when they're both making that level of power.
It's because under conditions where you aren't into boost that the smaller FI engine drops back into essentially NA mode that you make any fuel economy gains. So it makes sense to let the turbo engine stay out of boost during the emissions and mpg certification as much as possible.
Whether that happens in any random driver's daily driving is a different matter, though I'd guess that you (as a mfr) wouldn't want the boost to be too far out of reach during normal driving lest the car get exactly the reputation that the V8 side is viewing 4 cylinders in general with (a bog-slow low-compression NA 4). An enthusiast will tend to keep the engine at an rpm where boost will be available very quickly, while a non-enthusiast will tolerate longer periods of no boost and sluggish acceleration. Then again, when a 4 cylinder turbo is fitted with a manual transmission, even a non-enthusiast will tend to keep the transmission in each lower gear longer when driving in traffic.
I'm not going to chase down the data to argue overall cost of ownership other than to note (again?) that with an I4-T you have a simpler block casting, single exhaust with fewer catalytic converters and mufflers, and one less head with its valvetrain and associated controls. The financial advantage might still favor the V6-NA over the turbo-4, but it won't be by as much as the cost advantage that the V8-NA enjoys over the V6-TT, where the V6 here still has the V block casting, a more extensive intake, the same amount of exhaust plumbing, and the same number of heads, etc.
Norm