Re: Hi everybody
I agree with what bowtieguy said above.
Regarding intake and exhaust: Consider how much power the engine can make (and the flow required for that), then consider how much power (and flow) you are using for economic driving. It's a fraction of the maximum airflow and should flow quite freely. GM's engineers are no fools, with ever-tightening CAFE regulations and the continued popularity of big trucks they can't afford to leave any easy MPG on the table when they design the Cobalt.
Few modifications to the car will pay for themselves; modifying your driving will. Keep an accurate gaslog and experiment with different strategies.
It would never pay for itself, but if you transplanted the transaxle and PCM from a Cobalt XFE (I think those are the main differences, plus LRR tires) you could probably get a big jump in FE.
There's a good chance that your car has a wideband O2 sensor. If so, you may find that accelerating with WOT (and shifting low enough to control your acceleration) will provide better FE. If not, 70-90% would likely be best, as much as possible without hitting open loop. Doing it that way reduces pumping losses at the throttle* and reduces friction and reciprocating loss, which leverages increased volumetric efficiency available from your engine. 2500 RPM is almost certainly too much for the best FE.
* Going back your concern about pumping losses in the intake and exhaust: As long as the throttle is closed, it's a much worse restriction than either of those systems.
P.S. If your motivation for saving fuel is something other than saving money, then obviously the concerns about what will and won't pay for itself are irrelevant.
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