Whether you’re looking for better lap times or more miles from each gallon, every engine is happiest when operating at peak efficiency. While today’s new cars are usually pretty well maxed-out from the factory, things start to change after thousands of miles have been covered: parts age and wear, dirt gums up the works, and equipment falls out of spec. …
One thing i think a lot of people don't realize is just how little variance in power output there usually is between perfect and imperfect-but-still-runs-ok condition. For example, the difference between the same engine having 180psi of compression and 120psi of compression will be fairly small, single digit percentage. If it has an aggressive aftermarket camshaft and a carburetor it might show horrible drivability side-effects at lower rpm, but if its a stock fuel injected car, it wont show horrible side effects anywhere at all. Air fuel ratio being off? On the rich side, again you'll lose single digit percentage of power until you're practically at the point of fouling the plugs. On the lean side you'll get misfires or lose the ability to rev cleanly through the entire range almost before you notice the actual drop of power itself. Of course you can plug an intake filter or an exhaust system to the point that the car makes far less power, but again it will show up as the engine running ok under some conditions and not ok during others (i.e. it doesn't run ok at 5000 rpm if it wont rev past 4000...).
Basically what i'm getting at is if you think your car is way down on power but it runs 'ok' pretty much everywhere in the load/rpm spectrum, then it's almost certainly NOT way down on power, it's probably down less than 10%! Pretty much anything that causes large power losses also causes drivability issues and would fall more under 'broken' or 'needs repair' vs 'out of tune'.
Practically the only exception is ignition timing. You can tune practically all the power out of an engine and still have it run 'ok' (at least until it melts something) by having the wrong ignition timing. In terms of gaining power by changing the 'settings' of things, as long as it has enough compression to start then any air fuel ratio that won't break it will be in the ballpark on power, and the big swings will come from ignition timing. Chasing the optimal ignition timing might require you to do things to the air fuel ratio to ALLOW the optimum timing, but the power is coming from the timing.
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