rusty, in this there are 'experts' and 'expert experts' and...
read aufitt response as simple correction of terminology
being initial correction before further flow of meaning..
analogy - eating food for energy; relative amount of food ingested
= potential energy,, this large stomach aids ingestion of more food..
restricted mouth/throat determines how much food enters stomach..
impacted bowel similarly effects ongoing elimination thus process..
type of food eated, calories etc etc, effects energy potential of system..
valves/multiple valves thus also camshaft lobe shape = how much fuel
can enter, speed of entry and 'swirling' patterns as fuel enters..
analogous to mouth/teeth/tongue/throat for biting/chewing/swallowing food..
valve function esp at high revs also depends on precise fast opening and closing
[for max flow-thru of fuel/air] thus valve spring efficiency a factor, unless
valves are closed mechanically as for ducati desmodromic system..
bore and stroke ratios of piston/cylinder effect potential valve size/number
also speed of piston up and down thus time for sucking in more fuel..
compression ratio = how much bang you get from the system,
depending on type of fuel used to get the bang..
details of it include tuning fuel/air intake, also length etc of exhaust,
thus 'shape' of exhaust pulses exiting thru exhaust, which may
help suck fuel out and also may help restrict escape of fuel once in..
[thus exhausts themselves, rather than simply dumping it out
directly thru valves to atmosphere in racing applications,,
as some, restriction, helps retain fuel air in combustion chamber]
there is always some trade-off in designing internal combustion engines,
esp for typical engines in typical road vehicles, where small gains here
may mean large costs [in $, noise, etc] there..
as for your friends scooter engine, it comes down to that formula..
design engineers have already designed it according to their formula
which includes typical road use, incl economy, servicing and engine life..
you can always improve on the engine, in small ways [spark plugs etc]
but dont expect to greatly enhance a 161cc scooter performance
with typical engine add-ons.. [get her a cbr250r