I still play with mini bikes when time permits. Currently doing an 8HP OHV B&S motored Heald Super Bronc. Torq-a-verter drive, custom header (played with the formulas and re did it length and diameter 7 times), bored, flowed, and rejetted the carb, longer runner intake manifold, completely filled/reshaped intake runner, reshaped/filled exhaust runner, unshrouded combustion chamber, milled head, Cadillac NorthStar intake valve, V-6 Nissan exhaust valve,both the same stem size but cut down in length, 5+ angle valve job, advanced timing(made an offset crank key), indexed and side gapped spark plug, and whatever I forgot. Nothing at all done to the short block, never dissembled it, only top end. Original HP=7.8, new HP=15.9. STOCK CAM WITH STOCK PHASING (so far). Old top speed=34, new top speed=63+. I have a cop RADAR gun.
Sometimes its easier to try things on a single cylinder, or a 4 cyl. 2000 Pinto engine. You can learn something on anything... I taught myself porting with a 2300 Pinto engine, Dykem layout fluid and spray paint. Where the air goes, so does the fuel, fuel removes Dykem, Exhaust burns paint, where the exhaust goes, no paint. I figured that out when I was very young and couldn't afford a flow bench. Paint/Dykem the ports, make a full throttle blast down the street, remove head grind/reshape the clean spots. By the time I made an industrial shop vac flow bench, I had a reasonable idea how air worked. The Superflow 600 came later...
Study steam engines if you want to understand compression(expansion) ratio effects. Study Boyle, Hooke, Gay-Lussac and Hilsch if you want to learn about air. The Super Flow tells you what happened, studying the pioneers tells you why. TIMINATOR P.S. later I'll clue y'all in on my other mini bike project.