[ksa8907]

i think the vent in the cap is for a worst-case-scenario. like a fiery car crash, rather than building pressure and rupturing in an explosion, the tank will vent in a controlled manner.

i don't want to discourage creativity and ingenuity but you're not likely to make any measurable difference to efficiency when it comes to the engine. if i were going to do any changes to an engine for efficiency, id rebuild it and lighten the internal parts as much as possible without affecting strength and balance, use roller bearings, port and polish the heads, and port match the intake and exhaust.

these are tricks primarily used by performance/racing enthusiasts but they work great for efficiency too because they play entirely upon reducing lost energy. lost energy in the bearings, moving heavy internal parts, and volumetric efficiency.