How does cid make power?

dusterbing the peace said
Fuel, gasoline for these motors, has a BTU/lb rating of 11,500-13,000 typically. BTU/hr is a unit of power, readily converted to horsepower: 1 hp = 2,544.4 BTU/hr.

The more fuel you can get in the motor, the more power you can get out (to the limit of efficiency for the particular motor). You need air to burn the fuel, which is where air to fuel ratio comes in. You’ve probably heard numbers like 12.8, which simply means 12.8 parts air to one part fuel.

The more cubic inches displacement, the more fuel and air can be burned. Some of the examples being thrown around comparing 440 ratings are simply a difference in volumetric efficiency of the motor. A well built small block will out power a crappy big block all day, because the small block is extracting the energy from the fuel better than the big block. If the same design parameters were applied to the build of the big block it would win, simply because it has more fuel going through it.

In this regard, it is easier to burn more fuel with less build effort from a big block.

Let’s do 500hp
That takes 1,272,217 BTU/hr of fuel. If we use good gas with 13,000 BTU/lb energy, we need 97.9 lb/hr…but that’s at 100% thermal efficiency. A gasoline engine is closer to the low 30s percentage wise for thermal efficiency. So at 35% thermal efficiency we need 279.6 lb/hr fuel flow. At 12.8:1 AFR we then need 3,579 lb of air which ends up at 739 CFM. That is a 340 at 7,512 rpm, or a 440 at 5,805 rpm…or for the earlier example, 100 cid at 25,541 rpm. And those are at 100% volumetric efficiency…I’m done doing math and don’t feel the need to give examples of higher or lower values there.

Which one sounds easier to build?

Good summary.

I'd simplify it thusly:
Engines run on expanded air. Burning fuel expands the air. Not all the heat from the burnt fuel turns to power that's why exhausts are hot.

Torque is based on how much expansion you can get and is a function of rod throw and piston area (diameter) and total expansion (roughly related to compression and time).

Horsepower is how fast you can get through the power cycles.

If the head flows ****, you'll still make torque (maybe more because the air expands for longer since it can't get out) but you can't exchange the air and so can't rpm and thus **** hp.

Flow is dictated by engine architecture as RB says. Some architectures can't support enough valve, or port, or compression, etc due various constraints. Take two engines of the same displacement, and both will make similar PEAK torque numbers, but the one which flows more at the RIGHT time will keep making torque as the revs increase and thus make POWER.

When it comes to flow, there's a practical limit to what can be done with poppet valves. Airplane engine mfgs knew this decades ago. Go look up sleeve valve engines from the 40s and prepare to have your minds blown.