When I went to tech school I was taught that atmospheric pressure pushed air into the area of the cylinder as the piston was moving down on the intake stroke. Carbs have a throttle plate that controls the amount of air into this cylinder. With no load engine at 1500 rpm's, the throttle is only open slightly. This give us a high manifold vacuum. If the auto trans is put in gear (with your foot on the brake ) and then the engine rpm's are raised under load to 1500. The throttle plate is now open farther allowing a greater amount of air/fuel to fill the cylinder. More air, more fuel, different power output, different sound.
Diesels completely fill the cylinder on each stroke, no throttle plate to hold back atmospheric pressure. Power is controlled by fuel only.
Joe
