[DHARROD]

My quick answer is Cam, Crank and compresion ratio. Sounds like your seeing engines that are creating 'old shool' horsepower/torque - have a lot of 'space' for fuel, longer distance for the piston to travel to create power/torque, thus they can't be starved for fuel (can lead to larger carb and psi). At a low idle you'll hear the firing order more distinctly (slow heart beat) - give it fuel and the beast is alive (fast heart beat one on top of each other). An engine guro will jump in on this and give you more clarity. Often these cars have to have a higher RPM at idle to keep the heart beat sounding adequate.