Engine Room · Demo
Firing order and reciprocating balance — why a six purrs and a four buzzes.
Even firing every 180°, but the famous uncancelled secondary force.
Even firing means a power stroke every 180° of crank rotation — chosen for smooth torque delivery and even crank loading. The lit chip is the cylinder on its power stroke right now.
Why layout is character
One push per two turns is lumpy. Add cylinders and stagger their firing and the pushes overlap into something smooth — a four fires every half turn, a straight-six every third. But each piston also slams up and down, and whether those inertia punches cancel or add up depends purely on how the cylinders are arranged. That is why a straight-six purrs, an inline-four buzzes, and a boxer feels flat and calm.
First and second order.Piston inertia shakes at crank speed (first order) and — a consequence of the finite rod you met in Crank & Piston Geometry — again at twice crank speed (second order). The inline-four’s mirrored throws cancel the first but stack the second; the straight-six’s 120° spacing cancels both, and its symmetric layout cancels the rocking couples too. Big fours add counter-rotating balance shafts to fake what the six gets free.
Sound is geometry too.Even firing gaps make a smooth hum; uneven gaps (the cross-plane V8’s 90–180–270–180 pattern) make the burble. The exhaust note you can identify from across a street is the crankshaft’s geometry, made audible — hear it synthesized live in The Sound of Engines.
Reciprocating forces only. The balance meter shows piston inertia; it ignores rotating imbalance (crank counterweights handle that), torsional vibration and combustion torque pulses. Right for comparing layouts, not a full NVH model.