Mechanical Calculator

The Mechanical Calculator works out the engine’s hard numbers — the geometry that fixes its displacement, the compression ratio, and how violently the piston moves. Open it from Calculators › Mechanical Calculator. Every length can be typed in millimetres or inches (and volumes in cc), and the three tabs build on one another.

Figure 1

Tab 1 — Engine Geometry

Enter bore, stroke, rod length and the cylinder count, and it returns the engine’s basic geometry:

Per-cylinder volume and total displacement — the swept volume of one cylinder, V = π × (bore ÷ 2)² × stroke, times the number of cylinders.
Bore / Stroke ratio — bore ÷ stroke. Above 1 is “over-square” (a high-revving character), below 1 is “under-square” (more low-end torque), 1 is “square”.
Rod ratio — rod length ÷ stroke. A higher rod ratio leans the rod over less, easing side-load on the cylinder wall and changing how long the piston dwells near the top.

Tab 2 — Compression Ratio

The first section builds the static compression ratio from the actual parts. Enter the bore and stroke, the combustion chamber volume, the piston dome / dish (a dome is positive and reduces the space; a dish is negative and adds to it; 0 is a flat top), the head gasket thickness and bore, and the deck clearance (positive when the piston stops below the deck at TDC, as in most engines). From these it computes:

Vd = π × (bore ÷ 2)² × stroke
Vc = chamber + gasket + deck − dome
Static CR = (Vd + Vc) ÷ Vc

where the gasket and deck contributions are themselves small cylinders (π × radius² × thickness). In plain terms, compression ratio is the whole cylinder volume divided by what is left when the piston reaches the top.

The Dynamic CR section is more realistic: it measures compression only from the point the intake valve actually closes, using the IVC after BDC angle and the rod length — DCR = Static CR × V_at_IVC ÷ V_max. Dynamic CR is always lower than static, and it is the number to weigh against fuel octane and cam timing.

Finally, the Reverse solver runs it backwards: set a target CR and choose what to solve for — chamber volume, dome/dish, or gasket thickness — and it tells you the part value that lands on your goal.

Tab 3 — Piston Dynamics

Enter stroke, rod length, RPM and (optionally) the piston mass, and it reports how hard the bottom end is working:

Mean piston speed — V_mean = 2 × stroke × RPM ÷ 60, the classic durability yardstick; sustained values much past roughly 20–25 m/s are hard on a street engine.
Peak piston speed — the true maximum, which the rod ratio shifts away from mid-stroke.
Peak acceleration at TDC and BDC — where the piston is wrenched to a stop and flung back; this is what loads the rod and pin.
Wrist-pin force at TDC — if you give a piston mass, F = piston mass × acceleration, the inertial force trying to pull the assembly apart. High RPM with a heavy piston is where trouble starts.

Tip — rod ratio and mean piston speed are quick health checks on a build, and dynamic CR is the figure to pair with your fuel and cam choice — a big cam bleeds cylinder pressure and lowers it, which is why two engines with the same static CR can behave very differently.