Engine Displacement Calculator

Getting a number you can act on

1. 01
   Measure the bore, or take it from the specification

   Bore is the cylinder diameter in inches. On a rebuilt engine, measure it rather than assuming — a block that has been bored before is no longer at its nominal size, and that is exactly the case where the calculation matters.

2. 02
   Enter the stroke

   Stroke is how far the piston travels between top and bottom dead centre, which is twice the crankshaft throw. It is a property of the crankshaft, so a stroker crank changes it and boring the block does not.

3. 03
   Set the cylinder count

   Straightforward, but worth checking on unfamiliar engines. The per-cylinder figure the calculator gives is often the more useful number when comparing designs across different layouts.

4. 04
   Read the overbore table if the block is being machined

   Standard cleanup steps are +0.020, +0.030 and +0.040 inches. The table shows what each one does to displacement, which is usually less than people expect.

5. 05
   Check the bore/stroke ratio for character

   Above 1.0 the engine is oversquare and inclined to rev; below 1.0 it is undersquare and inclined toward low-end torque. It is the single most descriptive number about how an engine will behave.

What the calculator is actually doing

Nothing here is proprietary. If you would rather check it by hand, or explain it to someone at a counter, these are the same expressions the tool evaluates.

Why bore matters more than stroke

Look at the formula and one thing jumps out: bore is squared, stroke is not. That single asymmetry explains a great deal about how engines get built.

Doubling the stroke doubles the displacement. Doubling the bore quadruples it. So a small change in bore moves displacement considerably more than the same change in stroke, which is why boring a block is such an efficient way to find capacity.

It also explains why overbore steps are quoted in thousandths of an inch. On a four-inch bore, going +0.030 adds about 5 cubic inches across a V8 — real, but modest. Going a full quarter-inch larger would add over 40, which is why block wall thickness rather than arithmetic is what limits how far you can go.

The practical consequence for anyone rebuilding an engine: the displacement gain from a cleanup bore is a side effect, not the reason. You bore a block to get a true, round, straight cylinder. The extra capacity is a bonus you would not pay for on its own.

What the bore/stroke ratio tells you

This one number describes an engine's temperament better than its displacement does.

An oversquare engine — bore larger than stroke, ratio above 1.0 — has a shorter piston travel for a given capacity. That means lower piston speed at any given RPM, which lets the engine rev higher safely. It also allows larger valves, because there is more room in the bore to fit them. These engines make their power high in the range.

An undersquare engine — stroke longer than bore, ratio below 1.0 — has more leverage on the crankshaft throw and generally fills its cylinders well at low speed. Piston speed climbs quickly with RPM, so these engines are usually happier lower down. Diesel engines and older truck engines are strongly undersquare for exactly this reason.

The Ford 300 inline-six is a good illustration: a 4-inch bore with a 3.98-inch stroke is very nearly square, but it was tuned for low-speed pulling and became one of the most durable truck engines ever built. Meanwhile the Coyote 5.0 at 3.63 by 3.65 is nearly square too, and revs to 7,000. The ratio sets the possibilities; the cylinder head and camshaft decide what is done with them.

Why badge sizes rarely match the arithmetic

• A Chevrolet 350 calculates to about 350.0 cubic inches, which is unusually honest
• A Ford 302 is closer to 301.6, and a 351 Windsor to 351.9
• A 5.7-litre badge covers anything from about 345 to 350 cubic inches
• Marketing rounds to the nearest attractive number, and always has
• Metric badges round to one decimal place, hiding up to 50 cc either way
• For machining and compression work, use the calculated figure rather than the badge

Stroking, and why it is the other route to capacity

If bore is limited by the block, the remaining variable is stroke — and that is a crankshaft change rather than a machining operation.

The best known example is the 383: a Chevrolet 350 block bored 0.030 over, with the 3.75-inch crankshaft from a 400 fitted in place of the 3.48-inch original. Bore goes up slightly, stroke goes up considerably, and the result is around 383 cubic inches from a 350 block.

Stroking changes the engine's character as well as its size. A longer stroke lowers the bore/stroke ratio, moving the engine toward low-end torque and away from high-RPM breathing. That is usually the intention, but it is worth being deliberate about — a stroker built with high-RPM cylinder heads and camshaft is working against itself.

It also raises piston speed at any given engine speed, which is the real limit on how hard a long-stroke engine can be revved. Capacity is easy to add; the consequences arrive with it.