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Understanding Engine Displacement
Engine displacement is the combined volume of all the cylinders in an engine as the pistons move from Bottom Dead Center (BDC) to Top Dead Center (TDC). It is a primary indicator of an engine's potential power and torque output. In the United States, this is traditionally measured in Cubic Inch Displacement (CID), while the rest of the world and modern manufacturers typically use Liters (L) or Cubic Centimeters (CC).
The Displacement Formula
To calculate the cubic inch displacement of an engine, you use the following geometric formula:
CID = (Bore ÷ 2)2 × π × Stroke × Number of Cylinders
Alternatively, a simpler version for manual calculation is:
Bore × Bore × 0.7854 × Stroke × Cylinders
Key Definitions
- Bore: The diameter of the cylinder opening.
- Stroke: The distance the piston travels inside the cylinder from top to bottom.
- Cylinders: The total number of combustion chambers in the engine block.
Common Engine Displacement Examples
| Engine Name |
Bore (in) |
Stroke (in) |
Cylinders |
Resulting CID |
| Chevy 350 V8 |
4.000 |
3.480 |
8 |
349.85 (350) |
| Ford 302 V8 (5.0L) |
4.000 |
3.000 |
8 |
301.59 (302) |
| Chevy 454 Big Block |
4.250 |
4.000 |
8 |
453.96 (454) |
| Small Block Chevy 400 |
4.125 |
3.750 |
8 |
400.92 (400) |
Why Calculate Displacement?
Calculating CID is crucial when performing engine "overbores" during a rebuild. For example, if you "bore it .030 over," you are increasing the bore diameter by 0.030 inches. This tool allows you to see exactly how much volume you've added to the engine, which affects your compression ratio and air-fuel requirements.
Conversion Factors
- CID to Liters: Divide CID by 61.024
- CID to CC: Multiply CID by 16.387
- Liters to CID: Multiply Liters by 61.024
function calculateDisplacement() {
var bore = parseFloat(document.getElementById('bore').value);
var stroke = parseFloat(document.getElementById('stroke').value);
var cylinders = parseFloat(document.getElementById('cylinders').value);
if (isNaN(bore) || isNaN(stroke) || isNaN(cylinders) || bore <= 0 || stroke <= 0 || cylinders <= 0) {
alert("Please enter valid positive numbers for all fields.");
return;
}
// Formula: (Bore/2)^2 * PI * Stroke * Cylinders
var radius = bore / 2;
var cid = Math.pow(radius, 2) * Math.PI * stroke * cylinders;
// Conversions
var liters = cid * 0.0163871;
var cc = cid * 16.3871;
// Display results
document.getElementById('res-cid').innerHTML = cid.toFixed(2) + " cu in";
document.getElementById('res-liters').innerHTML = liters.toFixed(2) + " L";
document.getElementById('res-cc').innerHTML = Math.round(cc).toLocaleString() + " cc";
document.getElementById('engine-result').style.display = 'block';
}