Formula
Every cylindrical volume is the bore area times the height, converted at 16.387064 cc per cubic inch. The chamber total is the head, plus the gasket, plus the deck, plus the piston number. A dome is a negative piston number, so it comes back out.
cc = π/4 × bore² × height × 16.387064
chamber = head + gasket + deck + piston
static = (swept + chamber) / chamber
dynamic = (area × stroke after IVC + chamber) / chamber
Dynamic stroke is the piston travel from top dead center when the crank is at 180° plus the closing angle. Travel is r(1 − cos θ) + L − √(L² − (r sin θ)²), with r half the stroke and L the rod length. If the rod is not longer than half the stroke, or the closing angle is 0, dynamic compression is left blank. Deck clearance above the deck is not modeled: a negative deck height is treated as zero.
Worked examples
Bore 4.00, stroke 3.48, chamber 64 cc, dish 10 cc, gasket 4.10 × 0.040, deck 0. Swept 716.6 cc. Gasket 8.65 cc. Chamber 82.65 cc. Static 9.67:1.
The same engine with the intake closing 60° after bottom dead center and a 6.00 in rod. The piston has already risen part of the stroke, so the trapped volume is smaller and the dynamic ratio is lower than 9.67 and still above 6.
A flat top with 0.010 in deck. The dish goes to 0 and the deck adds a thin cylinder of bore 4.00 and height 0.010. The ratio rises compared with the 10 cc dish.
Reference
One cubic inch is 16.387064 cc. A 4.00 inch bore, one inch tall, is 206.4 cc.
| Piece | Volume |
|---|---|
| Swept, 4.00 × 3.48 | 716.6 cc |
| Gasket, 4.10 × 0.040 | 8.65 cc |
| Dish | 10.00 cc |
| Head | 64.00 cc |
| Static ratio | 9.67 : 1 |
Questions
What is the static compression of a 4.00 inch bore, 3.48 inch stroke, 64 cc chamber, and a 10 cc dish?
9.67:1 when the gasket is 4.10 × 0.040 inch and the piston is at zero deck. Swept volume is 716.6 cc. The gasket adds 8.65 cc. The chamber total is 64 + 8.65 + 10 = 82.65 cc. (716.6 + 82.65) / 82.65 = 9.67.
What is a positive piston volume?
A dish. It adds volume and lowers the ratio. A dome is negative: type −6 for a 6 cc dome. The head chamber is entered separately and is always positive.
What is dynamic compression here?
The same chamber, with the stroke measured from intake-valve closing back to top dead center. Enter the rod length and the closing point in degrees after bottom dead center. Leave the closing point at 0 to skip it. This is geometry, not a cam card recommendation.
Sources
- Static compression = (swept + chamber) / chamber. Chamber = head cc + gasket cylinder + deck cylinder + piston cc. 1 in³ = 16.387064 cc.
- Dynamic stroke is piston travel from TDC at crank angle 180° + IVC ABDC, using the rod-ratio formula. A sketch of the geometry, not a cam recommendation.