Static compression from bore, stroke, chamber cc, dish or dome, gasket and deck, with a gasket thickness table and the chamber or piston volume a target ratio needs.
Compression Ratio Calculator - Static CR from cc and Inches
Static compression from bore, stroke, chamber cc, dish or dome, gasket and deck, with a gasket thickness table and the chamber or piston volume a target ratio needs.
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Static compression ratio from bore, stroke and cc
Enter the bore, the stroke, the head chamber in cc, the piston dish or dome, the head gasket and the deck clearance. The compression ratio calculator adds up the clearance volume, divides the full cylinder by it and gives the static ratio to two decimals. It also shows which part of the clearance volume comes from where, what other gasket thicknesses would do, and, if you type a target, how many cc the chamber or the pistons need to hit it. Inches or millimeters for lengths; volumes are always in cc, the way heads and pistons are specified.
The quick start V8, tile by tile
Per cylinder, from the quick start inputs. The ratio is the first tile divided by the second.
The whole cylinder with the piston at the bottom: 727.4 cc swept plus 82.66 cc left at the top.
The clearance volume. The chamber is only 64 cc of it; gasket, deck and dish add almost 19 cc.
The gasket ring: a 4.100 in circle 0.039 in tall. Every 0.010 in of gasket on that bore is about 2.16 cc.
The gap above the piston at top dead center, 0.025 in over the full 4.030 in bore.
Filling the form from a parts list
- Units - inches or millimeters for bore, stroke, gasket and deck. Chamber and piston volumes are cc in both cases.
- Bore and stroke - the finished bore after any overbore, and the crank stroke. Together they give the swept volume.
- Number of cylinders - optional, only for the total displacement in cu in and liters.
- Chamber volume - the cc of the head chamber, from the head maker or measured with a burette. A milled head has a smaller chamber, so re-measure after machining.
- Piston volume - positive for a dish or valve reliefs, negative for a dome, the convention JE Pistons uses. Flat top with no reliefs: 0.
- Gasket bore and compressed thickness - from the gasket package. The compressed figure, not the free thickness. Leave the gasket bore empty and the cylinder bore is used.
- Deck clearance - how far the piston sits below the deck at top dead center. Negative if it comes out of the hole.
- Target ratio - optional. The result then says what chamber or piston volume would give it.
One change at a time
The quick start engine with a single input changed per row. Small cc differences move the ratio a lot because the clearance volume is small to begin with.
| Change | Clearance | Ratio |
|---|---|---|
| As entered | 82.66 cc | 9.80:1 |
| Gasket 0.027 in instead of 0.039 in | 80.07 cc | 10.09:1 |
| Block decked to zero | 77.44 cc | 10.39:1 |
| 58 cc heads | 76.66 cc | 10.49:1 |
| 76 cc heads | 94.66 cc | 8.68:1 |
| Stroke 3.750 in, same pistons and deck | 82.66 cc | 10.48:1 |
| 12 cc dome, deck zero | 60.44 cc | 13.04:1 |
The stroke row has a catch: a longer stroke on the same block usually needs different pistons or a different deck height, which changes the clearance too. Enter the parts you will actually run.
Facts that come out of the arithmetic
A ratio is per cylinder. The number of cylinders never enters the formula. A 77 x 64 mm single with a 22 cc chamber and a 0.6 mm gasket with a 78 mm bore is 12.98:1 whether it is alone or one of six.
Gasket and deck often hide more than the dish. In the quick start engine they hold 13.67 cc together, almost three times the 5 cc of dish.
An overbore adds displacement, not much ratio. The same V8 at a stock 4.000 in bore is 349.8 cu in; at 4.030 in it is 355.1 cu in.
Exact conversion: 1 cu in is 16.387064 cc, so a 44.39 cu in cylinder is 727.4 cc.
Gasket volume by gasket bore
Pure geometry: the area of the gasket opening times its compressed thickness. Use it to see what a thicker or thinner gasket is worth in cc before you open the calculator.
| Gasket bore | Per 0.010 in | At 0.039 in | At 0.051 in |
|---|---|---|---|
| 3.500 in | 1.58 cc | 6.15 cc | 8.04 cc |
| 3.750 in | 1.81 cc | 7.06 cc | 9.23 cc |
| 4.000 in | 2.06 cc | 8.03 cc | 10.50 cc |
| 4.100 in | 2.16 cc | 8.44 cc | 11.03 cc |
| 4.250 in | 2.32 cc | 9.07 cc | 11.86 cc |
| 4.500 in | 2.61 cc | 10.16 cc | 13.29 cc |
Who checks a ratio before ordering parts
Choosing a head gasket
The gasket table in the result shows the ratio at nine thicknesses. On the quick start engine, going from 0.039 to 0.051 in costs 0.27 of a point.
Picking pistons for a rebuild
Type a target of 10.5 and the quick start engine needs -1.09 cc of piston volume, a nearly flat top, or 57.91 cc heads with the current pistons.
Checking a published build
JE Pistons' worked example, 4.310 x 4.250 in, 118 cc heads, -18 cc dome, 0.041 in gasket, zero deck, gives 10.25:1 here, the same as JE's own calculator.
Lowering compression for boost
For a target of 9.0 the same V8 needs 8.26 cc more clearance: 72.26 cc heads, or a 13.26 cc dish.
Compression ratio questions
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