Boneyard Tools

Static vs dynamic compression ratio

How the static ratio from bore, stroke and chamber differs from the dynamic ratio the cam actually delivers, and why both matter for tuning.

Open the Compression Ratio Calculator →

What the static ratio measures

Static compression ratio is pure geometry. It takes the swept volume the piston sweeps between bottom and top dead centre, adds the clearance volume left above the piston, and divides that total by the clearance. Because it ignores valve timing and engine speed, the static figure is fixed the moment you machine the head, choose a gasket and pick a piston. It is the number this calculator returns, and it is the baseline every other compression figure is compared against.

Why dynamic compression is lower

In a running engine the intake valve does not slam shut at bottom dead centre. It stays open a little into the compression stroke so a fast moving intake charge can keep filling the cylinder. During that window some mixture is pushed back out, so effective compression only begins once the valve seats. The result is a dynamic compression ratio that is lower than the static one, and the longer the cam duration, the bigger the gap.

How chamber volume swings the number

Clearance volume is the lever that moves the ratio. Shave the head, run a thinner gasket, or fit a domed piston and the chamber shrinks, so the same swept volume divides by a smaller number and the ratio climbs. A dished piston does the opposite by adding clearance and dropping the ratio. Because the clearance figure is small, a few cc either way changes the result far more than an equal change in swept volume, which is why accurate chamber cc measurement matters most.

Choosing a ratio for your fuel and cam

A pump gas naturally aspirated build often lands near 10 to 11 to 1, while race engines on high octane or methanol run much higher. Forced induction usually drops static compression into the high single digits to leave room for boost pressure. A long duration cam that bleeds off cylinder pressure lets you run a higher static ratio without knock, so cam choice and compression ratio should be picked together rather than in isolation.

Frequently asked questions

Can this calculator give me dynamic compression?

No. Dynamic compression needs the intake valve closing point from your cam card, which is not one of the inputs. This tool reports the static geometric ratio, and you can pair it with a dedicated dynamic calculator that asks for cam timing.

Should I target the same ratio for boost as for naturally aspirated?

No. Turbo and supercharged engines add pressure on top of mechanical compression, so builders usually pick a lower static ratio, often in the 8 to 9 to 1 range, to keep combined cylinder pressure and knock in check.

How precise does my chamber measurement need to be?

As precise as you can manage. Because clearance is the small number in the formula, a one or two cc error can shift the ratio by a noticeable amount, so measuring the chamber by filling it with fluid beats trusting a catalogue figure.