The cylinder is not sealed at bottom dead centre
Static compression ratio assumes it is, which is a convenient fiction that makes the number comparable between engines but does not describe what happens in one. On any cam with real duration the intake valve is still open sixty degrees past bottom dead centre, and until it shuts, the piston coming up simply pushes mixture back out into the port. Compression starts at the closing point, not at BDC.
So the useful question is: how far up the bore is the piston when the valve finally shuts? That distance is the effective stroke, and dynamic compression ratio is the static sum run on it.
Where the effective stroke comes from
Not from a fraction of the stroke, and not from a cosine. The piston position for a crank and rod is the full slider-crank expression — the crank throw times one minus the cosine of the angle, plus the rod length, less the square root of the rod squared minus the crank throw times the sine, all squared. It is exact rather than approximate, and it is why rod length appears in a compression calculation at all.
On the defaults — 3.480 in stroke, 5.700 in rod, intake closing 60 degrees after BDC — the piston has come 2.813 in from top dead centre, which is 80.8 percent of the stroke. It handed back 0.667 in, and with it 139 cc of the 727 cc the cylinder actually sweeps.
What that does to the number
With 66 cc of clearance volume, the static ratio is 12.02 to 1 and the dynamic ratio is 9.91 to 1 — a difference of 2.11, or 17.6 percent. Nothing about the engine changed between those two figures except which crank angle you started counting at. That gap is the cam, and only the cam.
The table on the page sweeps the closing point either side of where you set it. Ten degrees of intake closing is worth about three quarters of a ratio on these numbers, and advancing or retarding the cam by ten degrees moves it by the same amount in the opposite direction, because advancing the cam closes the intake earlier.
Which cam card figure to use
This is where the number most often goes wrong. Cam cards quote duration and event angles two ways — advertised, at a low checking lift, and at 0.050 in of lifter rise — and the two intake closing points typically differ by twenty-five to thirty degrees. Put an advertised closing angle into a calculation that expected a 0.050 in figure and the answer moves by most of a whole ratio. The card states which convention it is using; read it before typing.
Whether the valve is meaningfully sealed at either of those lifts is a fair question and no arithmetic answers it. Both are conventions for describing a lobe, not measurements of when gas stops moving.
Rod length is not a rounding error
Rod length changes where the piston is at a given crank angle, and the direction is the opposite of what most people expect. A longer rod pushes the motion toward a pure sine wave, which part-way up the compression stroke leaves the crown a little further from bottom dead centre than a short rod would. More stroke has already been handed back when the valve shuts, so the dynamic ratio comes out slightly lower. On the defaults the rod to stroke ratio is 1.638 and the answer is 9.91 to 1; fit a 6.000 in rod and it is 9.88, a 5.200 in rod and it is 9.97. Across that whole span the effect is about 0.15 of a ratio — real, worth having the field on the form, and much smaller than a cam change.
The pressure line and what it is not
Raise the dynamic ratio to a chosen exponent, multiply by the intake pressure, and you get a number in psia. It is a model. The exponent stands in for heat lost to the chamber walls and gas lost past the rings while the engine turns over slowly, and different sources use different values for it, which is why the field is an input. What comes out is not a prediction of what a compression gauge on your engine will read: cranking speed, battery condition, throttle position, ring seal, chamber temperature and the gauge itself all move a real reading and none of them are here.
And nothing on the page says what fuel any of this suits. That depends on chamber shape, cooling, mixture distribution, altitude and ignition timing, and it is a question for the person building the engine.
Questions people ask
What is dynamic compression ratio?
The same ratio as the static figure, but measured from the crank angle where the intake valve actually closes instead of from bottom dead centre. Because the valve is still open well past BDC on any real cam, the piston has already travelled some way up the bore before compression starts, and the effective stroke is shorter than the real one. On the default figures a 12.02 to 1 static engine works out at 9.91 to 1 dynamic.
Which intake closing figure should I enter?
Whichever one your cam card gives, but know which it is. Cards quote events at advertised duration, taken at a low checking lift, and at 0.050 in of lifter rise, and the two intake closing points differ by twenty-five to thirty degrees on a typical grind. Mixing them up moves the dynamic ratio by most of a whole point, which is more than any other input on the page.
Why does rod length change the dynamic ratio?
Because it changes where the piston is at a given crank angle, and it moves the answer the opposite way to how it is usually described. A longer rod makes the motion more sinusoidal, which part-way up the compression stroke leaves the crown further from bottom dead centre, so more of the stroke has already been given back when the valve shuts and the dynamic ratio comes out slightly lower. On the default figures a 5.200 in rod gives 9.97 to 1 and a 6.600 in rod gives 9.82. It is a real effect worth a sixth of a ratio, which is why the calculation uses the full slider-crank geometry rather than a fraction of the stroke.
Does advancing the cam raise the dynamic ratio?
Yes, because advancing closes the intake valve earlier, which means the piston has travelled less of the stroke before compression starts. Four degrees of advance is worth about a quarter of a ratio on these figures, taking 9.91 to 1 up to 10.18. Retarding does the opposite. The page takes the advance separately from the card figure so you can see what the installed position is doing rather than only what the grind does.
Is the cranking pressure figure what my compression gauge will show?
No. It is the dynamic ratio raised to an exponent you chose, times the intake pressure you entered, and it is a model rather than a measurement. Real cranking pressure moves with cranking speed, battery condition, whether the throttle is open, ring and valve seal, chamber temperature and the gauge itself. The figure is useful for comparing two builds on the same assumptions, not for predicting a reading.