Engine Build Calculator (Beta)
Work out an engine build in your browser. Enter the parts you measured (bore, stroke, rod, chamber, piston dome or dish, head gasket and fire ring, deck clearance) and it gives you the displacement and a true compression ratio with every volume accounted for. It also works out piston position, speed, acceleration and rod angle right through the cycle with gudgeon pin offset included, plus airflow in CFM.
Engine
geometry and piston motion, exact for the dimensions entered
Where the compression ratio comes from
every input in the Compression group moves one of these lines
Gudgeon pin offset
the pin is off the bore centreline, so the simple slider-crank no longer holds
With an offset pin the rod is not vertical at top dead centre, TDC and BDC
no longer land on 0° and 180°, they are no longer half a turn apart, and the
crown travels fractionally further than twice the crank throw. Positive offset is
toward the major thrust side, the usual production direction; negative is a piston
fitted the other way round, which is a known way of chasing a little less side load
on the power stroke. A model that leaves the offset out puts the pin on the bore
centreline, which gets the rod angles and TDC wrong on any real piston.
Airflow
exact for the volumetric efficiency entered (naturally aspirated)
Airflow here is simply swept volume multiplied by engine speed and the VE you
type. VE is an input, not a prediction, so this page has no idea what your
head, cam or exhaust will actually flow. For a modelled torque and power curve, use the
dyno simulator.
Piston motion
Position below deck
Velocity
Acceleration
Motion table
361 rows, one per crank degree, 0 to 360° from true TDC, laid out as the
original sheet had it. “Below deck” matches the sheet’s own column to
within 10-9 mm. Scroll the box; the whole thing downloads as CSV.