Dynamic Compression Ratio

ScopeThe effective compression ratio measured from where the intake valve actually CLOSES (not from BDC), so it reflects what the cam does to cylinder pressure: from the clearance volume (set by the static CR) and the piston position at intake-valve-closing (slider-crank geometry off rod length and stroke), DCR = (swept-from-IVC + clearance) / clearance. A 4.030 bore, 3.75 stroke, 6.0 rod, 10.5 static engine with a 60-degree ABDC intake close runs about an 8.7 dynamic CR. A big cam closes the intake later, bleeds off charge, and drops the DCR -- which is why a high-static-CR engine with a large cam can still run on pump gas while a mild cam on the same short block detonates. Roughly 7.5 to 8.5 DCR suits 91-93 octane at sea level; altitude and head material shift it. An estimate off the geometry; the cam's actual seat timing, the octane, and the tune govern.

Formula or tabler = stroke/2; V_clear = (pi/4 x bore^2 x stroke) / (static_cr - 1); theta = (180 - IVC_ABDC) deg from TDC; piston_from_TDC = r + rod - (r cos theta + sqrt(rod^2 - r^2 sin^2 theta)); V_eff = pi/4 x bore^2 x piston_from_TDC; DCR = (V_eff + V_clear) / V_clear.

Source and editionClassical slider-crank engine geometry (Machinery's Handbook / engine-design texts), by name; the dynamic-compression concept measures compression from the intake-valve-closing point.

Free accessThe slider-crank piston-position relation is public first-principles geometry; the bore, stroke, rod, static CR, and intake-valve-closing angle come from the build and the cam card.

What governsEstimate. AHJ and licensed professional govern.

API field namesbore_in, stroke_in, rod_length_in, static_cr, ivc_abdc_deg, dynamic_cr, clearance_volume_in3

• IVC point compression is measured from intake-valve-closing (deg ABDC) at the checking lash, not from BDCdynamic-compression method
• Slider-crank piston position from rod length and stroke; the static CR sets the clearance volumeengine geometry