Blow-Off Valve Flow Area Calculator
Liguori Drag Racing • Forced Induction
All calculators / Forced Induction
Blow-Off Valve Flow Area
Estimate an equivalent effective air-relief orifice at one operating point.
Starting values are examples. Enter your measurements; results update as you type.
Method & assumptions
Use crankshaft horsepower, BSFC and target AFR for the actual fuel. Fuel mass flow is horsepower × BSFC; air mass flow is fuel flow × AFR / 60. Fuel specific gravity does not change this mass-to-mass calculation. Steady compressible ideal-air orifice model, vented to ambient, γ = 1.4 and R = 287.05 J/kg·K. Uses the choked or subcritical flow equation as appropriate. Effective flow diameter is not a manufacturer valve-port diameter. Valve lift, tested flow rating, plumbing, charge volume and throttle-closure transients require separate evaluation.