Automotive & Engine
Compression Ratio
CR = (Vc + Vd) / Vc
Static compression ratio from cylinder, chamber, deck, gasket, and piston-dome volumes.
Calculate →Crawl Ratio
Crawl = Gear × T-case × Axle
Low-speed crawl ratio from transmission low gear, transfer-case low, and axle ratio.
Calculate →Dyno Correction Factor
cf = 1.18·(990/Pd)·√((Tc+273)/298) − 0.18
SAE J1349 correction factor for dyno horsepower from observed dry pressure and temperature.
Calculate →Effective Gear Ratio
EGR = Axle × Tire_stock / Tire_new
Effective final-drive ratio after a tire-diameter change.
Calculate →Engine Equations
VE · CID · CR · head volume
Multi-equation engine builder set — displacement, CR, head gasket volume, VE, air flow.
Calculate →Engine Displacement
CID = π · (B/2)² · S · N
Engine displacement (CID or cc) from bore, stroke, and cylinder count.
Calculate →ET Method Horsepower
hp = weight · (5.825 / ET)³
Estimated flywheel horsepower from quarter-mile elapsed time and vehicle weight.
Calculate →Fuel Injector Size
lb/hr = hp · BSFC / (N · DC)
Fuel injector flow rate from target horsepower, BSFC, injector count, and duty cycle.
Calculate →Gear Equations
mph = RPM · tire / (gear · axle · 336)
Driveline speed from RPM, gear, axle, and tire diameter — and the inverse.
Calculate →Gear Ratio
GR = N_driven / N_driving
Gear ratio from tooth counts of driven and driving gears.
Calculate →Horsepower Equations
hp = T · RPM / 5252
Multi-equation horsepower set — rotating, trap speed, ET, power-to-weight, dyno correction.
Calculate →Horsepower from Elapsed Time
hp = weight · (5.825 / ET)³
Estimated horsepower from quarter-mile elapsed time and vehicle weight.
Calculate →Horsepower from Trap Speed
hp = weight · (mph / 234)³
Estimated horsepower from quarter-mile finish-line trap speed and vehicle weight (Hale).
Calculate →Horsepower from Torque
hp = T · RPM / 5252
Rotating horsepower from torque and engine RPM.
Calculate →Power to Weight Ratio
P/W = hp / weight
Power-to-weight ratio for vehicle and aircraft performance comparison.
Calculate →Suspension Coil Spring Rate
k = G · d⁴ / (8 · D³ · N)
Coil spring rate from wire diameter, mean coil diameter, active coils, and shear modulus.
Calculate →Tire Diameter
D = rim + 2 · (W · AR / 25.4)
Overall tire diameter from rim, section width, and aspect ratio.
Calculate →Trap Speed Horsepower
ΔHP = W · [(V₂/234)³ − (V₁/234)³]
Horsepower change between two quarter-mile trap speeds at a fixed vehicle weight.
Calculate →Volumetric Efficiency
VE = (CFM · 5660) / (CID · RPM)
Engine volumetric efficiency from airflow CFM, displacement, and RPM.
Calculate →Automotive performance calculators for engine builders, racers, and gearheads. Compute horsepower from quarter-mile trap speed or elapsed time, engine displacement from bore/stroke, compression ratio from head/piston/deck volumes, volumetric efficiency, gear ratios, and power-to-weight ratios.
Calculators here mirror equations from textbook racing-engine references and are used to set up dyno baselines, spec a final-drive ratio, or quick-estimate hp from a track slip.
When to use these calculators
Use Horsepower from Trap Speed to estimate flywheel hp from a quarter-mile finish-line trap speed and vehicle weight (the Hale/Roe formula). Use Compression Ratio when assembling a short-block to verify static CR from bore, stroke, deck height, head volume, and head-gasket volume. Use Volumetric Efficiency to check airflow against displacement and RPM after a dyno pull.
These are educational estimators — actual dyno measurement remains the gold standard for engine output.
Frequently Asked Questions
- Which horsepower formula is most accurate?
- Trap-speed (Hale) formula is the most consistent for full-pass quarter-mile data: hp = weight × (trap_mph / 234)³. ET-method is approximate at best. Engine dyno or chassis dyno measurement is the only direct way to measure hp.
- How do I calculate static compression ratio?
- Static CR = (cylinder_volume + chamber_volume + gasket_volume + deck_volume + piston_dome_volume) ÷ (chamber_volume + gasket_volume + deck_volume + piston_dome_volume). The Compression Ratio calculator handles negative-volume (dome) pistons and positive-volume (dish) pistons.
- What does volumetric efficiency tell me?
- VE = actual_airflow ÷ theoretical_airflow at a given RPM. NA street engines typically run 75-85% VE; race-prepped heads + cam can reach 100-115%. The VE calculator works backward from a dyno-measured airflow CFM and engine displacement.