The following equations can be useful when designing or building an engine and the following quotation can help keep everything in perspective.
"Today’s scientists have substituted mathematics for experiments, and they wander off through equation after equation, and eventually build a structure which has no relation to reality." Nikola Tesla
"Today’s scientists have substituted mathematics for experiments, and they wander off through equation after equation, and eventually build a structure which has no relation to reality." Nikola Tesla
Basic Airflow Rule: The airflow (cfm) to the engine (through the air cleaner, throttle body, intake manifold, and cylinder head) must support the horsepower trying to be achieved at the specific rpm and the port velocity must also be at the correct speed.
If the engine power is low, perform a compression and leak down test to confirm sealing. Analyze the camshaft, dynamic compression ratio, and the exhaust system for problems. Assure the tune-up is correct for the air/fuel ratio and the ignition timing. Test the battery and the charging system.
If the engine power is low, perform a compression and leak down test to confirm sealing. Analyze the camshaft, dynamic compression ratio, and the exhaust system for problems. Assure the tune-up is correct for the air/fuel ratio and the ignition timing. Test the battery and the charging system.
If you are not using math to design and build your engines you are probably missing out on potential power.
circumference of a circle = diameter x 3.14159
area of a circle = (radius x radius) x 3.14159
actual valve lift = (cam lift x rocker arm ratio) - valve lash
degrees of duration = opening + closing + 180
use positive numbers if opening is BTDC and closing is ABDC for the intake and opening is BBDC and closing
is ATDC for the exhaust, use negative numbers for other positions
intake centerline = (intake duration / 2) - intake opening
this assumes cam profile is symmetrical
exhaust centerline = (exhaust duration / 2) - exhaust closing
this assumes cam profile is symmetrical
lobe separation angle = (intake centerline + exhaust centerline) / 2
this assumes cam profile is symmetrical
the lobe centerlines are also the maximum lift points for symmetrical profiles
maximum flat tappet velocity = circumference of tappet face / 360
cubic inches = liters x 61.02398
cubic inches = (bore x bore) x stroke x (0.7854 x number of cylinders)
average piston velocity = (stroke x rpm) / 6
rod to stroke ratio = rod length (center to center) / stroke
total theoretical airflow (cfm) = (rpm x cubic inch) / 3456
theoretical airflow (cfm) is used to determine the volumetric efficiency (ve)
ve = (actual cfm / theoretical cfm) x 100
actual cfm is measured on a dynamometer
rpm peak horsepower = (1925 / cubic inch per cylinder) x cfm airflow @ 10"
cfm airflow is through the complete intake system
cfm airflow @ 10" = (rpm x cubic inch per cylinder) / 1925
cfm airflow is through the complete intake system
horsepower @ crank = (cfm airflow @ 10" x 0.4) x number of cylinders
cfm airflow is through the complete intake system
flow bench pressure conversion factor = square root of (new pressure / current pressure)
The formulas above using 10" are older formulas when many shops used a 10" standard.
port velocity = (rpm x (bore x bore) x stroke) / (pcsa x 260.9)
pcsa = minimum port cross sectional area in square inches, or the area of (intake valve diameter x 0.82)
690 ft/sec is ideal, 660 - 720 ft/sec is acceptable, Mach 0.55 - 0.60 based on 1200 ft/sec.
pcsa = (rpm x (bore x bore) x stroke) / 180,000
rpm peak horsepower = (pcsa x 180,000) / ((bore x bore) x stroke)
horsepower = (torque x rpm) / 5252
torque = (horsepower x 5252) / rpm
brake mean effective pressure (bmep) = (horsepower x 792,000) / (cubic inch displacement x rpm )
stock 125-145
modified street 150-185
race 185-230
bmep = (torque x 150.8) / cubic inch displacement
horsepower = (cubic inch displacement x rpm x bmep) / 792,000
good video showing relationship between drag and mph https://www.youtube.com/watch?v=F76-npz0CeI
circumference of a circle = diameter x 3.14159
area of a circle = (radius x radius) x 3.14159
actual valve lift = (cam lift x rocker arm ratio) - valve lash
degrees of duration = opening + closing + 180
use positive numbers if opening is BTDC and closing is ABDC for the intake and opening is BBDC and closing
is ATDC for the exhaust, use negative numbers for other positions
intake centerline = (intake duration / 2) - intake opening
this assumes cam profile is symmetrical
exhaust centerline = (exhaust duration / 2) - exhaust closing
this assumes cam profile is symmetrical
lobe separation angle = (intake centerline + exhaust centerline) / 2
this assumes cam profile is symmetrical
the lobe centerlines are also the maximum lift points for symmetrical profiles
maximum flat tappet velocity = circumference of tappet face / 360
cubic inches = liters x 61.02398
cubic inches = (bore x bore) x stroke x (0.7854 x number of cylinders)
average piston velocity = (stroke x rpm) / 6
rod to stroke ratio = rod length (center to center) / stroke
total theoretical airflow (cfm) = (rpm x cubic inch) / 3456
theoretical airflow (cfm) is used to determine the volumetric efficiency (ve)
ve = (actual cfm / theoretical cfm) x 100
actual cfm is measured on a dynamometer
rpm peak horsepower = (1925 / cubic inch per cylinder) x cfm airflow @ 10"
cfm airflow is through the complete intake system
cfm airflow @ 10" = (rpm x cubic inch per cylinder) / 1925
cfm airflow is through the complete intake system
horsepower @ crank = (cfm airflow @ 10" x 0.4) x number of cylinders
cfm airflow is through the complete intake system
flow bench pressure conversion factor = square root of (new pressure / current pressure)
The formulas above using 10" are older formulas when many shops used a 10" standard.
port velocity = (rpm x (bore x bore) x stroke) / (pcsa x 260.9)
pcsa = minimum port cross sectional area in square inches, or the area of (intake valve diameter x 0.82)
690 ft/sec is ideal, 660 - 720 ft/sec is acceptable, Mach 0.55 - 0.60 based on 1200 ft/sec.
pcsa = (rpm x (bore x bore) x stroke) / 180,000
rpm peak horsepower = (pcsa x 180,000) / ((bore x bore) x stroke)
horsepower = (torque x rpm) / 5252
torque = (horsepower x 5252) / rpm
brake mean effective pressure (bmep) = (horsepower x 792,000) / (cubic inch displacement x rpm )
stock 125-145
modified street 150-185
race 185-230
bmep = (torque x 150.8) / cubic inch displacement
horsepower = (cubic inch displacement x rpm x bmep) / 792,000
good video showing relationship between drag and mph https://www.youtube.com/watch?v=F76-npz0CeI