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Engine Math


Home Intake F100 Math Galaxie FEFox


These formulas use inches, but at the bottom of the page there is an inch to millimeter and back calculator below.

This is roughly how many ponies you gots:
Simple formula for figuring HP from 1/4 mile speed.
Enter MPH in Quarter Mile:
Enter Vehicle Weight with Driver:


Another way to finger out your HP
This one works off of volumetric efficiency per rpm.
Cubic Inches
Estimate Volumetric Efficiency
RPM


Correct HP to Sea Level
Not quite worth the drive to the beach, huh?
Current HP
Current Elevation


Reckon on how big your injectors needs to be:
This formula works by figuring fuel required at 80% duty cycle for the injectors at max HP.

Horsepower
Enter Number of Cylinders
BSFC

Injector Size LB/HR
Injector Size CC/MIN
Fuel Pump Flow GAL/HR
Fuel Pump Flow L/HR


CFM Required
How much air would your engine suck if it had 150% volumetric efficiency? Fat chance. Most stock engines have VE ratings around 75%, hot rods 90%, and racers 110% and up. Now, that's conventional wisdom referring to conventional V8s. Many modern engines surpass those figures. Having a little extra airflow capability makes good power, but having a bunch bigger carb or TB is a detriment to response. Most people fudge a little bit on the big side. And of course boost will affect VE if you're lucky enough to not be restricted to atmospheric pressure.
RPM
Cubic Inches
Volumetric Efficiency % (below)

Volumetric Efficiency
It goes both ways.

CFM used
Cubic Inches
RPM

CFM of EFI Throttle Body
This is only for EFI. Now that you know how much air your engine needs, how big of a throttle body will you need to satisfy it? It's my understanding that carburetors and fi throttle bodies are flowed at different pressures. This formula assumes 140CFM per square inch. Throttle shafts are assumed to be 3/8" dia.

Throttle Blade Diameter
Number of Throttle Blades

Engine Displacement in Cubic Inches

Bore
Stroke
Number of Cylinders

Cubic Inches to Liters

CID

Liters to Cubic Inches
Liters


Figure Static Compression Ratio:

Cylinder Bore
Stroke
Deck Clearance (how far the piston's down the cyl)
Gasket Thickness (usually .025 to .045)
Combustion Chamber Vol. CCs
Valve Relief CCs (- if Domed)

Rod Ratio
According to engine builders, rod to stroke ratio should be around 1.75:1. Let's try it.
Stroke:
Con Rod length Center-Center:


Mechanical Redline
RPM at which stock cast reciprocating assembly will self destruct. This assumes piston speed of 3800 feet per minute is terminal velocity with stock components. Now, my 460 comes out to 5922 redline, but I've spun a stock bottom end repeatedly as high as 7400. But then that engine later broke a couple of rods when I nuked the pee out of it. Go figure.
Stroke:


Tire Diameter from Aspect Ratio
Our example tire is a 225/60R15 which has a 25.6" diameter.

First number of tire size (ex:225)
Second number of tire size (ex:60)
Diameter of rim (ex:15)


MPH for RPM and Tire Diameter

RPM
Gear Ratio
Tire Diameter


Convert Inches and Millimeters

Inches:


MM:


E-mail me if you have any corrections for my formulas or new ones to add: