How do you calculate compression ratio? What’s the best way to figure out engine displacement?

In the automotive mechanics world, there are formulas and conversions for measuring just about everything—from horsepower and torque to carburetor cfm and gear ratio. The key is remembering them.

While cheat sheets might have frowned upon in your sixth-grade classroom, we strongly encourage them in the garage, shop, or pits. That’s why we’ve put together this list of 13 key performance formulas you should know when building or tuning your street or race vehicle.

Read it, print it out, and tape it to your beer fridge. (Quick—while your sixth grade teacher isn’t looking!)

**Racing Carburetor CFM**

Racing Carburetor CFM = RPM x Displacement ÷ 3456 x 1.1*Note: Summit Racing also offers this CFM Calculator to make the job easier.*

**Displacement**

Displacement = .7854 x Bore^{2 }x Stroke x Number of Cylinders

**Correct Compression Ratio (CCR)**

CCR = FCR (Altitude/1,000) x .2*Note: You can also take this Compression Ratio Calculator tool for a spin.*

**Tire Diameter**

Tire Diameter = (MPH x Gear Ratio x 336) ÷ RPM

**Rocker Arm Ratio and Valve Lift**

Gross Valve Lift = Camshaft Lobe Lift x Rocker Arm Ratio

**Horsepower**

Horsepower = (RPM x Torque) ÷ 5,252

**Torque**

Torque = (5,252 x HP) ÷ RPM

**Rod Ratio**

Rod Ratio = Rod Length ÷ Crank Stroke Length

**Average Piston Speed**

Average Piston Speed = Crank Stroke x RPM ÷ 6

**Rear Gear Ratio**

Rear Gear Ratio = (RPM at Finish Line x Tire Diameter) ÷ (MPH x 336)*Note: You can also save this link to a handy* Gear Ratio calculator.

**Volume (CCs) of Deck Clearance**

CCs of Deck Clearance = Bore x Bore x 12.87 x Depth of Deck Clearance

**Volume (CCs) of Head Gasket**

CCs of Head Gasket = Bore x Bore x 12.87 x Thickness of Head Gasket

**Compression Ratio**

Combined Chamber CCs + Gasket CCs + Deck CI CCs + (Displacement x (N))

**÷**

Combine Chamber CCs + Gasket CCs + Deck CI CCs

*(N) = 2.0483 for 8-cylinder. (N) = 2.7311 for 6-cylinder. (N)=4.0967 for 4-cylinder.*

good job specifications.

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Invaluable

Thanks, I needed that!

[…] included this formula, along with 12 other key automotive formulas, in our 13 Key Automotive Formulas You Should Know post. Check it our for valuable information on compression ratio, rear gear ratio, and more. You can […]

Re: Racing carb CFM:

Using 5000 rpm and my Ford 390, I get CFM of over 2 million!

i do not use silicon pistons, it cost me a race motor just replacing stock pistons with hypertectic ones? i thought i looked them over good? but looking after it blew up the wrist pin hole one the one that let loose was made too thin around the pin? it jerked it right out of the one piston, after looking at all the pistons together the one had less writ pin material around it, so be cautious using non factory/or forged pistons, even with low compression, like i had. lost a reliable motor over 100.00 pistons…..

Very good stuff

I really like this, but… Technically compression doesn’t start until the valves are all closed, so the stroke doesn’t start until well after BDC. so with variable valve timing we get a variable compression ratio. With fixed timing, a high calculated ratio is actually quite low when int closure is taken into account

Compression Ratio is a fixed “mechanical ratio”, not to be confused with cylinder “pressure”, which is what you are referring to. The ratio never changes, while the cylinder pressure changes constantly, depending on the many variables that are constantly changing through out the stroke of the piston, valve timing, etc.

And technically, compression “starts” while both valves are still open! (during the overlap cycle of the valves when the exhaust velocity causes the intake charge to get a head start on the cylinder filling, before the exhaust valve closes, and even before the piston starts it’s downward movement. The compression “ratio”, is a totally separate measurement, not even related to the compression itself.

If we were actually talking of “effective compression ratio” then it would be another story all together, which is where you are headed, and most people, even some professional mechanics, don’t fully understand.

I disagree with yours comment, technically compression”starts….during overlap, there is compression on the exhaust stroke forcing exhaust out, Intake stroke create vacuum, Cyl vacuum continues to bottom dead center of the intake stroke, compression “starts” after vacuum is diminished on the compression stroke which is a few degrees past bottom dead center. I’m a 1970 graduate of Nashville Auto Diesel College, they did not teach us how to fix anything, they taught us how things work, if you know how something works, you can figure out what’s wrong with it.

[…] or bookmark this page for quick access. And while you’re at it, you might want to check out our 13 Key Automotive Formulas Every Gearhead Should Know post, […]

I once had a book that had all of the engine formulas in it including determining the proper diameter, length, and length of collector. of exhaust pipes by where you wanted the torque to peak, same for intake runners. It also had formulas for selecting cam shafts, for proper torque peak tuning.

Looking at a compression ratio is showing minimum or maximum displacement through the head just goes to show how smart these guys in here are wanting to argue with math

This site has a bunch of useful ones too,

http://www.gtsparkplugs.com/AutomotiveCalculators.html

Explain why I need to factor in number of cylinders to get the compression ratio… I would think all the cylinders are balanced which would kill the need to know the other 7 on a V8…I.E. Deck clearance + head gasket thickness + combustion cc +/- piston dish ÷ cylinder cc = Compression ratio…

Looks like that formula uses displacement as the known swept vol. So it uses number of cylinders vs displacement to get the swept vol down to one cylinder.

I’m a retired mechanic. I have known of a few formulas that are needed in shop work.

I have many car related books and magazines so I do not need to remember most.

I have always remembered cubic inches formula by memory.

[…] on the walls in the ol’ dream garage. The whole article has some good points so make sure and give it a read. The equations in their raw form are below. Note a few of these are inverse, like tire diameter vs […]

[…] included this formula, along with 12 other key automotive formulas, in our 13 Key Automotive Formulas You Should Know post. Check it our for valuable information on compression ratio, rear gear ratio, and more. You […]

[…] bookmark this page for quick access. And while you’re at it, you might want to check out our 13 Key Automotive Formulas Every Gearhead Should Know post, […]

For figuring displacement, I always use the formula I learned in 7th grade Geometry Class for the volume of a cylinder – (pi)RxR(H), with R = 1/2 the bore and H = stroke, then multiply by the # of cylinders. I always did better when I understood what all the numbers and letters meant.