Anytime a new powerplant is being assembled the main question is “How much power and how much torque?” Sure, that’s what we want to hear most so we can boast at shows, races and bench races with our buds. But behind those big power numbers are a long list of other numbers consisting of minute measurements, calculations and math. Engine building is all about numbers.

By using a few measurements of your new engine, you can answer a lot of technical questions such as total displacement, compression ratio and more thanks to some simple math formulas.

If you ditched algebra to get to shop class early, you may want to creep back into the classroom. There are loads of formulas used in engine building as well as other areas of power and how to use it. Speedometer gear ratios, tire height, differential ratios, transmission gearing and much more can all be decided through a few calculations.

By understanding and knowing a variety of measurements and specifications of your engine you can determine displacement, compression ratios, stroke and bore as well as useful measurements to use during the machining process for decking the block or heads and even the pistons.

We hopped over to the Lunati Cams website to learn a few basic engine calculations. These examples just scratch the surface of what you can derive. For even more information, be sure to cruise by lunatipower.com.

Engine Displacement

We’ll start with one of the most common questions that results after some machine work; cubic inches. When you start boring a cylinder and modifying the length of stroke of the piston, the cubic inches of the engine increases (or can even decrease). Each cylinder’s capacity can be determined by the following:

• Bore x Bore x Stoke x 0.7854 = One Cylinder’s Cubic Inch
• Then, simply arrive at the total cubic inches by multiplying it by the number of cylinders.
• An example is a 4.250 bore with a stroke of 3.5
• 4.125 x 4.125 x 3.5 x 0.7854 = 46.77 x 8 = 374.19

Stroke Length and Bore Diameter

With the formula shown above for cubic inches, you can now determine the stroke or the bore of an engine by filling in the other values. The stroke of an engine is calculated dividing the engine displacement with bore information:

     Stroke Length:  cubic inches / bore x bore x 7.854 x number of cylinders

     Bore Diameter: Square root of the answer to Engine Displacement / (stroke x 0.7854 x number of cylinders). Lets follow along with an example of an engine using a 3.48” stroke in a 358 ci engine.

• 3.48 x .7854 x 8 = 21.86
• 358 / 21.86 = 16.37
• √16.37 = 4.046 bore diameter

Decking the Cylinder Head to Reduce Total Chamber CC

By using the cylinder head volume, you can determine how much can be removed when decking the heads or even the final compression ratio of the engine.

Do you have a goal or rule that limits the cylinder head chamber volume? If so, there’s a formula that will give you a guideline when decking a head. On a small block Chevy, the head will need 0.006” deck removed for each cc in chamber volume reduction. An open chamber big block head require about 0.005 per cc. Using these numbers will guide you to the ballpark, but the head should always be cc’d the chamber volume for complete accuracy.

• Deck Material to Remove = Current chamber volume – desired chamber volume x deck material per cc

(86.07 – 71.66) X 0.006 = 0.086” to remove from the head

Compression Ratio

A common question when building an engine is what the compression ratio will end up at. There is a simple formula to determine this ratio, but first we need to explain two terms used in the equation; Swept Volume and Total Chamber Volume. The compression ratio is the relationship of the swept volume to the total chamber volume.

Swept Volume is the area that the piston travels through from bottom dead center to top dead center.  The inches value must be changed to cc by multiplying the quotient by 12.8704.

• Swept volume (cc) = Cylinder Bore Diameter (inches) x Cylinder Bore Diameter (inches) x Stroke (inches) X 12.8704
• Swept Volume = 4.000 x 4.000 x 3.477 x 12.8704
• Swept Volume = 716.00 cc

Total Chamber Volume is the space above the piston at TDC including the area above the piston in the cylinder, the area of the compressed gasket, the combustion chamber, valve pocket and the dome of the piston so there is a bit more calculating to take care of.

With the Swept Volume of 716.62 cc and the Total Chamber Volume determined at 86.07 cc, we can proceed to determine the compression ratio:

• Compression ratio = (Swept Volume + Total Chamber Volume) / Total Chamber Volume
• Compression ratio = (716.00 + 86.07) / 86.07
• Compression ratio = 9.31:1