Engine Calculators

Engine work is a chain of numbers that depend on each other. Displacement sets the starting point, compression ratio decides how hard the engine can be run on the fuel you use, piston speed shows how much stress the bottom end sees, and torque and rpm give power. Plan these together, because changing one moves the others.

Which tool answers which question

• Cam timing calculator: Turn the numbers on a cam card into the four valve events, overlap and advance. Then send the intake-closing point straight to the compression calculator.
• Compression ratio calculator: Static and dynamic compression in one place, plus an estimated cranking pressure. It also sorts out the step most builders get stuck on: which...
• Coolant mix and freeze protection calculator: How cold your current coolant will protect to, and how much to drain and replace with straight antifreeze to reach the mix you want.
• Cylinder head flow to horsepower calculator: How much power a set of heads can support, based on their intake port flow. Useful when comparing heads, as long as you remember it's a ceiling, not a promise.
• Engine displacement calculator: Cubic inches, liters and cc from bore, stroke and cylinder count, with what common overbores do to it. Your numbers are saved to My Build so the carburetor...
• Engine rebuild cost planner: Add up machine work, parts and labor for your rebuild with a cushion for surprises. Fill in your own quotes. Every line is optional.
• Header primary tube calculator: Starting-point primary tube length and diameter for a set of headers, from your cam's exhaust opening point and the rpm you want peak power.
• Horsepower and torque calculator: Horsepower and torque are the same measurement viewed two ways, tied together by rpm. Enter either one and the rpm to get the other, in both US and metric...
• Piston speed calculator: Mean piston speed is the quick sanity check for how hard an engine is working. This also finds the true peak speed from your rod length, which is where the...
• Rocker ratio and valve lift calculator: Your cam card lists lobe lift, or valve lift at a specific rocker ratio. This shows the lift your valves will actually see with the rockers you're running,...

Displacement, compression and piston speed

Displacement is the swept volume of all cylinders, worked out from bore, stroke and cylinder count. Compression ratio compares the cylinder volume at the bottom of the stroke with the volume left at the top, which depends on the head chamber, gasket thickness, deck height and piston shape. A higher ratio makes more power from the same air and fuel but needs more fuel octane or careful tuning to avoid detonation.

Piston speed rises with stroke and rpm. Many builders treat figures above roughly 4,500 to 5,000 feet per minute as a point to use stronger rods, pistons and bearings, and the stress rises quickly beyond that.

Power, cam and headers

Horsepower is torque multiplied by rpm and divided by 5,252, so an engine's power curve follows its torque curve. Cam timing, header primary size and cylinder head flow all shift where the engine makes torque. These tools give planning estimates, and a dyno or a tuner shows what your engine actually does.

Common mistakes

• Changing one number in isolation. A bigger cam, a higher compression ratio and the fuel you can buy all interact.
• Using nominal sizes. Measure the real bore, stroke, deck height and gasket thickness.
• Ignoring cooling. More power makes more heat, so check the coolant mix and the radiator.
• Treating estimates as measurements. Verify clearances and specifications with the parts maker.

Guides for this section

Want the background behind these tools? These guides explain the thinking and the numbers in more depth.

• Compression, Cam Timing and Pump Gas: Making the Combination Work
• Budgeting an Engine Rebuild: Machine Work, Parts and Surprises
• Turbo and Supercharger Basics: Boost, Pressure Ratio and Reliability
• All guides