CC to HP Converter (and HP to CC)

Cubic centimetres (cc) measure how much air and fuel the pistons sweep on each turn, which is the size of the engine. Horsepower measures how fast that engine can do work. Two engines with the same cc can make very different power depending on how often they fire, how fast they spin, how well they breathe and where the power is measured. That is why every honest cc to hp converter gives an estimate, and why this one shows a range instead of a single number.

The range comes from real engines. For each engine type, the converter takes the published horsepower of the engines in the table, divides it by their displacement, and uses the lowest, the middle and the highest result. For 4-stroke small engines that works out to 2.8 to 3.4 hp per 100 cc; for 2-stroke handheld engines it is 4.6 to 8.6. Add a row to the table and the range, the calculator and the figures on this page all move together.

A 2-stroke engine fires on every revolution of the crankshaft; a 4-stroke fires on every other one. At the same size and rpm the 2-stroke gets twice the power strokes, and it spins faster as well: a chainsaw makes its rated power at around 9,000 rpm or more, while most 4-stroke small engines are governed near 3,600 rpm so a generator holds its frequency and a mower blade stays under its safe tip speed.

The result shows in the table: the typical 2-stroke handheld engine makes 5.8 hp per 100 cc against 3 for a 4-stroke small engine, about 1.9 times as much. Lumping the two together, as most cc to hp calculators do, produces a number that is wrong for both: too high for a lawn mower and too low for a chainsaw.

Racing 2-strokes go further still. A motocross engine is built for peak power at high rpm with a tuned expansion chamber, and even measured at the rear wheel, after the gearbox and chain take their share, it makes 16.7 to 24.8 hp per 100 cc. Outboards sit close to handheld tools at 5.7 to 7.1, because they are tuned for torque and long hours at steady throttle, and their rating is taken at the propeller shaft.

The Harbor Freight Predator 212 is the engine most people mean when they search for 212cc to hp. With the 4-stroke small engine type selected, the converter estimates 6.4 hp, with a likely range of 5.9 to 7.3 hp. Harbor Freight rates it at 6.5 hp: inside the range and a little above the middle, which fits a budget engine rated by its seller rather than to the SAE net standard Honda uses.

The same arithmetic for a 2-stroke: the Husqvarna 450 Rancher chainsaw displaces 50.2 cc. As a 2-stroke handheld engine that estimates to 2.9 hp (2.3 to 4.3), and Husqvarna rates it at 3.2 hp. Put the same 50.2 cc through the 4-stroke setting and you get 1.5 hp, less than half the real figure, which is exactly the mistake a one-size-fits-all converter makes.

To estimate the size of engine behind a horsepower figure, the converter divides instead of multiplying. A 6.5 hp 4-stroke usually means about 216 cc (likely 189 to 232 cc), which is why engines sold as 6.5 hp run from 203 to 212 cc, like the Briggs & Stratton Vanguard and the Predator in the table. A 25 hp 2-stroke outboard works out to about 408 cc (354 to 439).

As a rule of thumb, a 4-stroke small engine needs about 33 cc per horsepower, a 2-stroke handheld engine about 17 cc, a 2-stroke outboard about 16 cc, and a 2-stroke motocross engine fewer than 6 cc.

The ratings in the table are not all measured the same way, which is why the measurement column is there. Honda rates its engines to SAE J1349: net power at the crankshaft with the production muffler and air filter fitted. The Briggs & Stratton Vanguard figures are gross, taken without those losses, so they read higher for the same engine. Stihl and Husqvarna quote output-shaft power to ISO 7293. Outboard makers rate at the propeller shaft, and dirt bike makers publish no horsepower at all, so the bike rows are magazine dyno tests at the rear wheel.

Many walk-behind lawn mowers show no horsepower any more. After a US class action over overstated mower horsepower labels, settled in 2010, most makers switched to listing cc or gross torque in foot-pounds. If your mower engine only shows cc, the 4-stroke setting is the best way to get a fair horsepower figure from it.

The displacement is usually on the engine label, in the manual, or stamped near the model number. If it is missing, measure the bore and stroke and work it out with the engine displacement calculator, which links back here with the cc filled in.

Knowing the horsepower changes nothing about fuel: a 2-stroke saw, bike or outboard still needs the ratio on its fuel cap, usually 50:1 for handheld tools and between 32:1 and 40:1 for dirt bikes. The oil mix calculator works out the ounces or millilitres for any amount of gas.