Small Engine Guys I'm Confused HP vs. CC


Senior Member
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I cant seem to understand the difference in cc vs. horsepower. I know that its cubic centimeters instead of inches but here's what I dont get. I am looking at gokarts here's what they are:

One is listed as 150cc, the horsepower spec is 13hp.

The other one I'm looking at has a 13hp honda engine. It is listed as having 390cc. How is this? Which engine has more power? How can they have the same horsepower but one with way more cc's? Some help would be great.


Senior Member
CC's and Hp have only a limited correlation.

CC (actually CCD) is cubic centimeter displacement. This is a function of bore diameter and stroke. Simplified, it is how much water in cc's the cylinder will hold with the piston all the way down.

Horsepower is how much energy the engine can generate at a given rpm. While cc's have some bearing, so does engine timing, fuel type, volume and method of introduction into the cylinder along with many other factors. Even altitude above sea level can effect Hp.

As an example, a 1967 Camaro with a 327 cubic inch (english equivalent of CC) engine could be bought stock with horsepower from about 230 to 325Hp, depending on how it was set up.

Frequently, small engine makers will produce diff Hp engines with the same CC's simply by making changes in the carburetor.

Hope this helps.


Senior Member
Engine displacement can often have very little to do with how much power it produces. Your power is in the efficiency of the combustion process. Most of today's engines....small engines included...are built to meet the EPA's regulations in regards to emissions. You could easily take a specific small engine, say the 390cc engine....increase the valve lift and duration (usually with a bigger camshaft), increase the compression ratio, and increase the intake and exhaust flow.....and possibly double the engine's power.

Example: In 1980, a Chevrolet 350 cu in engine that came stock in a Camaro, was only rated at 170 HP and 270 ft lbs of torque. The same cu in engine in 1971 was rated at 330 HP and 360 ft lbs of torque. The main difference were the specs of the internals I mentioned before.
How many RPM's are the engines turning to produce that HP. How many cylinders?

I'll bet that 150 is screaming at peak HP.

I just looked at a 4 cyl. 2.5 liter Japanese engine that is rated at more HP than the 4.9 in-line 6 in my truck --- my truck hits peak HP at about 2500 RPM and the Japanese rice burner hits peak HP at about 4500-5000 RPM. The Jap engine may turn out more HP due to breathing, exhaust, other tuning issues, but is isn't going to be doing it as long as my big 6 which is doing half the work. Also, my engine is producing a lot more torque-- which is what moves loads.

I can increase the HP on my engine by about 30% simply by modifying the intake (increasing breathing) and exhaust. The engine was purposely de-tuned by Ford to increase the life.

Also, I'm not sure that small engines have a standard for rating HP as do auto engines and marine engines -- where and how is the HP of each engine measured?


Senior Member
The design of the motor makes a big differance in horsepower, but I am willing to bet money that the larger cc motor will produce twice the torque of the smaller one, a 150cc 4 cycle is most likely making the power in the higher rpm range with no torque and will require a gearbox with 4-5 gears to make up for that. The larger engine won't need the multiple gears to get the power and a simple torque converter setup will work just fine.


Senior Member
This is all good info, to add one more example in dirtbikes (motocrossers) a 125cc 2 stroke is kinda comparable to a 250cc 4 stroke.
And a 250cc 2 stroke is kinda comparable to a 450cc 4 stroke. But very different in torque range.


Senior Member
the 13hp 150cc will be faster because it is most likely lighter
If the track is tight and twisty or has uphill sections then the bigger motor will come off the corners harder because of the better lowend torque inherent in large displacement low hp engines.

Now if the track is such that the smaller motor can stay up in it's most effective rpm range then the potentially lighter weight could be an advantage.
390 is 13 hp cc.s are wierd to calc. but some where around 27-32cc.s in a horse kinda like liters 61 ci in a liter but a 305 and a 302 are 5.0 liter