Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

With out going through all the formulas to figure out what the hoursepower of an item is rated at, is there a rule of thumb or rough way to convert cubic centemeters to horsepower? An example 205cc=?, 208cc=?etc,etc,

No. Too many variables in most cases to make a universal "rule of thumb". Engine compression, reciprocating efficiency, rpm, valve size, carb size, cam lobe profiles, stroke/bore dimensions, etc., etc..... all have an effect on engine performance.

all I can remember  - when I was a kid  that corvettes were at 350 cubic  inches  and then all of a sudden it was 5.7 liters..   Sooooooo    i guess you could convert cubic inches to metric.   but that does not answer request.  You cannot convert cc s to horsepower-   for example a porche might have 3 liters and 400 horspower....  and a  cobalt might have a 3 liter  and 150 horsepower.

good luch with the numbers..

John the bulbman

One way to do it would be to convert the foot pounds or Newton meters to hp for any specific motor. That would then take into account all the things done to a given cc engine, carb, jets, muffler, and extra anything. That would take getting a graph of RPM to ftlbs/nmrs. Graphs like that plot the ftlbs/nmrs to RPM over the useable range of RPM which for snowblowers is roughly 500-3600 RPM.   

I”ve seen some spec’s online and in some manuals but they are never a graph. They only give a single torque figure for a single RPM value, for example, 7.5 nm @ 2300 RPM. As far as I know numbers like that are completely useless.

Power out is non-linear curve and at the higher RPM values the curve flattens right out. You can’t divide and get a nm figure for 1 RPM then multiply it by 3600 to get max nm then covert it to hp.

The only way to do a conversion, for a specific engine, (is not with cc’s) but what happens with those cc’s i.e. carb, jet settings, muffler, piston diameter & whatever to produce a tourque value. Since they don’t give max values (ratings at 3600) or at least I’ve never seen a company that did give them, you can’t tell much.

tima wrote:  is there a rule of thumb or rough way to convert cubic centemeters to horsepower? An example 205cc=?, 208cc=?etc,etc,

I'll risk a guess that this is an effort to compare snowthrowing capability, now that we have older units rated in hp and newer ones rated by torque and/or cc.

Engine design is a major factor in comparing an older used unit to a new one. For instance a 195 cc L-head (or side valve) Tecumseh is rated at 5 hp (2003 model HSSK50), but most newer 5 hp engines are only 179 cc because they use overhead valves which process the combustion mixture more efficiently.

Impeller design is the second important consideration in throwing distance. My neighbor's older MTD has 312 cc, but uses the traditional small, high bypass impeller with a large 14 mm gap from blade tip to housing.  This gap provides stone damage protection but bypasses snow inside the impeller housing with each revolution instead of throwing it. The impeller housing and chute clogs and the traditional answer has been to apply more power. (Stuff a big enough Pratt & Whitney into an F4 Phantom and it will eventually fly.)

Some manufactureres have now finally modernized impeller design. Toro's Power Max system has a very tight 3 mm impeller housing gap for efficient throwing and a large channel on the downstroke side that recirculates excess snow back to the auger. Feeding it back to the auger keeps the impeller from jamming up, thus maintaining throwing power.

The net result is my modern 250 cc Toro throws fresh powder and EOD farther than my neighbor's 312 cc traditional design.

For the average winter of 8" snowfalls a 205 cc with advanced impeller should do. With the 20" blasts the northeast has gotten this year 250 cc would be better.