How many cc's to make each HP in stock small engines?

[Orson_Yancey]

Hi,
I have been asking myself the question for a few years: How many cc to make each HP in a small
engine commonly used on yard and outdoor power equipment.
Based on measurements and browsing through a lot of manuals and books, this I what
I think. A stock OEM four-stroke small engine at 3,600 rpm, running one 87 octane gasoline
makes ONE horsepower for

About each 40 cc for a flat head (L-Head, side-valve)
About each 32 cc for an overhead valve (OHV).

The think this formula is pretty accurate for brands of stock four-stroke small engines, commonly found on yard
and outdoor power equipment -- Briggs, Tecumseh, Lauson, Clinton etc.
Notice that an OHV of the same displacement will put out about 25% more HP than a flat-head. Moreover,
a flat head and an OHV of the same displacement will weigh about the same. I like flat heads a lot for reasons of
legacy and nostalgia. But when I am looking for higher HP to weight ratios, I look for OHV's.

Do you have any thoughts or comments on this? I take with a gain of salt the advertised HP by the small engine
manufacturer of yard equipment. Instead, I always looks at the displacement and do my own calculation.

 

[massacre]

I have a flathead that makes ~25hp at the tire. Yes I put it on a chassis dyno. Under 300cc and I have spun it over 9000 rpm repeatedly

There are too many variables for that formula to work out for everything.
If you are comparing stock to stock then you may be on to something though.
But to just say flatheads don’t have good hp/wt ratio is kind of shortsighted imho
OHV has more moving parts more things to break they both have pros and cons

 

[Orson_Yancey]

I edited the thread to emphasize stock OEM yard and outdoor power equipment engines.
Yes, a high performance 10,000 rpm race four-stroke probably puts out as high as ONE HP for each 10cc.
And a high performance 12,000 rpm race two-stroke such as a liquid-cooled motocross or snowmobile
race engine puts out as high as ONE HP for each 5cc.
Impressive numbers.

 

[massacre]

Thanks