What would be the difference in HP if they were both tested under the same test standards?
Not nearly as much as the "advertized" difference when both are operated at the RPM they are governed to for their application.

Gearing makes all the difference in "pulling power".

HORSEPOWER is not the "tell all" figure or descriptive for comparing work capabilities of machinery (contrary to popular myth and marketing strategies).
The term "horsepower" is actually an archaic thing and really only refers to the RATE, or SPEED at which a specific weight moving task is accomplished.
TORQUE or twisting power is a much better description of the AMOUNT of real power output an engine can develop.
Very slowly.......the automotive world is coming around to utilizing torque values to describe engine power output, but old habits do die a slow and argumentative death.
In recent years, using the term "horsepower" in marketing has come under scrutiny (and under fire) to the chagrin of engine manufactures slapped with class action lawsuits for "misleading advertising" in how they made claims of "horsepower" of their products.

Here is a link that might be useful: wikihorsepower

A choice between 16 Clydesdales and 26 miniature horses?

Or how many miniature horses would it take to pull the Budweiser Wagon?

Yeah back in the old days, a 12 hp would easily run a 46" deck/tractor, the newer engines? Who would know?

I have replaced a Kohler Command 26h.p., with an old 18h.p. Briggs L-Head, and the customer is still running it and happy with the performance!!!

"Or how many miniature horses would it take to pull the Budweiser Wagon?" As long as they pulled it off a cliff... :)

When it comes to powering a deck, my experience has been that a 10HP single running a 42" deck was to move very slowly through moist long grass without bogging down where as a 20HP twin powering a 48" deck flies through.
I know; I know, I shouldn't be cutting moist long grass but I have 16 years till retirement unlike you guys.

"*** I have 16 years till retirement unlike you guys."***
Are you sure you have ONLY 16 years to go?
They keep moving the carrot further away for me :^)

"***Are you sure you have ONLY 16 years to go?
They keep moving the carrot further away for me :^)"***

Yes but I think Wheely Boy is already 64... yikes! :)

FWIW, when Briggs gave me a new engine to replace the one that had air cleaner issues a while back, they gave me a 26HP rather than the 22. I had the two engines "side by each," totally and completely identical. The only difference was the decal reading 26 rather than 22. The 26 doesn't seem any more powerful or anything but that's what, 8% more so hard to detect. The only time the engine is really loaded is mowing and with a 50" deck either engine didn't know it was working.

Personally, I'm starting to like the "cc" rating. It at least gives you something to compare which might actually mean something. Yes, I know that the same cc engine "power" can vary widely depending on carburetion, etc. and more so.

As mownie points out, HP doesn't mean much and I feel it's more an advertising point than anything else. Let's face it, "bigger is better," so the bigger number probably generates more sales. Also, depending on how an engine is tested you can get a variation on "HP."

EV