Roughly 16.5 HP
Based on everything I have read from Brigs and Stratton, and talking with some small engine repair guys I know, the 342cc engine puts out about 11-12hp, and the 420cc engine is about 15hp. Hope this helps.
13.5
divide the cc by 15 to get the horsepower.
14 hp in 420cc you take 420cc devide by 30 = 14hp
There isn't a formula for equating horsepower to displacement. That'll be determined by the injectors, the displacement, stroke, and compression of the pistons, and other factors.
You cannot convert cc to horsepower. cc is a measurement of the size of the engine. In other words cc is the bore x stroke x number of pistons. A 420 cc engine could have 15 h.p., 30 h.p, or any other amount.Displacement doesn't correspond to horsepower. Lot of other factors involved.
I have had mine up to 55 mph on a paved road. =)
17.5
I could be anywhere from 10 to 16 hp. Determining factors: compression ratio, bore and stroke combination, cam profile, carburetor and exhaust system. All these thing play an important role in determining the horsepower. I've pulled nearly 30hp out of a little 212cc and seen some 420s fall on there face at 10. If you really want to mod a 420 you can easily get over 100hp. Just depends on all the above factors mentioned.
420 cc how fast
The best answer is "it doesn't work that way." Two engines of the same displacement can have very different power ratings. As a rule of thumb, assume that a "modern" engine (anything designed in the last 10-15 years) has far more power than anything from the 1960s or 1970s of the same displacement. An GM "Big Block" engine, such as the Rocket 455 (7.5 L), from the early 1970s only produced 300-400 HP or so; modern engines can easily achieve that at much smaller sizes.