Igor, if your 625 has 6.25 lb ft of torque, and maintains it up to the maximum speed of 3,000 rpm, it would have 3.6 hp rather than 5. (That is still a major improvement on 2.6 hp, of course.) I suspect B&S may be gradually changing to a system of names that comes a bit closer to complying with "truth in advertising" standards where they exist (Australia has some of the toughest ones around. Australian law (the Trade Practices Act) has required that only net horsepower be advertised ever since that Act was introduced some decades ago). You might care to read these:
http://www.newsinferno.com/legal-news/lawsuit-alleges-lawnmower-horsepower-fraud/http://www.yourlawyer.com/topics/overview/Lawnmower_Horsepower_FraudI notice that B&S quote SAE J1940 in their advertising. J1940 still allows gross HP to be quoted instead of net HP, plus according to the internet B&S still tests their engines at 3,600 rpm even though they govern the vertical shaft ones to lower speeds than that. B&S is currently advertising both gross torque and gross horsepower:
http://www.briggsandstratton.com/engines/push-mower/detail/?series=550+Series&id={7AF24CC0-43AB-42CB-8282-4CE6C538D6D6}
"*All power levels are stated gross torque per SAE J1940 as rated by Briggs & Stratton.
*All power levels are stated gross horsepower per SAE J1940 as rated by Briggs & Stratton."
http://www.toro.com/home/mowers/recycler/20066.html"*The gross torque of this engine was laboratory rated by the engine manufacturer in accordance with SAE J1940. As configured to meet safety, emission, and operating requirements, the actual engine horsepower on this class of lawn mower will be significantly lower."
Gross power and torque can be measured without air cleaner and muffler. If J1940 follows other SAE power testing standards, they can also be measured with mixture and spark timing optimised rather than being controlled by the carburetor and magneto. Given B&S's fixed ignition timing, this could make a large difference. Some other remarkable tricks are also permitted for gross measurements.
The formula for calculating horsepower is 2 times pi times RPM times torque, divided by 33,000. Because 2 times pi divided by 33,000 is 1 divided by 5252, you can calculate horsepower as RPM times torque (in lb ft), divided by 5252.
Generally, square intake ports are used for low to medium speed engines because they may improve torque in that speed range. Round ports tend to suit high speed engines such as modern motorcycles (not mobile air compressors of the Harley breed) and racing engines. However a much more important factor is the size of the port, and the size of the intake valve. In this case the main factor is likely to be which is easier and cheaper to make. As a wild guess, when B&S was getting ready to move engine manufacturing to China it redesigned the products to suit today's market.
