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.) .........
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.
Interesting points, Grumpy. (I meant to reply to this post a while back, but life gets in the way, doesn't it)
Using that formula gives you a "snapshot" of one point in the rev range, not an overall indication of available horsepower. Both hp and torque work in curves rather than straight lines. Stationary engines are very different to auto engines in regard to the shape of the torque and power curves, as well as the intersecting points.
Many of the older side-valve engines had a much flatter torque curve than some of the new ohv engines. Given equal amounts of torque, a flatter curve means a much more flexible engine, able to maintain power under load as the revs drop.
This is why B&S (and others) have such a wide range of engines available - a mower engine will have a different torque curve requirement than, say, one destined for a grain auger.
One of a number of findings made by Direct Connection (the Mopar factory performance group) in the "muscle car" era, was that simply lengthening the intake manifold could have a marked effect on the amount of torque available at lower rpm.
In the case of my 625, the horsepower is nowhere near maximum at the same rpm that torque peaks. It is quite possibly only producing 3.6 hp at that point. The hp keeps climbing as the revs rise (to 3600), whilst the torque peaks at 3060 rpm, then drops away.
An engine that peaks both torque and hp at the same rpm would be extremely limited in its application - as soon as the engine came under load, the power would "fall off a cliff" and the engine would stall very rapidly.
Mack's range of Maxidyne engines are a good example of hp/torque applications. The little (237hp) motor was revolutionary in the early/mid 1960's. 237hp at around 2000 rpm, but maximum torque (906 ft/lbs if I remember right) delivered at 1200 rpm. Most of the other Mack engines of the time (Thermodyne's etc) had to be kept in a fairly narrow rev range to work well (shifting down at around 1750 rpm), hence the need for the 18 speed "quad-box" transmission. Maxidyne engines were mated with a 5 speed for highway use, and a 6 speed (extra low crawler gear) for severe duty. I have personally pulled road trains (up to 70 tons) with a 237 Maxidyne - an amazing engine!
Large books have been written regarding relationships between hp & torque in all sorts of applications. I suppose we could (over)simplify the subject, by saying that the 625 series is a much better engine for my usage, than the 45 series.
