Same displacement, different HP ratings

[jlasater]

I've noticed with both Briggs and Honda that identical displacement engines may have a 16hp or 18hp rating. Assuming they are all governed at 3600rpm, how do they account for the difference? I'm thinking the Vanguard 16 and 18hp v-twins, and the Honda GX610/620 units.

Do they use different size carbs? Intake manifolds? Do they juggle the bore and stroke ending up with the same displacement but different HP?

[Tarheel]

 A while back I worked on a 16 house vertical shaft Kohler engine. The trouble was in the ignition system and replacement parts were far to costly to invest for the condition of the mower. After a while I learned that I could change the flywheel and the ignition coil (getting away from the spark advance) And lose one or 2 horsepower. (Those parts came from an engine of that same lower HP)

 The flywheel was a different size and there was no spark advance. I can't say for sure how much difference it made in usable power because before the parts change the engine seldom ran long enough to really get a feel for it. But I can say the mower doesn't suffer from lack of power. 

 Anyway, Besides bore and stroke, cam lift and duration as well as cam timing, compression ratios and so on. Rotating mass is more a function of torque IMO but I would think it plays a part in HP as well. ( I have never truly understood how torque is figured on a running engine. I had always thought torque to be the ability to do work at zero RPMs and in an internal combustion engine that would be zero with an exception or two)

 Anyway, there are a great many ways to change horsepower while keeping the same displacement. Hope this helps.

 

[jlasater]

Agreed...I'm just curious how Honda and Briggs do it. Just wondered if anyone had the inside line (i.e. "The 18HP Vanguard has a larger carb than the 16HP") or somesuch.

[Chris727]

There was a big lawsuit over HP ratings in recent years and many manufacturers switched to marketing the “CC”s of displacement. 

[ShaunE]

I think valves are different sizes in some of the old 32xxx Briggs blocks also changing the HP designation.

[CarlH]

On the smaller displacement Commands (18HP-22HP) both the carburetion and cams could be different for the same bore/stroke.

[oldsarge]

If I remember  Right from my parts counter days, the short block s for the old B & S flathead 16 & 18 horse twins were the same !!!!

[SmilinSam]

I have swapped single cylinder Kohler commands around from one tractor to another quite a bit. Looked up the internal specs on the 9hp - 18hp blocks. The specs are all the same  for a half dozen or so HP ratings of those engines. Outer difernces include a couple of different ignition systems,  2 or 3 different carbs, and I do note that the RPM's seem tobe set lower then 3600 on some.

 I took a 9hp off a STX38 put the carb on it from a 16hp, set the rpms at 3600, and used it in a Toro Wheel Horse that used to have a 18 hp command twin to drive a 48" deck.  Worked fine.

[Brent_Baumer]

Just depends on which models came with a turbo, twin turbos, a supercharger or nitrous.

[MikeES]

The older cast iron 32 cubic inch Briggs (14hp, 15hp, 16hp) where the same engine.  The 14 and 15 hp had a little tab on the throttle to prevent the throttle butterfly to fully open.  Break off the tab and you have a 16hp.

The Kohler boxer twins  KT17, KT19.  The Magnums with the exact same bore/stroke/cam were M18, M20.  Only difference is the Magnums were magneto ignition vs. battery ignition.   Apparently the slight drag with the ignition magnet is less loss than the cam/plunger/points on the KTs.