When I was young, my dad bought a new Sears riding mower. It was, compared to today, a bare bones machine. It was 8hp, 3 speed transmission, pull start, etc. That 8hp mower would cut through the highest grass without bogging down. It would climb some serious hills, too. And if I would put it in 3rd gear, hold in the clutch, and throttle it up, once I dumped the clutch I could get the front wheels off the ground. Now my 20hp mower bogs in grass that's not that high. That 12hp difference. I don't understand what's changed. I go from an 8hp single cylinder to a 20hp v-twin with what seems like less power. Have they come up with a different way to calculate hp?
The engines are rated to 75% of the total load or to put it another way the engines are 25% bigger than they need to be.
However Hp is measured on a dyno and is simply the amount of power an engine can generate under a specific ( low ) load , often almost no load at all .
Dynos can not put a shock load on the engine that is the job of a water brake and the water brake tests shows the inertial characteristics of the engine and these are not directly proportional to Hp but are proportional to torque .
To understand this google Hp vs Torque , lots of good stuff on the web to explain the difference.
This is why push mower engines are now rated by their torque which is a measure of the amount of work an engine can do .
A 6 Hp 2 stroke has near twice the torque as a 6 Hp 4 stroke which is why my 125 cc 5.2 Hp Victa 24" mower can power through stuff that a 160cc 21" Honda simply can not cut
Down side is it uses double the fuel per hour so you have to push fast to get good fuel economy.
Older engines were originally designed with slide rules so were way over engineered which is why they last for so long .
Older engines have much heavier crankshafts and few have counter weights to damp out vibrations so all of the torque the engine generates is available to the mower.
Those anti vibration counter weights gobble up near 30% of an engines power so the get the same power aa anti-vibe engine needs to be around 25% bigger capacity
Mechanical gear boxes and blade engagements consume far less power than hydro drives & electric PTO's
So old mowers with shakey engines and all mechanical drive trains could spin bigger decks with smaller engines.
Clearest example of just how much is the MTD line up
If you look at all their mowers with the same deck size
The one with the variable drive & manual deck will have a much smaller engine
The exact same mower with a hydro or electric PTO will have a bigger engine
And the one with the electric PTO & hydro will have an engine that is bigger still
Electric PTO's pull 2 to 4 amps and that energy has to come from the engine on top of the load from the blades pumping air & cutting grass and the tranny pushing things along .
Then there are things like the reduction of blade tip speed .
Faster spinning blades have smaller flutes thus less air drag and due to the higher speed more inertial energy so are less affected by the resistance the grass presents.
Then there are other things.
MY 8/32 Cox , the 8/30 Rover & Greenfields all have solid cast iron engine pulleys that go from 10 to 16 lbs so again they store a massive amount of energy .
The killer is the fixed timing because fixed timing relies on the stored inertia to accelerate the engine when extra power is required which is why the Kohlers with variable timing that JD used cut so well.
Down side is they are expensive & prone to electrical failures on an engine with a fairly primitive electrical system and small battery
