Some newer common rail turbocharged and intercooled tractor engines are often being run at over 35 HP/L continuous rating. This would give an engine with the displacement of the 4066's the potential for 10+ more HP than it is currently rated at. For example, Deere sells standalone versions of its tractor engines and provides the various maximum power levels the engines can make depending on duty cycle.
EWX 2.9L/3029H (2.9 L three cylinder, two valve heads, single fixed vane turbocharger, intercooled): 74 HP (25.5 HP/L) continuous full-load power rating with no intermittent ratings given. 75 HP is the cutoff where the EPA essentially requires urea (DEF/SCR) to meet emissions and Deere does not make a DEF/SCR version of this engine and thus limits its power to 74 HP.
PSS 4.5 L/4045C (4.5 L four cylinder, four valve heads, series turbocharged/intercooled): 140 HP (31.1 HP/L) continuous, 173 HP (38.4 HP/L) intermittent.
PSS 6.8 L/6068C (6.8 L six cylinder, four valve heads, series turbocharged/intercooled): 250 HP (36.8 HP/L) continuous, 300 HP (44.1 HP/L) intermittent.
PSS 9.0 L/6090C (9.0 L six cylinder, four valve heads, series turbocharged/intercooled): 325 HP (36.1 HP/L) continuous, 425 HP (47.2 HP/L) intermittent.
PSS 13.5 L/6135C (13.5 L six cylinder, four valve heads, series turbocharged/intercooled): 500 HP (37.3 HP/L) continuous, 601 HP (44.5 HP/L) intermittent.
PSS 13.6 L/6136C (13.6 L six cylinder, four valve heads, series turbocharged/intercooled): 585 HP (43.0 HP/L) continuous, 684 HP (50.3 HP/L) intermittent.
PTO horsepower is often underrated but it is difficult to say exactly what the losses should be unless the tractor is tested, particularly for a hydrostatic transmission unit. Generally tractors smaller than utility tractors do not get formal testing performed. The University of Nebraska tested the predecessor to the Deere 4066, the later 4720. It also had a 66 HP rated engine and a hydrostatic transmission, although it was Deere's 2.4 L skid loader engine (4024T) rather than a Yanmar engine. It was rated at 56 PTO HP and tested at 59 HP for an 11% loss. No hydrostatic Massey Ferguson or Iseki tractors have been tested by Nebraska.
A decent rule of thumb for a gear transmission is about an 8-10% loss at the PTO vs. flywheel. There doesn't seem to be a rule of thumb for hydrostatic transmissions other than it's more than a gear transmission due to hydrostatic pumping losses.
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