# About tractor power ratings



## Hayjosh (Mar 24, 2016)

I did not realize this until tonight, but why is tractor PTO hp rated at engine speed (highest engine rpm), instead of PTO speed?

My Ford 4610 is advertised as 63 hp gross, 52 hp PTO. It was only when I was looking at the Nebraska data when I see that 52 hp is at engine speed (2200 rpm), at PTO speed (1800 rpm) actual PTO hp was around 46 hp.

Why would PTO hp not be rated at PTO speed?


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## hog987 (Apr 5, 2011)

Some test results show both rated pto horsepower and maximum speed pto horse power.

But in your case its to make tractor seem bigger than it is. Same reason they went to engine horsepower to sell tractors instead of pto power


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## Trillium Farm (Dec 18, 2014)

Hayjosh said:


> I did not realize this until tonight, but why is tractor PTO hp rated at engine speed (highest engine rpm), instead of PTO speed?
> 
> My Ford 4610 is advertised as 63 hp gross, 52 hp PTO. It was only when I was looking at the Nebraska data when I see that 52 hp is at engine speed (2200 rpm), at PTO speed (1800 rpm) actual PTO hp was around 46 hp. Why would PTO hp not be rated at PTO speed?


Could it be that it matters to those pulling an implement, to them PTO hp doesn't matter. I think they ought to have Max HP at the drawbar and hp at PTO speed at the PTO


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## mlappin (Jun 25, 2009)

All depends on the tractor. Some of them PTO speed and max engine speed are one and the same.


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## Gearclash (Nov 25, 2010)

Not all tractors are rated that way. But yes, it does make a difference. Years ago we ran our big square baler with an IH 1066, an early one that hit rated pto hp at 2400 engine rpm. However pto speed was 100 rpm or so slower. The 1066 was replaced by a CIH 5140 that had a lower rated pto hp, but it hit that power at pto speed. The 5140 could output the 1066 on the baler.


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## andys (Jan 23, 2018)

It's because horsepower and torque peaks are never at the same RPM. HP peak is always going to be a little bit above the peak, look at any dyno chart for just about any engine ever produced. It has less torque at the HP peak than it does at the torque peak. Because of that, if you load an engine at the HP peak, once you put a load on it that takes more torque than it has, the governor can't make it up to maintain RPM. It will slow down until the torque available matches the load at some RPM between the HP peak and the torque peak. If you geared the PTO so that it were turning 540 at the peak HP speed, when you load it down all the way it would have to slow down to reach the peak torque, and the PTO wouldn't be running a true 540. If you gear it to the peak torque, then that's it. The governor just regulates it and the speed stays the same.

If you're running a snow blower or something that doesn't need to maintain speed, then you care more about horsepower and don't care if it's a little above the rated RPM.


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