Right size implement for tractor

Folks just looking at implements and saw how a few articles/sites reference size of implement being recommended based on horsepower (HP) of tractor? My question is that tractor HP the overall engine HP or the "drawbar HP"?
 
Up until fairly recently it was uncommon to refer to, or even know the value of, a tractor's
engine horsepower unless talking about large 4WD tractors. Historically the most common way to
classify tractors was by PTO power and before that, belt power. This value will be a little
less, maybe 5%, than engine power and will be around 10% greater than drawbar power. Drawbar
power is the lowest due to the loss of power going through the transmission and the relative
inefficiency of putting the power to the ground through the tires. Drawbar power isn't often
referenced in literature since it is highly dependent on the ground surface whereas PTO power can
easily be checked with a dynamometer. Recently it has become more common for manufactururers to
reference engine power, probably because the higher number looks better on a spec sheet. Engine
and PTO power ratings are close enough to use either when selecting implements for a particular
tractor.

In the early days of tractors the belt and drawbar power ratings were often part of the tractor's
model designation. For example, a Case 22-40 was rated at 22 drawbar hp and 40 belt hp. On
these early tractors the drawbar power was often half of the belt power due to the terrible
efficiency of the drivetrains of that era and also since so much power was required just to move
the tractor that there wasn't much left for doing work.
 
The reason for the question was driven by I was looking at disc harrows online and some sites listed implements recommended for tractors within a certain HP range (25hp-35hp, 30hp-45hp, etc) and was wondering which HP they were referencing? Totally get picking right implement is based on numerous factors (task, soil, weather, speed, etc) but assume the power of the tractor does play a big part on choosing the right size of the implement to optimize job
 

I'd be going off "engine power"

However, all engine power is not equal.

A 3500lb 50 HP compact tractor on R4 tires will not do the same work a 7000lb 50 HP utility tractor on R1 tires will do.
 


There are more variables to consider...

Will you add a Spike-Toothed Harrow behind the Disc Harrow.?

Will you be using this Disc Harrow in Plowed ground..?? Traction is definitely a problem in Plowed Ground and you may well need Duals..

Since this is an implement that pulled behind the tractor, I would sure use the Drawbar HP as a reference ..

We always used an IH #37 10' Disc with our WD-45 Allis and never was short of power, but did use Duals ( 14.9x28" ), we puled a 12 ft wide JD Harrow behind it on plowed ground.. always in 3rd gear..
A WD-45 with Loaded tires would weigh around 5,000 lbs, I'd guess..
 
The common hp rating for tractors for decades was the pto rating. That is what I would go with.

Of late tractors are mentioned with their engine ho because it sounds bigger.

Myself, I try to distill everything to pto hp and then it is easier for me to compare machines in my head.

Paul
 

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