Engine HP vs PTO HP

Is there any rule of thumb in the reduction of PTO vs Engine horse power. Which is more accurate? Or do you just have to look at both when selecting a model?

I would guess it would be about 90% of engine HP.

If you look at the Nebraska tests they measured the hp in two ways, drawbar and pto. Pto hp is always higher because there is less loss straight out the back than through the final drives.
Engine hp is probably a less accurate measure of what a tractor will do because I doubt they take into account the loss from all the add ons like hydraulic and power steering pumps, charging systems, heck, even the water pump and fan could be disconnected for the test if you supplied an outside coolant source.
But those items combined do eat up a bit of hp.

ASABE standard estimate is 90% of net engine, 83% of gross engine.

(quoted from post at 13:13:11 01/25/18) I would guess it would be about 90% of engine HP.

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I think you are being generous. 50% to 60% is more realistic.

I am sure some will disagree (always are), but, I do not see how there could be a set formula. PTO is a gear reduction device. Seems that there are so many different gear ratios on so many different tractor pto it could not be possible to get a set percentage without including those gearing factors. For instance: a U moline is said to have a 540 rpm pto, but, my dyno book declares it to be 585 rpm. And some are 620, 1000, or 1120. Just saying.....

This stuff varies a lot. I have dynoed several JD 2 cyl. models and they all seem to do on the PTO what Nebraska tested them at on the engine belt pulley.
Our JD 750 compact is only rated to be like 18 hp ? but it makes something 24 to 25 hp ? I always thought it had power about like a JD B.

PTO horsespower is usually around 92% of "SAE Net" horsepower. Note I said "net." Not ""gross horsepower. I don't know what you mean when you say "engine horsepower."

I agree. I have tractors where the PTO rated speed agrees with the engine rated speed. I also have tractors where the engine RPM is much lower at PTO rated speed than it is at engine rated speed. Annoying.

Engine horsepower is max at engine with no fan or water pump to take power, the to get to usuable horse poer you have to ad in what the generator, water pump, power steering pump and clutch and travsmission take and subtract to get actual usueable horsepower. That is why the compact tractors are rated at bare engine that is more than a farm tractor is rated at but that farm tractor will pull twice the load. And a hydrostat takes more power to operate than a gear driven tractor. And then most engines put out maximum power at a speed way higher than they have to be run to get pto speed so slowing down the engine to get the pto speed you loose power. Each transmission-rear end takes a different amount of power. And a lot of the tractors now are not tested so they cannot be sold in Nebraska and they just therfore use a bare engine horsepower in advertising to get people to think they are getting a lot bigger tractor. That is why a tractor advertised as a 45 engine horsepower tractor cannot do the work of a Ford N series putting out 22 horsepower on the PTO. And the pto-belt hotsepower is always higher by quite a bit than drawbar power because it does not loose the power to move the tractor. So if you have a hydrolic pump engaged the engine will put less power to a drive shaft than you do not have a hydrolic pump. Way to many variables to have a formula. And pto rated hp is 85% of max pto hp. drawbar is 75% due to wheel slippage and gear train among other factors.

When you dyno a tractor to check for HP complaints/ to verify HP, you're supposed to load it down to rated engine speed. Doesn't matter what PTO speed is. If the engine is rated for 145 HP @ 2200 RPM, you load the engine down to 2200 RPM. Whether that's 1065, 1021 or 1000 PTO RPM is irrelevant. 585 is the speed the PTO is at when the engine is at rated speed. Presumably, the PTO is capable of spinning 600+ RPM at high idle.

Sounds about right for the over 100 HP tractors. I remember my old Steiger KR1225 was 225 engine HP and 190 PTO HP.

Yes, but you missed my point. With so many different pto speeds, don't see how there can be a specific percentage formula for engine horsepower. That is the original question.

+1 We use the ASABE standards at Iowa State.

I can't help with a rule of thumb, but I have a 23HP engine in my
8N and a 23HP engine in my lawn mower. Only one will run a 5 ft
rotary mower. So I can say that the engine HP rating isn't what I
would compare if I were looking to compare machines.
Tractor to tractor, I would compare PTO horsepower or draw bar
horsepower depending on my intended use for that tractor.

(quoted from post at 19:36:33 01/25/18) I can't help with a rule of thumb, but I have a 23HP engine in my
8N and a 23HP engine in my lawn mower. Only one will run a 5 ft
rotary mower. So I can say that the engine HP rating isn't what I
would compare if I were looking to compare machines.
Tractor to tractor, I would compare PTO horsepower or draw bar
horsepower depending on my intended use for that tractor.

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And the lawn more runs twice the speed roughly so that means to get the same hp rating the 8n needs twice the toque

"Net engine HP" is so miss-leading..Basically, as the RPMs go up, so does HP. and I do not like That measurement either..

Many Manufacturers over the years have used the same basic engine in their "Newer Model", turn a few more RPM to it and yep..there ya go...

NO cam changes, marginal compression changes, same bore/stroke..

If a Prony Break (sp?) or Dynomometer is used, the Brake operator can load the engine/ tractor for Peak Torque RPM. Max HP at exactly what RPM.. Even measure how well an engine..(Like a JD "R") will handle an Overload and for just How Long..
The results are surprising..

There were many models of the "JD B"...from very ;ow numbers ( 11 or 12HP).

The "Late Styled" JDB ..(Gas) has something like 24+ at the drawbar ( will pull 2x14" about anywhere.. the "B" has a BIG flywheel that will help care engine RPM's up to get thru a tough spot.. I think the Compact tractor is a little too Delicate for Heavy abuse....( use!)..

This is an odd thing to imagine:

Why is it that a working steam locomotive makes the Most Power at the point that it is doing the LEAST work..?

Steam was awesome...!!

(quoted from post at 14:36:20 01/25/18) PTO horsespower is usually around 92% of "SAE Net" horsepower. Note I said "net." Not ""gross horsepower. I don't know what you mean when you say "engine horsepower."

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What do you mean, you don't know what he means? Engine horsepower is engine horsepower: You hook the engine's crankshaft directly to a dynamometer and measure its output at rated RPM.

I don't know what you mean by "gross" and "net."

For some reason tractor manufacturers started using engine horsepower as a marketing tool. I would not be surprised to find out that many/most/all of these engine horsepower ratings are gathered with the engine screaming well above rated RPM. This gives them an inflated HP number to boast about, much in the same way Briggs & Stratton, Tecumseh, Toro, and other small engine manufacturers did for decades. The small engine manufacturers got sued, and lost. Notice how small engines no longer have HP ratings on them, only displacement.

The small engine manufacturers got sued, and lost. Notice how small engines no longer have HP ratings on them, only displacement.

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Kohler, Kawasaki, Honda, Predator all list the engine HP. Briggs list their engine HP on all except the push mower engines.

.....and they specify "Brake" hp where the engine hp is measured at the given rpms with nothing attached including air filter muffler and
anything else that would consume or limit hp...probably with premium fuel, maybe even AV gas. grin.

On a tractor, PTO is what you are interested in, forget engine hp. Dyno is connected to the PTO shaft and measures the power. Drawbar is
nice to know too but way more difficult to measure and lots more variables in getting the answer so the industry has settled on PTO.

Thanks for all of the replies, good information. The reason I asked is because I have and want to get more PTO driven implements. I'm looking at a newer tractor with a 60HP rating and a 53HP PTO rating. The thing to I think is to pick implement that will work within that 53HP PTO rating. Thanks again.