Horsepower Question

When we farm boys talk tractors, and talk about horsepower, we talk in PTO H.P., correct? In other words, when I say my litte Ford Jubilee has 31 HP, I am talking rated PTO H.P. I have been told that 75-85% of the PTO HP is a close estimation of drawbar H.P. Is this accurate? I have been told this by a few dealers, some use 75 some 85 %. Either way, without a dyno, it is still a guess.

Do you concur that we usually speak in PTO HP most of the time? Is my 75-85% number an accurate figure?

Thanks

Andrew

"When we farm boys talk tractors, and talk about horsepower, we talk in PTO H.P., correct?"

NO, not all the time. We know tha a "M" is belt and drawbar rated, not pto.

On the specific question you are asking, ratio of drawbar to pto hp, if you go through the Nebraska test book there are a couple of tractors that made 98% and there have been a few a lot lower than the figures you quoted. Where the average is, I wouldn't venture a guess.

Drawbar HP on the other hand is the horsepower available to do “pulling” work. It is the engine net flywheel horsepower LESS losses in the transmission, tire friction/slippage loss, and the HP required to physically move the tractor over the ground. The last two items can vary considerably depending on the gross weight of the tractor, the nature of the ground being covered (soft and loose vs hard and firm), tire design/tread pattern, etc. Your suspicion these losses can consume 15% to 25% of a tractors gross flywheel HP not far off (look at a few Nebraska Tractor Test summaries sometime!)

Bottom line: If you want to compare a tractor’s ability to do pulling work in the field, drawbar HP is the number you want to be looking at. Drawbar HP is what truly “hauls the mail”.

For example the lowly Farmall A and SA, IH never had the SA tested at Nebraska. Most folks will tell you that due to live hydraulics the SA really never delivered as much hp to PTO or wheels as the A did, and that is precisely why it was never tested. IH decided to let rest on the A record. A lot of farmers that owned both will tell you the A actually had an edge.

I don't know as I ever knew the exact particulars, however a 300 with TA, IPTO and live hydraulics was another prime example. I expect most of the loss came from IPTO. I do know that back in the 50s if one was plowing or disking and disconnected the seasonal disconnect on the belly of the tractor for IPTO, it was noticable on drawbar pull.

On early PTO's (eg. the Farmall H and M) PTO power passed thru only a single pair of gears and a dog clutch. Losses between the flywheel and the PTO stub shaft were probably only 2 or 3%.

More modern tractors use all manner of multiple planetary gear sets, hydraulic clutches, band brakes, etc. While they provide soft-starting and smooth live PTO, they do so at the expense of wasting 10% or more of the flywheel HP.

For what it's worth, the first car I owned was a 1960 Chev. The 283 cubic inch engine was rated at 185 horsepower, but when I put that car to the test on the floor dynamometer at the auto mechanics school, we could measure only 60 horsepower at the rear wheels, and my instructor said that was normal.

HP is a function of Torque x RPM and right out of the engine is wayyyyy y more then AFTER gears n trannys n differential n alllll l that friction n heat losses.

John T

A JD B will pump out 12 HP or whatever continuously for maybe 20 or 30 years without busting a sweat.

However that motor in that Murray mower will will likely be pretty well used up by the time it's reached 30 MINUTES of operation at it's "rated" 20 HP!

One heard of BOTH Drwabar HP and Belt Pulley HP etc on the older tractors and it obviously requires HP to drive the tranny n differential gears n friction yielding less HP on the drawbar versus any belt pulley and the PTO also requires gears n friction yeilding less HP on it versus HP right out of the engine.

Im unsure of your 75% estimate, but experience tells me that may not be too far off due to the friction and gears n tranny n differential losses of engine HP to the time it gets to the ground????? ????? ?????

John T