Smokstak Bulletin Board

Old Engine Torque

I was just curious to see if anyone had ever figured out the foot pounds of torque an old engine would put out, compared to modern day engines. Say like, what would a 225 H.P. Fairbanks Y put out compared to a modern day 225 H.P. V-8, or any two engines of the same H.P. for that matter. This just crossed my mind and I was wondering if anyone had ever hooked up an old engine to an engine dynamometer. -- Tanner

Funny you should ask. Back on November 3, Dave asked about "old" engine efficiency. My brother-in-law and I went to Terre Haute Indiana to pick up an engine that I had bought from an instructor in the Mechanical Engineering Department at the Rose Hulman Institute there. He gave us a nice tour of the facility and showed us several engines connected to dynamometers. The students were comparing late model V8 engines to a 2.25 HP Associated engine that the instructor owned. This information was gathered back in 2001 and the instructor is making a search to find the data collected by the students. When or if he finds it, I will let you all know and try to make it available to all who would like to have it. Keep your eyes on this board for that information! -- Don

Power equals Torque times RPM. For engines with same HP, at the same RPM, torque must be the same. Advertised HP means nothing unless you specify the RPM. -- Brent

I'm just guessing, but I think the torque would be about the same as any other engine of the same horsepower. Then again, those heavy flywheels will carry a lot of torque once they get spinning. The thing to remember is that the old engine made horsepower at low RPM. If they could stay together at say 2,000 RPM, they would have a lot more horsepower. -- Vernon

I don't see how that can be possible. Look at how many rpm a modern day 6hp engine has to run to get the same hp as my 1925 6hp F/M. You turn the rpm down to match my F/M and I don't think you're going to get the same torque. I had always thought with the old engines the torque is generated by massive flywheels and in the modern engines torque is generated by high rpm. I don't see how the two can be compared by saying that at the same hp and rpm they must have the same torque. -- David

Unlike an electric motor, HP and torque are not related by HP=T/5252. The torque a gas engine produces is primarily a function of the distance between the main bearing center line and the rod bearing centerline (crank offset). This dimension multiplied by the force (cylinder pressure x piston area) applied to the connecting rod gives the torque output. There are other factors that affect the RPM where the torque peak occurs like valve lift and duration that can be changed by the camshaft. The "flywheel effect" is inertia. Newton showed us that an object in motion tends to stay in motion unless acted on by an outside force (in our case friction). The inertia is the energy the flywheels impart to the system after being acted on by the torque and acceleration of the crank shaft. Now, concerning a modern V8 engine comparison, I'll use my 13 HP Allis Chalmers B as a comparison. Put a modern 16 HP Briggs, Honda, Kohler, etc. in place of that and let's see if it will pull a 14" single bottom, or better yet, 12" 2 bottoms. Given the same ground speed of my B vs. a Sears garden tractor, if my engine RPM are the same and gear reduction the same, I will have the same ground speed (gear reduction accounts for tire size difference). If my gear reductions are the same, my torque multiplication to the ground is the same. Does the Sears have enough torque to the ground to plow like the B? I don't think so. Old gas engines were (are) torque monsters. They produce very low (relative) HP but gobs of torque. -- John

Well if the torque curve has a transfer curve equal to the coefficient of the radius. Then the RPM emission will be produced with the pulley diameter X P=3+RPM. You must have this to determine the factor point. But the escapement will keep the equivalent RPM foot pounds. I guess what I'm saying is it all depends on the piston return spring. -- Monte

Brent is correct. Engines at equal RPM and equal torque produce equal horsepower. Horsepower is calculated by entering torque and RPM into a formula. If you plug identical numbers into the formula, you get identical horsepower. Period.

I made a big mistake when I took college physics. I tried to use my powers of reasoning to solve problems and would get lost in a sea of words every time. I eventually learned it was as simple as plugging the numbers into the formulas.

HP = k X torque X rpm

To get equal horsepower from an engine that is running at half the RPM, the torque must be double.

To get equal horsepower from an engine that is running at 250 rpm, its torque must be ten times that as an engine running at 2,500 rpm.

Just plug the numbers into the formula.

BTW, in the formula, above, k is a conversion factor that is the same for every calculation, therefore, when doing these comparisons it can be disregarded. -- Orrin

Hi Orrin, I see the confusion (not mine). Brent’s does state that hp=torque x rpm. But, below that he does not state that engines with same torque and same rpm have the same hp (hp=torque x rpm). He states that engines with the same hp and same rpm have the same torque (torque=hp x rpm). That's like saying that 2x4=8 must then mean that 8x4=2. I suspect that Brent's fingers got a little bit ahead of his thinking. Gee, that never happens to any of us, does it? -- David.

Brent was correct in saying that if you have two engines of equal HP and equal RPM then they also produce the same torque. Again as somebody else said HP=k X torque X rpm. If you input a constant HP, RPM and constant k then you have no way for the equation to come out other then with equal torque. -- CJ

Hi CJ, it just hit me what the problem here is. The original question was comparing an old engine with a modern one. You can't, it's impossible. If you were to lower the rpm on a modern day engine to match the rpm on my 1925 one lunger then it would no longer be producing 6hp. It needs the high rpm to produce it. The formula would apply to two 1925 one lunger flywheels or two modern day engines, but you can't use it comparing the two different types of engines because a modern engine will not produce 6hp at 500 rpm. A different method would have to be used. -- David

Here goes the same old argument about the horsepower of old engines. If you look at the hp formulas, you find the time element as in feet per minute. If you take x ft lbs. of torque at 100 rpm and x ft lbs. of torque at 3000 rpm the result is more hp. I also disagree with the theory that huge flywheels "give" these old engines torque. The fly only stores and releases the power from the combustion in the cylinder. This is easily proven by observing how long the flywheels have "torque" if the engine is not firing. I think we know the answer to that one! -- Larry

What I was meaning to say about big flywheels having more torque is that they store more energy. Spin a small flywheel at the same speed as a big one and see which one stops faster. Be it that this momentum won't last long, it is good for a short burst. You can stall an engine with a small flywheel faster then you can one with a big flywheel. The reason for big flywheel in the fist place was to help keep the RPM steady during the non power producing strokes. If the engine isn't producing power during these strokes, then torque must be coming from someplace else, or the engine would stop during those strokes. I understand the engine put that stored energy in the flywheels to begin with. If you could take torque readings during the non power strokes, you'd get higher torque readings from large flywheels, then from small flywheels. Of course small flywheels recover quicker then large flywheels, (there is always a trade off). Sometimes we get confused when comparing horsepower and torque. HP has a time element, torque doesn't. If you gear it right, you can make the second hand on your clock have as much torque as a dragster, (if you could get rid of all that friction from the gears). The only problem is, it might take 2 weeks to measure it because it would turn so slow. With HP, you add a time element, so while you could get the high torque, the horsepower would be so low, that it would be hard to measure. I forget, HP is 1 ton moved one foot in one minute? Torque is how hard it is turning. I think I have stirred the pot enough for now. – Vernon

I am not sure what you mean by saying "Advertised HP means nothing unless you specify the RPM." HP is the ability to do work. Without having an engine's RPM then you can't calculate the torque, but you can still calculate its theoretical ability to lift weight up or perform any other "work". The RPM that an engine runs has nothing to do with its ability to do work. We also can't forget that there are different ways to measure HP. Modern small engines are rated at the maximum HP at rated speed. Old tractor engines were rated at Belt or PTO HP which takes into account transmission frictional losses etc. In a way comparing a 13 HP antique tractor to a 13 HP Honda engine is like comparing apples to oranges. The ratings are considered in two totally different ways. I would personally take the tractor. -- CJ

1 HP is equal to 550 ft-lbs/sec. In other words, 1 HP is the amount of power required to lift a weight of 550 lbs one foot in one second. If it were to take two seconds to lift that 550 lbs one foot, then you only have 1/2 a HP.

The problem you run into is the way some manufacturers choose to rate their engines (and motors, for that matter). Hercules was very careful to underrate theirs slightly, probably for fear of being sued. Glenn Karch could elaborate on this, but in his book, he shows how they had an independent laboratory (some university, I think) actually check the HP. They then advertised them at slightly less.

Nowadays, they find some loophole and take advantage of it. Like rating it at some peak HP that the engine or motor can't possibly sustain for more than a split second. That's why my "6-1/2" HP air compressor is really only about a 3. -- Hal

I did do some calculations once on the torque developed by my 20 Hp. JC oil-field engine. This wasn't to long after it had thrown me off the trailer while trying to start it. We looked at the momentum of the various moving components, flywheels, crank, cross head, etc, of the engine at a speed of 180 RPM. This was a few years ago, but if I remember correctly it had a torque at that speed in the range of 1800 ft-lbs. -- Steve

Thank you Steve, your answer is what I wanted to hear. All you other guys are getting too technical. I just wanted to see if anyone had ever figured out the torque of their engine. -- Tanner

Many times, over the years, I have run across literature that showed Prony brakes used to calculate power output. I never thought much about them until after I messed around with my 3 hp F-M Z (1920). I had an 8' long solid maple 2x4 that I was prying up under the belt pulley to put some load on the engine.

I was curious how the governing would actually function on this engine and what kind of power it actually had. I also wanted to put some firing pressure on the rings so I loaded her every which way I could think of with all variations and constant loads. As tough as this maple is, there was fine sawdust steadily produced as the pulley wore into the maple. (I was cautious at first so that I didn't allow the board to wack me, ever.)

It can be an eye opening experience, at least it was for me. I was a little surprised how easy it was to bog things down, even given all the dimensions of the fulcrum point, pulley diameter, etc. A few hours of this had the engine well covered with maple 'flour'.

Essentially, the Prony brake is just a lash-up that places a weighing scale under my stick, so to speak, and measures the number of pounds on the end of the stick. A quick glance in an old engine book that covers the Prony brake shows how they can be built, quite simply, with mostly wood parts and a platform scale (a Fairbanks, perhaps).

A few calculations using numbers derived from simple measurements produces an actual foot/pound number.

This could be a fun demonstration unit at any engine show. Maybe when I get a little free time. -- Kid

The tangent force at a 2 foot diameter pulley or the lbs of "pull" at 1 foot radius (or ft-lb) = 5254 (hp)/ RPM... So a 10 hp old engine operating at 150 rpm produces a steady torque of 350 ft lb. Of course, the operating characteristics of engines vary, as you load them, the rpm drops and the torque may keep up to maintain the rpm. This formula is only approximate, based upon pure mathematics. It says my diesel truck will produce 965 ft-lb at 365 HP and 2000 rpm. it only does 565 based upon the torque HP curves. -- Paul

I hate beating a dead horse and maybe I am missing some thing, but I do not believe that you can call all horsepower/torque ratios relative. How about this: We have a 1915 Economy engine with an Webster mag. Factory rated HP is 12. Lets also say that we have unlimited resources and decide that those flywheels are just too darn light weight. So, we hire a foundry to cast identical diameter flywheels but with a face width of 7". You go ahead and crank it up, with the help of a friend now that we have added several hundred pounds to the flywheel weight, and run it at the same factory rated speed of 300 RPM. OK, this engine isn't creating any more energy now than it was before, but I promise that the measurable torque and the amount of force/resistance required to stop those flywheels, which would make any oil field engine lover envious, would be far more than a stock 12HP Economy engine. OK, that being said, I think the critical part of the equation in determining the relation of our antique engines to modern day, light weight, high RPM engines is in fact the total amount of "Stored" or kinetic energy that they have. I am no mathematician nor do I doubt any of the formulas discussed in this thread. BUT, I can not help but believe that in comparing an old hit & miss flywheel engine to a modern engine of equal HP, that the stored reciprocating energy of the flywheels is a determining factor that proves that our older engines have far greater torque than their modern counterpart, which have very little stored kinetic energy. How do you measure this stored kinetic energy? I have no idea, but it definitely has a place in any mathematical equation that compares these two very different torque ratios. -- Keith

IF you put those HUGE flywheels on the same engine it would not be off and running with a few pops of the exhaust. Rather, it would probably take 15-20 power impulses to get the engine up to speed. Where did all that energy go? Into the flywheels - a VERY efficient place to store energy for short intervals. The only things you have accomplished are lengthening both the startup and shutdown times, nothing more I'm afraid. There is only so much power in each gasoline explosion and NOTHING, everything else remaining constant, can change that. It will pull you through a tough spot though but the energy used AT THAT TIME will have to be replaced. -- Craig

Keith: On the same engine and operating at the same speed and conditions, heavier flywheels will not cause it to produce more average torque. Let's say the engine produces 100 lb/ft of torque with the original flywheels. It will still produce the same 100 lb/ft of torque with the heavier flywheels but will have more "instantaneous" torque. Sure, if you suddenly put a large load on it, it will take longer for the engine to stall because the load is -discharging the flywheel weight. Once this is discharged, the engine will stop. -- Elden

Somebody else already mentioned Prony brakes. It's too bad they are relatively scarce at shows. The folks at Rollag have one and it's quite an education to see how it is used and how horsepower is calculated.

A Prony brake calculation of horsepower requires two measurements: 1) RPM; and, 2) Foot-pounds of torque.

The "pounds" portion of torque is measured by a scale and the "foot" measurement is the length of the lever between the brake and the scale.

The length of the lever on any Prony brake doesn't change, so, to measure horsepower only two readings are taken: 1) RPM and 2) Pounds

It's is simple as that. Two simple measurements.

Understanding horsepower only requires that a person knows that it's a measure of only two things. Thousands of words of logic or reasoning will only serve to obscure a very simple thing.

The scale and the RPM meter could care less about such things as: size of flywheel, old engine or new engine, single cylinder or multi-cylinder, diesel or gas, internal combustion or external combustion, etc. etc., you name it. -- Orrin

OK then, you say that: "A Prony brake calculation of horsepower requires two measurements: 1) RPM; and, 2) Foot-pounds of torque." And "The scale and the RPM meter could care less about such things as: size of flywheel," So this can only go to prove that a modern, say 6HP engine, is a high RPM/low torque 6HP and an old Hit & Miss is a low RPM/high torque 6HP, right? Thus answering the original question of how does our old iron compare to modern machinery in the torque aspect? So in fact, just because it (an engine) is creating 6HP of energy be it new or old does not automatically mean equal torque! -- Keith

There was a real good article in Gas Engine Magazine a few years ago by someone who built a Prony brake. And I can't find it. Does anyone recall what issue it's in? -- Al

Well to further add to the confusion.... If you geared the 3600 RPM engine down to 500 RPM it would have the same torque as the 500 RPM one minus gear losses. No Machine is 100 % efficient. True the 500 RPM engine had much larger flywheels but they are only turning a fraction of the speed. Old engines are probably a little more conservatively rated though. Horse power has the same relationship as Watts in electricity. One horsepower is about the same as 746 watts and one horsepower will raise a weight of 33,000 pounds at the rate of one foot per minute. If you cut the RPM in half the torque will double and still be the same horsepower. -- Ken

The answer to this conundrum was given very early in the discussion. The limiting factor is the pro bono misconception affecting a 6 HP yellow Hired Man (Associated) in Fresno, times the foot pounds of pork through the muffler valve. This must equal the circuitous entrails of bivalves as stated in the plethora of formulae contained herein. Any questions? -- Gary

Well, How about this: If you stall a Steam Engine in the right place it will deliver Maximum Torque and not be developing any horsepower. This is the only engine that will make maximum Torque at stall. But, just let this baby get some RPM and then watch the horsepower... -- Ken

Correct. Note that electric motors have a torque curve that is maximum at zero RPM and drops off with increasing speed. That’s why they would be so good for cars. Put an electric motor at each wheel. Now you just need a really long extension cord. -- Brent

I saw the factory specifications on a 40 H.P. valveless Franklin and it had a rating of 325 BHP (brake horse power). Does this help answer anything or did I just post in the wrong thread? -- Fred

HP is simply the torque (force) expended at a given RPM (time). Work: HP always equals force per unit time. A jet plane may develop 180,000 lbs of thrust, but at the start of take off it is producing near zero HP. By the end of the runway, it has expended a lot of force over a great distance so the HP is now large. HP is just the rate at which torque is delivered. A large engine with a lot of stored inertia in the flywheels has the capability to produce a very high amount of torque over a short time and hence a high instantaneous HP. Large flywheels do store a lot of energy to even out the pulses of our old iron, but given a steady load, they all slow down. -- Paul


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