Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

It’s confusing the way torque and power are talked about.  Maybe someone can square me away?  The confusion is people referencing max torque of engines from torque curves.

 

Related to snowblowers only.  The following are not statements of fact.  They are my understanding.

 

Torque is the guy that tosses the snow, the lifting energy.   Power (speed) is also a factor.  Together they make up the needed elements for throughput related to torque (how much can be gobbled up) and power (distance of toss from engine/impeller speed). 

 

Torque and power have to be considered together at the useful “range” which for a snowblower is max throttle, max RPM.  For the RPM part that is max under load so 3600 drooping to ?? unsure but a guess 3000. 

 

People talk about how much torque is available for a given engine and those figures usually reference torque curves. Referencing torque curves seems useless for a snowblower because it’s never operated in the range of RPM where max torque happens.   Any references to these numbers for a snowblower are meaningless.

 

The only part of a torque curve that is the meaningful for snowblowers is the far end (not max torque from the curve) and that happens at 3000 to 3600 (the far end).   

 

Is the above right?  Max torque from curves is not useful for a snowblower.  It’s what is at the far end of the curve that is meaningful for snowblowers.  The 3000 to 3600 section of the curve.  No one runs their engine at 2400 RPM. 

trouts2

I would say that max torque value on the curve is useful in some discussions.  Where it is that value that is being discussed. For example... when a blower gets bogged down and the rpm starts to drop.  As it does, and will if the blower is being driven at to fast a speed into deep very wet snow.
When the rpm drops in that situation having the torque value actually going up is a very good thing.
So comparing max torque shown on the curve can be a way of comparing how well different engines will perform under an extreme snow load. (one that normally you would try and avoid by slowing down the travel speed).
 I would agree with you that the torque rating given for the normal operating rpm. Is a more useful spec. The problem that sometimes arise in discussions about torque ratings. Is that people sometimes like to mix the max torque rating up with the normal operating torque rating, and you can't do that.  If two different engine have the same torque rating at 3600 rpm.
But one has a higher max torque rating.  If I had to choose I would certainly choose the one with the higher max rating. Nothing like having a little more guts when you really need them, couldn't hurt.
But having said that, I would also add that. Looking at a torque curve can give a person an insight into how a particular engine will perform as it's being loaded up. I believe that the closer the max torque rpm is to the operating torque rpm the better.
17lbs at 2500 and 13lbs at 3600. (Would be better in my opinion). Than 17lbs at 2100 and 13lbs at 3600.  You'd only have to drop 1100 rpm to reach max torque in the first example. But you'd have to drop 1500rpm to reach max torque in the 2nd example. In some ways the figures are only really useful if a person is going to constantly be pushing there blower to max effort.  Which isn't really a good idea of needed unless your in a really big hurry to get the job done.  The travel speed for the depth and density of the snow is what really determines how a blower performs. You can blow 12" of snow with a worn out 3hp single stage if you go slow and don't take a full bucket cut.  Which still beats using a shovel, at least for me.

If a person wants to clear a big driveway fast get a larger bucket with a big engine. A 32" clears and extra 8" per pass over a 24". Four down and back passes clears an additional 5'+ of drive, that's a lot. OR you could go to the ultimate and get a Tool Cat with a 5' blower and do a 10' wide drive in two passes. Your coffee wouldn't even have time to get cold.

I'd have to agree with jrtrebor.  The only engine I own that came with more literature than I care to read was the 6 hp Robin-Subaru commercial engine on my Ariens Pro 21 inch lawn mower.  That engine came with an operators manual, a parts manual, and a service manual.  The service manual notes that max hp is achieved at 3600 rpms and that max torque is achieved at 2800 rpms. 

At 3600 RPMs the engine produces 6.0 hp and 8.6 ft/lbs of torque.
At 2800 RPMs the engine produces 5.0 hp and 9.4 ft/lbs of torque.

I know that cutting grass and blowing snow are different but the question is if 2800 rpms are close to where the engine is operating under a load going through grass.  I would think yes but that's just a guess.  In any event I'd have to say that as long as the max torque was achieved somewhere close to normal operational RPMs then it would likely come into play under the heaviest loads...and be something to consider. 

remember that the engine rpms isn't always gonna be at 3600 rpm all the time, for example when the engine is put under load the rpms will drop and this is where u want a good amount of torque to get the engine back up to rpm. the way i understand it is (torque is reponsiable for the bringing up of the rpm, and horsepower is responsiable for maintianing rpm once there) they work hand and hand. ideally in theroy it would nice to have equal amount of horsepower and torque but not at the same time. this is just my opinion, that l believe the rpms of an engine when put under a substantial load could easily drop to the low 2000 rpms. l think i will put my meter on a engine and test how much of an rpm drop there really is depending on load of course, lm curious now:).

I think we are all in agreement.  It’s not so useful to consider max torque alone without brining in its value in the useful operating range of a machine.  The specs at HD or dealers just give max ratings without RPM or curves so not much info about the engine. It would be nice if they included some.    

 

Niper99: I’m with you Niper99.  It’s time to  surf around for a clip on tach,

 

Paul7, may be the Subaru has a better curve in the higher RPM range than similar cc Briggs.  They are supposed to be good engines.  Maybe I should hunt down some curves for Honda and Yamaha.

The other thing is you can load an engine and have it run hard as in working very well not straining but under governor control control.  You can here when it's running well loaded but not struggling. 

Then there is running with loading that boggs down the engine heavily.  Engines at this point almost sound like thumping on old train engines.  Running a motor in that range for very long would bring a quick end.  It will still perform well but seems too taxed.  I've always wondered where I am in the curve at that point.  Time to get mobile tach. 

I was looking at one of these unit. Has both a tach and an hour meter.
http://www.tinytach.com/tinytach/index.php

trouts2 wrote:

The problems most have with snowblowers is the transfer of the engines torque to the drive mechanism, either via Hydrostatic Transmission or a friction wheel in contact with a drive plate being turned by the engine.  If the machine hits a particularly solid block of ice the engine may have sufficient torque to crush and throw it via the auger and impeller, but the drive may not let allow that to happen because the belt can slip, the tires can slip or the rubber faced friction wheel can slip, all of which keeps the machine motionless.  3000 pounds of torque is the equal of a 12" long Breaker Bar on a bolt and you applying 3000 pounds to the handle.   That is a lot of force.  Getting it where you need it is usually the only problem. This is why adjusting all these things before the season, and replacing the rubber friction wheel 'tire' or the belt from the engine crankshaft to the drive plate matters so much because all that irresistable torque can just as easily slip a belt that is stretched or a glazed friction wheel tire rather than drive the wheels and the machine.  So a better question to ask of any new snowblower is "How Wide is the Friction Wheel Rubber ?"  and "How Large is the Diameter of the Friction wheel ? " compared to other competitive snowblowers because it is quite literally how the Torque gets to the meet the Road under the machine. Lacking gears in a transmission these friction wheels are what makes the wheels turn or not.  The ones that better utilize the engines torque have never been determined, a failure of Consumer Reports who claims to evaluate and compare such machines IMHO. What CR tests they test fairly, but they fail to address and compare these critical drive systems that utilize the Torque you are concerned with.

trouts2 wrote:
trouts2 -  the commercial Robins (Subaru) on my mower is by far the best engine I've ever owned...quiet, one pull starting, very smooth power and perfectly balanced.  However I think that Robins make different models of varying specs and quality.  Probably not accurate to rate their entire line based on one model.  The OHV one I have has an oil pump so that mowing on slopes isn't a problem, an oil filter, auto decompression, etc. 

My guess is that just like Briggs and Honda, Robins-Subaru likely make a top of the line model all the way to a basic utility model.  Honda built a well deserved reputation with their GX line which debuted in 1987.  Then they came out with the GC line in 1997 and a lot of people bought equipment just because it had a "Honda" engine.  Most people never realized that it wasn't the same engine that built Honda's reputation.  Honda also has a GS model that falls somewhere in the middle of the GX and GC.   Anyway my sense is that not all Robin-Subaru's are created equal and different models may have different hp - torque curves.

   According to Snowman yours is a bit beefier because Ariens ordered it that way.  If I ordered the same model as yours I would not get the same engine. 

I've got a TinyTach on my CubCadet 930 (357cc Chinese motor). Full throttle @ no load shows 3700rpm. A heavy load pulls it down to 3000 rpm. Having the torque peak below the set RPM is a real benefit since as you load the engine down, it becomes harder to bog it down further. I wish I could find a torque curve for this motor to see where things sit. Horsepower is a simple equation HP=(Tq * RPM)/5252. So mathematically HP and Torque numbers can ONLY equal each other at 5252 RPM. However the HP and torque peaks have almost no relation to each other. The RPM of the torque peak is almost entirely dependent on the cam profile.

You have it basically correct. Torque is the twisting force. Its the real unit of how "powerful" an engine is. Horsepower is effectively a linear multiple of torque at a given RPM:

HP = rpm x torque / 5252

For an electric motor, torque is essentially flat across the RPM range, but the same is not true of an internal combustion engine, so when we deal with gas/diesel engines, we talk about a torque curve because its way more interesting. Every internal combustion engine is effectively optimized to produce maximum torque at a place the designer/engineer decides it would be most advantageous. Diesels produce all their torque down low. A honda VTEC tends to produce it up high in the RPM range (which means these engines produce lots of horsepower, but little torque, which at first doesn't make sense.)

For these OPE engines, torque is a way more interesting number than HP, particularly in the range of 2500-3600 RPM because that's where the engine does its work. So ideally, you want these engines to have their max torque in this relatively wide band, because if it falls off too quickly, as soon the motor bogs down under load its out of its power band and you're cussing because that big 13HP motor bogs down in the pile of snow.

The relevant formulas are here:

http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm

Even for car engines, people should be far more concerned about the torque curve, but that's too complicated, so people fixate on BHP in their cars, which is largely irrelevant. This also explains why a BMW inline 6 can often produce more useful power and fast times 0-60 than a V6 despite having 40-60 less max HP, since the torque curve on the I6 is often very broad and a lot of power is available from 2K RPM on up to the redline. But people like to argue peak horsepower, which again, is almost irrelevant.

Honda will show you there torque curves at this site:

http://engines.honda.com/models/model-detail/gx340

Subaru/Robin have it here, although the torque curves are missing from the snowthrower engines:

http://robinamerica.com/pfeatures.aspx?pid=199

I can't find the equivalent for Briggs engines, but I found this web site:

http://greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=00Al4s