# A question... peak torque vs peak hp



## pfn (Dec 24, 2010)

Good Morning All,

I recently install a tack/hour gauge on my Ariens 926043 (32" Pro). I have noticed that the engine seems to run at around 3100 rpm, not the 3600 that everyone seems to think is the proper speed. I haven't yet checked the butterfly in the carb to insure that it is opening fully so I haven't checked the obvious yet.

My question is based on a reading of the engine manual that states "Torque values are derived at 3060 RPM; horsepower values are derived at 3600 RPM.".

It seems to me that a snowblower would operate optimally when it runs at peak torque, not necessarily at peak horsepower. 

So is my machine spinning at 3100 RMP is doing what it should to do? 3600 or 3060... What say ye?


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## RedOctobyr (Mar 2, 2014)

My $0.02: 3600 RPM, assuming the engine is rated for safe operation at that speed. My reasoning: 

- Higher RPM means you'll throw light snow further. 
- You'll have peak power available. 
- As the load increases and the RPM drops, remember that your RPM will sag towards a range with *more* torque. So the system will have a better chance of "reaching equilibrium", without just wanting to bog and stall. If you'd started at 3060, then as soon as the RPM dropped, your torque would also be dropping, putting you in a worse position (kind of stacking the deck against the engine). 

Put differently, starting at 3600 RPM, you'll still have the benefits of 3060 RPM available, because as the engine starts to bog, it'll move you closer to 3060 anyhow. To me, this kind of seems like the best of both worlds. This isn't a generator or similar application, where the RPM needs to be completely stable and unchanging. Having it drop some from 3600 isn't the end of the world, if you were otherwise going to start at a lower speed anyhow. 

I'm not quite sure what you mean by checking if the carb butterfly is opening fully, to check the obvious. It shouldn't be open fully at full throttle at no-load. If it *was* fully open in that condition, the RPMs would run away, and you'd blow the engine. 

Now, if you mean watching the throttle plate while putting a big load on the engine, that's different. The throttle should open fully as the RPMs start to drop. 

On my 24", 10hp Tecumseh, I removed the carb shroud, and observed that the throttle was not opening fully, even when the RPMs dropped from 3600 to 3100, under a heavy load. I replaced the governor spring, and re-adjusted the governor, and now it is better. The RPMs will still drop some, but not as far, and the throttle opens further, with a heavy load. But I found it difficult to apply a load large enough to fully open the throttle plate. So it was difficult to gauge how much of the engine's potential I was really able to take advantage of. 

But before replacing the governor spring and adjusting the governor, I can say that I was not able to use all 10hp that the engine can offer, since it was bogging significantly, and *still* wasn't opening the throttle fully, and so was not making the max output. Had I not watched the carb during heavy operation, with the shroud removed, I never would have realized, which is kind of frustrating. It makes me wonder if this same thing is happening on other peoples machines.


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## pfn (Dec 24, 2010)

RedOctobyr said:


> My $0.02: 3600 RPM, assuming the engine is rated for safe operation at that speed. My reasoning:
> 
> - Higher RPM means you'll throw light snow further.
> - You'll have peak power available.
> ...


I like your point about running at 3600 and letting the load dictate the operating RPM. I can modulate that with the speed of the blowers forward travel. Excellent point.

I'm an old racer and we didn't do governors so I completely forgot about that aspect. 

I suppose it's time to adjust the governor unless someone has a better reason to do anything else.

Thanks!


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## Town (Jan 31, 2015)

Torque is a measure of the potential for work (force) while HP is a measure of the actual work (force over time) the engine can perform. A snowblower needs to run at the speed that gives the most work, which the manufacturer often says is 3600 rpm. By governing the engine to run at 3100 rpm you are not getting the maximum amount of work out of the engine. The governor linkage is adjustable to control engine speed under load and noload conditions. Under load the engine sound will change significantly but the speed should be maintained until the work to move the snow exceeds the work capacity of the engine.

On an overhead valve engine the torque curve is quite flat so the difference in peak torque and the torque at 3600 rpm is quite small. This is due to the good breathing of OHV engines.

I don't know how the governor is set up on your engine, but generally on an internal governor engine the angle of the governor shaft to the shaft that operates the carb butterfly is adjusted one way to increase rpm and the other way to decrease rpm. 

Good luck.


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## YSHSfan (Jun 25, 2014)

I'd look at the owners or service manual for the proper specs and adjustment precedure.
I'm guessing it will be 3600rpm with a worm engine and no load.

I was checking the manual for my Yamahas (I know is a different engine, but this is just for reference) and that is what it states except that the engine on the YS624 is set at 4000rpm and the engine on the YS828 is set to 3800rpm.

Most gas air cooled small engines are typically set to 3600rpm incliding Honda (except GX390, and possibly gx340) that are set to 3500rpm (I might be wrong with the last part :facepalm_zpsdj194qh).


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## pfn (Dec 24, 2010)

I looked thru my manuals (B&S 210000) and couldn't find a recommended operating RPM. I call B&S Monday and see what they have to say. 

A I am a bit concerned about the accuracy of my very cheap tach. I think it was 10 bucks delivered.


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## Town (Jan 31, 2015)

pfn said:


> I looked thru my manuals (B&S 210000) and couldn't find a recommended operating RPM. I call B&S Monday and see what they have to say.
> 
> A I am a bit concerned about the accuracy of my very cheap tach. I think it was 10 bucks delivered.


The Ariens Pro model owner manual specs list engine speed as 3600 +/- 100 rpm for the Briggs & Stratton Polar Force engine. So I would set the speed for 3700 rpm. You will be a lot happier with a fast (to spec) engine than your current 3100.

Those cheap tack/hour meter instruments got a lot of praise from a lot of people in a recent thread on Tachs.

Good luck.


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## tpenfield (Feb 24, 2015)

Typical torque / HP curve 










Usually the torque reaches its max before you peak out in HP. Keep in mind that HP is the product of both Torque and RPM. So, it is not until the torque that an engine can create decreases at a faster rate than the increase in RPM that you get a reduction in HP . . . meaning that you are past the peak HP. 

Since these utility engines that we use for our snowblowers, etc are governed to about 3600 RPM, the peak HP is often at full throttle, as there is still decent torque at that engine speed, even though the torque may be past its peak.

You want to run the engine at/near full throttle to get to most power. What you will notice is that when the engine gets loaded up, it will tend to slow down into the range of its maximum torque. If the load is too great and remains such, the engine will stall.


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## [email protected] (Nov 21, 2011)

I was told _torque is what gets you to (X speed) and horsepower keeps it there_

So you set the engine to 3600 rpm (FAST), the augers bite into some heavy EOD, the speed drops, and the governor cranks in more air/fuel to create torque to twist all the moving parts to get back to 3600 again (or as close as possible).


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## pfn (Dec 24, 2010)

Thoughtful responses from all.

Thanks!


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