# Why 3600 RPM?



## tdipaul (Jul 11, 2015)

.

Why are almost all outdoor power equipment engines limited to more or less this rpm?

emissions
safety
noise
cost
other?

I believe that mower blade tip speed is limited to ~19,000 rpm so that answers that

but what about other types of machines?

a bump up to 4600 might really improve the productivity of a snowblower

or would it become a death trap?

.


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## nwcove (Mar 2, 2015)

just a guess......but i think these machines are tested at 4000rpm to make sure they will stay together, then the mfgs back it off by 10% to make sure they last a little while.


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## UNDERTAKER (Dec 30, 2013)

*Anything above 3600 rpms is a recipe for all ELL breaking loose. small engine blocks and their parts are not made for high RPMS. unless you go high perf like some BRIGGS Motors that are made for the app. k:k:k:k:k:k:*


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## RedOctobyr (Mar 2, 2014)

Sped up, the engines designed for 3600 would be more likely to tear themselves apart, yes. 



But one reasonable explanation is that 3600 is the RPM required for a conventional (sometimes called "contractor") generator in the US (that's 60 Hz AC). So any generator engine must run at 3600, regardless of the size. That provides at least 1 required RPM for small engines, which you can't "get around" with a shorter blade, or a different pulley size (generators are direct-drive), etc. 



(Inverter generators are different, they start with DC, and create the proper 60 Hz AC output regardless of the engine's speed. The GX100 engine in my Honda EU2000i runs between 3000 and 5000 RPM, varying its speed based on the electrical load.)



Also, a blade tip-speed limit would likely be in Feet Per Minute, not RPM. It needs to be a linear measurement, since a small-diameter blade is allowed to turn at a higher RPM than a larger-diameter blade.


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## 1132le (Feb 23, 2017)

35 yrs I've been running 3700 to 3850 never lost a motor
1 engine popped when the gov let go and over reved
flat heads run 3750 no issue with clean full oil
bad ownerrs and tecs equal blown engines
current 8hp tech 20 plus yrs old runs 3725 with the ability to run 3800 for eod and it sounds great starts 1/2 a pull everytime

EOM


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## HCBPH (Mar 8, 2011)

Can an engine be made to safely run more than 3600 rpms - yes. I once had a racing go-cart engine that would run somewhere in a 6k-8k rpms range (best guess based on specs at the time). They are small, light weight and hardened parts to take the abuse. 
Snowblowers are not designed for high HP and high RPMs. Look at car vs truck engines. You can find the same size engine yet one has high rpm limit, higher listed HP (typically the car), while the truck has lower hp but higher torque. It's also things like lift and overlap on the cam in addition to things like forged vs cast cranks. It's torque that pulls the load, not HP. You want as much torque as you can get to pull the machine and move the snow.


Best I can describe it. Look into Race Engines vs Street Engines, there are a lot of writeups out there and you'll start seeing similarities here.


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## tabora (Mar 1, 2017)

3600 RPM also happens to be where the horsepower curve flattens out, while the torque curve has not dropped appreciably. Remember, the application for generators, snowblowers and mowers is a constant RPM for LONG duration with periodic changes in load that require peak torque to overcome. Gocarts rev freely for short periods of time. Very different...




[email protected] said:


> 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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## JLawrence08648 (Jan 15, 2017)

I run my snowblower at 3,600 rpms because they throw better than less rpm than that. I don't run it more than that because smarter period than I on here says you should not.

I run my residential lawnmower at 2,600 because I cut my grass often and I want the engine to last. I set the rpm of the residential mowers I sell at 2,800-2,850 because I don't know how high "their" grass is and I don't want the mower to come back because it didn't cut.The

It seems the Kawasaki engines on the commercial walk behinds are set fine with the governor besides having a hand throttle control so I don't adjust those.


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## RedOctobyr (Mar 2, 2014)

JLawrence08648 said:


> I run my snowblower at 3,600 rpms *because they throw better than less rpm than that*...
> 
> I set the rpm of the residential mowers I sell at 2,800-2,850 because I don't know how high "their" grass is and *I don't want the mower to come back because it didn't cut.*



It would seem to me that the same principle would apply to both types of machines. If the mower is supposed to be set to 3,400-3,600, for instance, and you had set it to 2,800, I would find that an excessive reduction. It's a lower blade tip speed, so it likely won't cut as well, and the airflow into a bagger (or just dispersing from the chute) will be reduced. Personally, as a buyer, I would consider this doing me a disservice, rather than a benefit. Especially if I didn't have a chance to decide whether I wanted the factory RPM. This is not intended as insulting, but as my perspective. 



Small engines are pretty reliable, in terms of blowing apart despite proper oil maintenance. I wouldn't want my mower doing a worse job of cutting every week, to (using hypothetical numbers) extend the engine's life from 20 seasons to 30 seasons. The rest of the machine may rust out before then, and getting a worse cut from a mower doesn't seem like an acceptable trade-off, to me. 



At some point, it's been discussed that lower RPMs are actually harder on small engines. Most tractors will tell you to mow at full throttle, or at least a high throttle point, for instance. 



The engines still have to open the throttle a bunch to try and maintain that RPM (granted, a snowblower is more likely to be bogged down than a mower). But the lower RPM means the flywheel is not moving as much cooling air, so they can actually run hotter than at full RPM. Oil flinging in the crankcase is also reduced, impacting lubrication. Idle speeds for small engines are set in part so that they don't stall, but also in part to ensure they're still getting adequate splash lubrication. 



For my machines, I set them to the factory RPMs, for high-speed, and for idle.


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## tdipaul (Jul 11, 2015)

_But one reasonable explanation is that 3600 is the RPM required for a conventional (sometimes called "contractor") generator in the US (that's 60 Hz AC). So any generator engine must run at 3600, regardless of the size. _

This makes sense for a generator, for sure

But a beefed up snowblower engine capable of running at 4,500- 5,000 all day long could have its advantages. 

Unless there is diminishing returns with impeller speed, dunno 

But Honda's turn faster than most and they perform great

Imagine if upping the impeller speed on your average Joe Blower could match Honda's throwing distance?

There doesn't appear to be anything fancy about the Honda impeller design - its all about speed

I can read it now: "For only $250, select the innovative new high speed engine for unsurpassed snow processing and throwing distance" 

Sign me up

I know there is more to this but it is fun thinking about it

.


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## Spectrum (Jan 6, 2013)

Gilson snowblowers shipped with engines set as high as 4000 RPM. The commonly accepted governor default of 3600 RPM could be rooted in generators or a conservative de-rated actual maximum.


https://gilsonsnowblowers.com//snowbulletins/gsbs_10.pdf


Pete


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## 1132le (Feb 23, 2017)

when raised from 3400 or 3500 to 3800 the eatability at the end of driveway and throwing distance increase is astounding
torque might be lower but the faster impeller more then makes up for it
there is no debating it only those that have not tried it would try debating it


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## tabora (Mar 1, 2017)

I have my Honda HSS1332 set at 3650RPM with max throttle (110 jet to overcome the CARB restrictions that made the motor run lean). When I hit the snow with it, it revs to 3750 and then settles back to 3650 while it blows the snow a loooong way away.


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## Stick (Feb 11, 2015)

I've wondered the same -- higher impeller speed. It can be done by changing the output belt pulley to a larger diameter, and of course, then a special (longer) belt. 



It would affect the walking speed also on some machines that use a single belt. 



I guess I never tried it, due to the "special" belt. The JD belts seem to be rated for more abuse than the aftermarket belts. So getting one in a 1 or 2" longer length might be impossible.


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## tadawson (Jan 3, 2018)

My take is run the engine at it's peak output RPM, and set the belt pulley ratio to get what you want for impeller speed. That way, both goals are met, and no power/torque is left on the table (and far less engines destroyed).


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## Stick (Feb 11, 2015)

tadawson said:


> My take is run the engine at it's peak output RPM, and set the belt pulley ratio to get what you want for impeller speed. That way, both goals are met, and no power/torque is left on the table (and far less engines destroyed).



That's pretty much what I was hinting. Leave the engine at the mfr's 3600 recommended optimum speed, but go with a lager sheave on the output shaft of engine to get the impeller to tach at a higher speed.. 



Do you know anyone that has done it on their snowblowers?


Stick


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## RedOctobyr (Mar 2, 2014)

People here have done it, yes. Ariens kind of does it themselves on their SHO models, which use a higher impeller RPM. 



I'm tempted to do it on my machine, since it usually has more power than needed (10hp, 24"). But figuring out a different impeller pulley and belt sounds like a bit of a pain, plus the cost involved. So I haven't done it yet. And I'm not comfortable with pushing the RPM on the Tecumseh OHV engine beyond the factory spec, so I can't just do the free & easy solution (adjust the governor to maybe 4000). 



tadawson, you say that no power/torque is left on the table, if you go to a larger pulley. I guess it depends on how you meant that. You will maintain the engine's current power/torque output. But you will lose effective torque to the sped-up impeller & augers, since they're now using a higher gear ratio. So you'll throw further, and process the snow faster, until it gets deep/heavy, and the geared-up impeller & augers more easily overload the engine, and the RPM drops. 



If you only raised the engine RPM, instead (no pulley change), even if the torque curve dropped off at the higher RPM, you'd still just fall back into the normal RPM (and therefore greater torque) range, if you overloaded it. A nice solution (free, and just takes a few minutes), *if* you're confident your engine will hold together.


If I was using a Harbor Freight engine, for instance, I'd be more open to trying >3600. People have done lots of performance testing on their 6.5hp engine, trying higher RPMs, etc, so I'd be more comfortable trying it. Plus, worst-case, a replacement is pretty cheap.


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## DriverRider (Nov 20, 2016)

tdipaul said:


> .
> 
> Why are almost all outdoor power equipment engines limited to more or less this rpm?
> safety
> ...



3600 RPM is the theoretical max an engine would have to turn (direct coupled) to a generator for 60Hz not to mention 3600 is a SAE standard for HP ratings back in the day. Then there is a question of spark timing which is static and has to be able to pull start and be more or less dead nuts on at its' anticipated running speed. The faster an engine turns the more advance it needs. Too much advance built in for very high speeds would likely give a kick back when starting.


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## tadawson (Jan 3, 2018)

RedOctobyr said:


> People here have done it, yes. Ariens kind of does it themselves on their SHO models, which use a higher impeller RPM.
> 
> 
> 
> ...


I was simply referring to staying on the 'sweet spot'or the engine power/torque curves, nothing more . . .


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## Town (Jan 31, 2015)

In the Ariens applications that use the 414 cc LCT engine the recommended maximum engine speed is 3600 rpm +/- 100 rpm. So 3700 rpm is within spec. LCT recommend, for the same engine used by Ariens, 3850 rpm +/- 50 rpm. So 3900 rpm is within spec. 

A Subaru SP170 OHC engine that I use in the summer has a spec for continuous rpm rating of 3600 rpm for 4 hp while the maximum hp is 5.7 hp produced at 4000 rpm. The label on the engine proudly advertises the engine as 6 hp. Somewhat like Honda describing/implying their 390 engine as 13 hp but the specs provided in various posts in this Forum define 11.7 hp or so.

Equipment manufacturers and engine manufacturers power and engine speeds are founded more in marketing hype than anything else.


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