Increasing RPMs to compensate for power loss from altitude, Why Not? - Snowblower Forum : Snow Blower Forums
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post #1 of 48 Old 02-11-2020, 08:45 AM Thread Starter
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Increasing RPMs to compensate for power loss from altitude, Why Not?

At 6000' my snowblower produces 18% less power, or so I've been told. Why not increase the RPMs 18% so that the power output is about that same as it would be at sea level? If the snowblowers RPM spec is 3,500 rpm, increasing 18% would be 4,130. I'm wondering why this isn't a common mod?

Go Kart owners say the RPMs of our Honda snow blower engines can easily be increased to 6000 rpms. Higher rpms require stiffer valve springs. So, it seems Honda engines are ballanced and precision enough to handle much higher rpm's and since the increase in power is not beyond the typical power at sea level, it seems to me to have no disadvantages so why isn't it more common?

Last edited by dugt; 02-11-2020 at 10:06 AM.
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post #2 of 48 Old 02-11-2020, 10:42 AM
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I am only throwing in my 2 cents, but I would assume these engines were designed to run at a certain RPM, and increasing them by say 20% would put increased stress on the internal parts.

JMHO

BTW, what does your Snow Blower Manufacture say?
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post #3 of 48 Old 02-11-2020, 11:47 AM
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There are some threads/posts in the Honda Forum that suggests the Honda engine can be run to 4,000 rpm in stock form although the posters actually used 3,900 rpm as the max. The governor was set to a lower rpm (than 3,900 rpm) but the throttle had an additional setting to get the 3,900 rpm (extra power on demand manually). This is at sea level I think. Again the performance engine tuners were used to confirm the high rpm and the posters had 1332 machines with 3,500 rpm spec. So 4,130 rpm would be a stretch. And they were using larger main jets.

The other aspect is that engine power to engine speed is not a direct and linear relationship, so the 18% increase in rpm would not equate to an 18% increase in power.

So if the 1332 Honda is lean at sea level it may be OK for the thinner air at 6,000 ft. It is possible that the dealer automatically swapped in the Honda recommended jet for 6,000 ft. In order to get more power it may be worthwhile seeing what main jet is installed in your machine and checking with Honda's recommendations for main jets at different heights above sea level. The Honda Forum may have some experience with your concern for more power and can advise.

Good luck.

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post #4 of 48 Old 02-11-2020, 12:00 PM
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Quote:
Originally Posted by dugt View Post
At 6000' my snowblower produces 18% less power, or so I've been told. Why not increase the RPMs 18% so that the power output is about that same as it would be at sea level? If the snowblowers RPM spec is 3,500 rpm, increasing 18% would be 4,130. I'm wondering why this isn't a common mod?

Go Kart owners say the RPMs of our Honda snow blower engines can easily be increased to 6000 rpms. Higher rpms require stiffer valve springs. So, it seems Honda engines are ballanced and precision enough to handle much higher rpm's and since the increase in power is not beyond the typical power at sea level, it seems to me to have no disadvantages so why isn't it more common?
you may be playing with fire. i have lived in same area for 23 years and my old honda's do just fine as stock. i just make sure the rpms are up to spec. if you have the new HSS model you can re-jet carb. check re-jetting thread in Honda forum here.

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post #5 of 48 Old 02-11-2020, 12:32 PM
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I believe rpm is measured at the crankshaft. These small engines only have one cylinder, and you are making them run at 6000 rpm. That is like 24,000 rpm on cars. 3600 rpm is already a lot, unless it is a two cycles engine. They can't breath at higher rpm. Also, the connecting rod, valves springs and everything else can't handle that.

Go cart community is like street car community. They always push things to the limit and don't care much about reliability.
It costs a lot to modify anything, just so you know. They don't just simply increase the engine rpm. I did rather spend that money on a new bigger engine.

If you can just simply increase the engine rpm to get more HP, then every one else would do so. There will be no completions among engine markers. All engines rpm were set from factory at the safe limits. You can only play with it a little by adjusting the fuel jets and slightly increase the engine rpm.
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post #6 of 48 Old 02-11-2020, 12:39 PM
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Quote:
Originally Posted by Town View Post

The other aspect is that engine power to engine speed is not a direct and linear relationship, so the 18% increase in rpm would not equate to an 18% increase in power.

Good luck.
Yep, HP on those engines would be very likely to drop, or stay flat after 4000 rpm.
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post #7 of 48 Old 02-11-2020, 01:35 PM
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Like they said. You likely won't see power increase linearly as you increase RPM, it depends on how the engine is breathing, etc. You would significantly increase the stress on the internals, which increases greater than linearly.

And because you (presumably) aren't gearing down the engine's output, it may produce more power, but you're also putting a bigger load on it. It's now spinning the augers and impeller faster, for instance, which requires more power. So even if the power output went up, the demands on the engine are also increasing, unless you changed the pulleys and belts to maintain the stock RPMs for auger, impeller, etc. So it may not really be the net-benefit that you'd be hoping for.

I would first make sure you have the proper jet for your altitude. Then, with a tachometer, set the engine RPM to the top of the governed RPM range spec. Or, if others have shown that your engine can safely handle 3900 but the spec is 3700 (I'm making up numbers), and you're comfortable with that, then set it to 3900.

But it's more involved than just increasing RPM % by the amount of power you've lost. And the risk is blowing the engine if you increase it too far.

Now, if you want to crank it up, but you have a spare larger engine ready to install if needed, that's different. Just ensure that you don't spin fast enough that you shatter the flywheel, or someone could really get hurt. For instance, the stock Predator 212cc engine flywheels are only considered safe to a certain RPM, then they risk coming apart if you go beyond that.

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post #8 of 48 Old 02-11-2020, 03:33 PM
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Quote:
Originally Posted by dugt View Post
At 6000' my snowblower produces 18% less power, or so I've been told. Why not increase the RPMs 18% so that the power output is about that same as it would be at sea level? If the snowblowers RPM spec is 3,500 rpm, increasing 18% would be 4,130. I'm wondering why this isn't a common mod?

Go Kart owners say the RPMs of our Honda snow blower engines can easily be increased to 6000 rpms. Higher rpms require stiffer valve springs. So, it seems Honda engines are ballanced and precision enough to handle much higher rpm's and since the increase in power is not beyond the typical power at sea level, it seems to me to have no disadvantages so why isn't it more common?
Chances are the engine will have a short life with 4130 RPM..the rods break on these things... Flywheels fly apart which is very very dangerous.
Depending on your engine you may actually have less HP at higher RPM s.
It just depends where your peak HP is..might be a little above the 3600 mark or even a little behind it...Some of the smaller engines tend to have a peak higher up though in the 4000 RPM range.
The engines are designed to have a peak torque in the 2400 to 2800 range ... this gives us a wide enough window to use our engine without a sudden power fall off.
Peak HP is just what's it's going to be..they don't care if it's 3600 or 4200 RPM.

Also doubling impeller RPM consumes 4 times as much power passing the same amount of snow....So any gain you may have is going to be offset some as well..unless you reduced your impeller speed.
The Honda and Loncin engines seem to handle the abuse of overspending well enough..But yet I wouldn't over due it..I am surprised Honda is going past the 3600 or 3750 mark.
None the less where 3900 RPM originates kinda goes back to mini bikes and the four wheel brothers...There wasn't a no load RPM per say as they had centrifugal clutches..the engine wouldn't see 3900 RPM as it was under a load..You could take off the chain and set no load to 3900..which put you back in the 3600 RPM range while topping out your speed with a person going straight and bent over to reduce drag.
For ages 3750 was considered the maximum allowed RPM with 3600 preferred..However a generator needs 3600 RPM ...so needs to have a higher no load RPM then get pulled back down to 3600..hence 3750
I would still consider 3750 to be the max I would want to go..and not play around beyond.. Yeah there is a small area above to play with..buts it's not much and life will be shortened... drastically if the rod breaks.
Also to consider...let's say you have top no load set to 3750 RPM...You are working the machine then let go of your handle to stop the impeller...the governor overshoots briefly on the correction speeding you past the 3750 mark..we touched that 3900 RPM mark for a brief moment just by unloading the engine.
Now let's start to run out of fuel or the carb clogs a bit and the engine hunts..It will be bumping that 3900 RPM quite a bit


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post #9 of 48 Old 02-11-2020, 04:06 PM
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As a side note... It's a Honda with a large engine and a fast impeller.. really isn't a need to mod as it should be great right out of the factory... Essentially 'pre modded'

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post #10 of 48 Old 02-11-2020, 10:55 PM Thread Starter
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Thanks for the replies!

To clarify, the blower that I want to mod first is a 15 year old HS724. I bought it used and don't know what jet is in it but since it is pre-CARB, if it has the original jet, it might be a little rich for 6000'. I should check the jet size.

Net Power Output (HP) 5.5 (4.1 kW) at 3,600 RPM
Net Torque (lbs./ft.) 9.1 (12.4 Nm) at 2,500 RPM
Max. RPM 3,600

I only use this blower on a 22'x11' deck that gets deep snow because snow blows off my high and exposed roof onto this deck. A bigger blower would be cumbersome turning on this small deck. (I have a new HSS1332ATD for my 80’x30’ driveway.)

I realize that increasing RPMs would increase wear caused by any imbalanced components in the engine. However, I read some posts in a Go Kart forum and they run Honda engines up to 8,000 rpms but 6k rpms is more common. Engine precision doesn’t seem to be a problem because these are well made engines. Predator engines are becoming more popular because they cost one third as much, $100 vs $300 for a Honda GX200. (These engines are cheap!) However Predators are known to break flywheels and rods. Honda’s are relatively tough at higher rpm’s.

Some have mentioned diminishing returns of power at RPMs above spec, 3600 rpm in my case. Then why do Go Kart racers push them up to 6000 rpms with few if any mods other than adjusting the governor?

Aren’t Go Karts raced at elevations much closer to sea level? At sea level and at 6000 rpms they are making much more power than I’m trying to get from a blower at 4200 rpm’s and at 6000’ elevation. I think increasing power output way beyond spec by increasing rpm’s way beyond spec would be much more wearing on an engine than increasing rpm’s to increase power to just barely meet the factory power specs.
For me, this discussion is somewhat academic. I just finished doing the impeller mod on my 724 and that might be all the improvement I need. But, I just got a tachometer so I will increase the rpm’s too. I’d rather not mess with the governor because the gas tank has to be removed to get to it.

Thanks again for the input and conversation!

Last edited by dugt; 02-12-2020 at 01:27 AM.
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