Onan Generators at high elevation

[DonTom]

Do others here notice a difference in output capacity with their Onan Gennys at high elevations?

Here, I am above 7,000' elevation. My 4KW (33 amp) genny (continuous capacity 30 amp?/ 3600 watts?) dies at a load of above around 13 amps (1,560 watts) or so here. Under heavier loads, the RPM/frequency drops low enough to kill the genny. At lower loads, all is fine. No problem using my converter to charge back up my 300 AH lith, so not a real big deal.

I noticed the issue while trying to charge my electric motorcycle. I can charge it at 1,400 watts, but not at 2,400 watts. I have charged it at 2,400 watts from this same genny at other locations.

This reminds me of how cars in the late 60's and early 70's (or even my 1971 motorcycle) ran at 7,000'. Perhaps will go uphill at 35 MPH max, but over 100 MPH at a similar hill at sea level. Even my 2022 RV 6.5 KW genny uses a carb and I don't think it has any elevation compensation. But I am in my old RV here with my old 4K Onan.

-Don- Tom's Place, CA

 

[CharlesinGA]

Just as any normally aspirated engine does, they lose power the higher you go. Many of the Onan carbs have altitude adjustment on them that at least allows them to run at the proper fuel/air ratio or close to it, rather than being over rich.

 

[DonTom]

Just as any normally aspirated engine does, they lose power the higher you go. Many of the Onan carbs have altitude adjustment on them that at least allows them to run at the proper fuel/air ratio or close to it, rather than being over rich.

Yes, I know about that adjustment. I have the cheap Chinese carb in there, but with the same adjustment. My genny seemed to work best set for the highest elevation up here. Trying to adjust it, the plastic thingy came loose and then I used a screwdriver, screwed in all the way in where it seemed to run best. Then to make things worse, I turned the metal thingy (seems to be an adjustable jet) in so tight that the outside section of the jet broke off the carb. Can no longer be adjusted, but it's set at the best place for here anyway, which is all the way in. That gives the carb less gas.

Now, that I cannot adjust it anymore, I wonder how it will work at sea level where it will now be extra lean. I have a spare carb at home, and I know how to change the carb, so I am not too concerned about that.

Yes, I understand even after a proper elevation adjustment, there will be less power at higher elevations. But how much less? Could it be below half at 7K feet? Such seemed to be the case with the old cars designed about the same way as these gennys.

I would think by now, they would use EFI in these gennys.

-Don- Tom's Place, CA

 

[DonTom]

Many of the Onan carbs have altitude adjustment on them that at least allows them to run at the proper fuel/air ratio or close to it, rather than being over rich.

It looks like your Onan carb cannot be adjusted for above 5K feet. Perhaps that explains why mine has to be all the way in (CW) tight for above 7K feet.

However, my Chinese carb label is 10K max. and that is where I now have it stuck at.

-Don- Tom's Place, CA

 

[Ex-Calif]

Yes, I understand even after a proper elevation adjustment, there will be less power at higher elevations. But how much less? Could it be below half at 7K feet? Such seemed to be the case with the old cars designed about the same way as these gennys.

You should have somewhere from 65-75% of the original HP at 7,000 feet. That's just the altitude change. If the temperature is hot as well you will lose more HP. Hot air is les dense. In the summer above Denver I bet you are looking at 50-65% of the sea level standard day HP.

 

[DonTom]

That is an interesting chart. But I figure the Onan is even worse than that because Onans do not use EFI with air pressure sensors. Onans cannot advance the timing and such for higher elevations.

BTW, even my 1984 Yamaha Venture motorcycle runs great at high elevations. It uses carbs but has an air pressure sensor to advance the timing for high elevations. Onans are a very poor cheap ancient design, IMAO. 1960's technology even in their new gennys such as in my 2022 RV. But I have not yet tried that genny out at 7K feet.

-Don- Tom's Place, CA

 

[Domo]

First thing I'd actually check if noticing decreased performance due to changes in ambient surroundings (such as altitude) would be the air filter as any little decrease in air passage while in a rarified atmosphere could impact very quickly. After that I'd start pulling out screwdrivers to make carb tweaks.

 

[Mark_K5LXP]

The "rule of thumb" is 3% loss per 1Kft. So 7K*3=21%. Face value output, 4K*-21%=3160W. Other variables such as spark timing, temperature, whatever might explain any difference. I've done generator testing where I'll use a floor heater as a bulk load, then start adding incandescent bulbs of various wattages to see where the thing starts to get wonky. It might not reveal the "why" but you'll know the "what". If you end up within say 10% of what you'd expect then there probably isn't much to gain but if you're way off, say 30% or more then there's something going on.

Mark B.
Albuquerque, NM

 

[Reinigm]

"Here, I am above 7,000' elevation. My 4KW (33 amp) genny (continuous capacity 30 amp?/ 3600 watts?) dies at a load of above around 13 amps (1,560 watts) or so here. Under heavier loads, the RPM/frequency drops low enough to kill the genny. At lower loads, all is fine. No problem using my converter to charge back up my 300 AH lith, so not a real big deal."

Interesting. Mine does that at sea level. I can make it work, but once it stops, it won't restart until the temperature goes back to cold again. Maybe I just need to adjust the altitude setting.

 

[Ex-Calif]

That is an interesting chart. But I figure the Onan is even worse than that because Onans do not use EFI with air pressure sensors. Onans cannot advance the timing and such for higher elevations.

BTW, even my 1984 Yamaha Venture motorcycle runs great at high elevations. It uses carbs but has an air pressure sensor to advance the timing for high elevations. Onans are a very poor cheap ancient design, IMAO. 1960's technology even in their new gennys such as in my 2022 RV. But I have not yet tried that genny out at 7K feet.

-Don- Tom's Place, CA

It's not so much timing as it is air density. EFI could offer a more precise mixture with some more complexity.

Best bet is to get a small wet/dry vac, put it on blow mode and pipe it into the carb. Poor man's supercharger - LOL...

 

[DonTom]

One thing I just noticed is the DC voltage makes a difference. The genny needs the ~13VDC for the fuel pump and control board (& other stuff?) and if I go direct from the genny without the converter on, the genny gives noticeably poorer performance with the slightly lower DC voltage. However, the converter eats up some of the wattage until the RV house battery is fully charged. So the best way to do this is to first charge the RV Lith battery to 100% SOC, so it draws almost no current (less than 2 amps) & keep the converter on for the higher DC voltage and then I can charge my bike with more power.

I wouldn't expect the DC voltage to make so much difference, especially since my house battery is a higher-voltage lith, but the change is rather obvious.

I noticed even if I use my inverter to charge the bike, the heavy load on the DC stops the genny, which is still above 13 volts. I don't have these weird issues at lower elevations, unless a new problem is popping up. I will know for sure when I get this RV back to Auburn (1,400' elevation).

-Don- Tom's Place, CA

 

[DonTom]

Maybe I just need to adjust the altitude setting.

Yes, it sounds like you're running way too rich, which I think is a common problem with Onans.

-Don- Tom's Place, CA

 

[Larry N.]

But I figure the Onan is even worse than that because Onans do not use EFI with air pressure sensors.

Dan is right, it's air density. On an aircraft you adjust mixture (not timing) to get the best from your engine at higher altitudes (typically above 3K feet), but I do recognize that in the old days a timing adjustment was common to get a car performing better at altitude.

However, that chart works fairly well with CARBURATED aircraft engines as well- I always figured around 70-75% of max at 7K feet, or so, for a quick ball park look for any piston aircraft. For actual flight planning (and operation) I'd use the aircraft manual, of course.

The manual for a carburated 1970's vintage Beech B-19 Sport shows 73% max power available at 7500 feet under standard conditions (15ºC/59ºF, 0% humidity, 29.92" Hg altimeter setting (corrected barometer reading)), and that's with mixture properly leaned.

 

[DonTom]

It's not so much timing as it is air density. EFI could offer a more precise mixture with some more complexity.

I realize there is nothing that can be done with rarefied air to get back the lost power, but compare a powerful large engine car from the late 1960's to a modern EFI car with a smaller engine uphill at 8,000' feet and see which is fastest. There's no comparison. I have no doubt if my Onan was EFI I would have no problem up here. And it would probably solve most Onan issues.

Even new Onans are an ancient design, IMAO.

-Don-

 

[Mark_K5LXP]

Some of their smaller and larger models are EFI, and inverter. Maybe the 4K will be updated at some point.

QG5500:

• Electronic fuel injection – industry first for maximum performance.
• Quietest generator set in its class.
• Exceptionally low vibration levels.
• Integrated design with enclosed muffler for easy installation.
• Self-diagnostic capability to simplify troubleshooting
• Runs two air conditioners with power to spare.
• Digital voltage regulation for stability as appliance loads change

Mark B.
Albuquerque, NM

 

[DonTom]

QG5500:

• Electronic fuel injection

I cannot remember the number of the Onan in my new RV. It is either 5.5 KW or 6.5 KW. I forgot which.

I am now wondering if it is a QG5500 with TBI. Throttle Body Injection can look just like a carburetor, so perhaps it has no carb and I was mistaken when I looked at it last. I will have to take a better look the next time I am in Auburn, where it is now parked.

But FWIW, for 2022 I even consider TBI to be an ancient design. Even this Y2K RV GM engine uses MPFI (Multi-Port-Fuel-Injection) which I consider to be "true" fuel injection, unlike TBI.
But even TBI is a big step above a stupid carburetor, IMAO. The Onan in this RV doesn't even have a CV carb. These Onans are 1960's technology at best. Perhaps I shouldn't expect it to work up here. Wally, Old_Crow tried to start his Onan a couple of days ago, he could not even get it to start! At least mine works fine up here to keep my RV house battery fully charged, as long as that is all I use it for.

And I can charge my motorcycle at 1,400 watts, but that is all.

I used my inverter to charge the bike today. But I cannot use the genny under load and the inverter at the same time. My Genny wants a full 14 volts DC up here, even with only a 1,400 watt total load. I don't think I could even use the genny here at all with lead acid batteries.

No matter what I try, I cannot get above 1.4KW total AC output from the genny without it dying often, including when I use the inverter. 63 hz with no load drops to low 50's under a little more than a 1.4 KW load and gives a low frequency alarm. And the genny even dies when I try to draw more than 1.4 KW from the genny and 1.0 KW from the inverter at the same time or vice versa. I spent a lot of time with such experiments today. Using the inverter to charge the bike means my 300AH battery dischargers somewhat fast and after it takes a lot of genny time to get back up to 100 SOC.

I have no idea why the DC voltage to the genny becomes so critical up here.

-Don- Tom's Place, CA

 

[Old_Crow]

It's not so much timing as it is air density. EFI could offer a more precise mixture with some more complexity.

Best bet is to get a small wet/dry vac, put it on blow mode and pipe it into the carb. Poor man's supercharger - LOL...

You must be a Roadkill fan. Did you see the episode where they used like 4 leaf blowers to supercharge a little crackerbox car and then ran it down the drag strip? I love those guys.

 

[Mark_K5LXP]

I've run my 4K microquiet at 9K feet and it always started and ran OK, but I've never done any kind of comprehensive load testing. The only time it's konked out is when the A/C is running and DW puts something in the microwave. I'll add it on my list of things to do some saturday to put the hurt to it, and see how it does here at 5K feet nominal. While it is a "legacy" design I still consider them pretty good, think about just how many have been built and successfully operated for literally decades. Seems with a few of their models they're moving to EFI and inverter, so the writing is probably on the wall for the 4K. For a single cylinder engine I don't consider TBI fundamentally different than port.

Mark B.
Albuquerque, NM

 

[DonTom]

see how it does here at 5K feet nominal.

I think there will be a noticeable difference between 5 and 7K-feet elevation.

If the genny is only used to run the converter, it will seem fine at 7,000 feet. At least mine does. It's under a heavy load where the difference is really noticed, and my Onan reminds me of the muscle cars of the late 1960's / early 1970's that could not go more than 45 MPH over Donner Summit (I-80, near Soda Springs, CA, 7,240' elevation & uphill) that modern small engine EFI cars can now easily get a speeding ticket by doing well over 75 MPH at the same location. Makes me wonder if the big reduction in max possible power at 7K' power is normal for my 4K Onan.

Now that I think about it, you're probably right, on a single cylinder engine TBI should work about as well as true injection to the single cylinder. The difference at least, shouldn't be noticeable.

-Don- Tom's Place, CA

 

[DonTom]

I cannot remember the number of the Onan in my new RV. It is either 5.5 KW or 6.5 KW. I forgot which.

I just now looked at it. It is an Onan QG5500 Model 5.5HGJAB-1038M. 45.8 amp capacity at 120 VAC.

Antique 1960's design for sure. It's a carb. Complete with an elevation adjustment. That was how I knew it was a carb and not TBI to begin with.

Motorcycles had CV carbs by 1970. Nothing to adjust for fuel-air mixture.

The Onan can only be set up to 5K' elevation. I guess they just let it run way too rich at higher elevations. Even my $40.00 Chinese carb in my old RV says it adjusts to 10K feet according to the label. Not that the adjustment helped much with a 2.5 KW load at 7K feet elevation.

But I did notice one little improvement over my Y2K Onan. The 5.5K Onan has an electronic choke instead of exhaust heat activated.

-Don- Auburn, CA