cc to HP conversion

on a side note, i have to say that i don't agree with the part about generac being a good machine. if you can find one at a decent price look for a honda genny. to put it in the perspective of tractors, a generac is a box store unit while a honda is a medium duty dealer unit. they have their usefulness, you just have to know what you are getting and what you need it for.

I don't know what kind of conversion table you came up with and I don't really see how they could even convert cc to hp,just like cubic inches to hp,the 2 aren't related.B&S used cc for awhile on a few of their engines,I believe a 5hp B&S engine is 206cc,that's just off the top of my head so,I'm not positive that I have that correct.At the same time a 250cc,2 stroke engine puts out about 30 or 35 hp stock.

Everlandfarm said:

Hi Folks,

Googled "convert cc to HP" and came up with "15-17 cc = 1HP.
Ev

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That's about 1 HP per cubic inch!!!
Not likely in OPE engines. You'll probably get about .6 HP or so per CI.
Your typical Briggs 31 CI OHV engines run about 16-20 HP.

I HP = about 750 watts. (74?)

OK, I agree that Honda is the way to go, however, $$ does enter the picture. The Generac is $449 with free shipping. A Honda EG series 3600Watt is $999 and you pay for shipping (Northern Tool catalog, page 12. Or there's a Honda EM series 3800 watt at the "discount price" of $1899.

My application is purely emergency use when we get an ice storm and lose power for up to 7 days. This has happened twice in the 5 years since we built the new house here. Odds are it'll happen again this winter or next. Would be nice to keep the frig and freezer running and perhaps use the internet. Oh, from amp readings taken at the load center the start up current for both is 24 amps which is about 2800 Watts so that's within the size of the unit I'm considering.

Looking at my GT 500 it has a 724 cc also marked 26HP, so doing that math, 27.84 cc/HP. Again, looking at the Northern catalog, page 11 there are three honda generators 3800, 5000, and 6500 watt, so that's 30.25, 30.63, and 29.92 cc/HP respectively. So, it would seem that 27-30 cc/HP is in the ballpark for these engines.

Going back to the Generac at 207 cc and dividing by 27 = 7.6 HP.

Now, I'm starting to realize that this is trying to compare apples to rhutabagas or something. However, the Honda EM series 3800 watt has a 8HP (242cc). So a Honda has a 8HP @3500W and Generac has a 7.6HP @ 3250W so it's at least in the ball park.

What other generator would anyone recomend in the 3250 watt size? Yep, I know Honda is a wonderful machine, much quieter, longer lasting, etc. This is for purely emergency use probably once a year.

The research I've done on Generac is that they're a reasonable machine and if you buy a Briggs portable generator, it has a Generac engine not a Briggs. (that's a fact that I'm still trying to get my head around....) They manufacture their own engines and generators here in the US. The Generac has low oil cutoff, circuit breakers, wheels, and a handle for maneuverability.

Apologies, I guess I answered my own question by looking around here-maybe a moral there somewhere? However I am interested in any recomendations on generators or if anyone has had bad experience with the smaller Generac units.

Thanks for your time,

Ev

I've got a 4000 watt Onan that I took out of a '73 motorhome. I have it built into a custom box outside the house and we use it for our backup. It works just fine to run the fridge, microwave, lights and TV. I see them on CL every so often.

As for the computer, you have to be very careful about the voltage fluctuation of a generator. You should really use a digital inverter type generator to provide clean power.

keep in mind tha tif you need to use a portable genny for a week you better have a lot of gas and a lot of OIL on hand. you have to change the oil in them every 25 hours of run time, just like a lawn mower! that means 1x a day if you it continuous.

what we did during our ice storm of '94 was run the genny long enough to keep the fridge and freezer cold, then shut it down. if they are full they will stay cold a while. a deep freeze if left closed and rather fully stocked should stay below freezing for 16 hours or more, a fridge usually about 8.

for optimal fuel usage you don't want more than about 50% of rated load on the genny. the more electrical load you run, the more gas you will use per hour.

also, if you plan on running electronics off the generator i highly recommend getting a good UPS. power the UPS from the genny and the electronics from the ups. you can potentially fry them them if you don't.

davidg said:

also, if you plan on running electronics off the generator i highly recommend getting a good UPS. power the UPS from the genny and the electronics from the ups. you can potentially fry them them if you don't.

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I've got an APC Smart UPS 1400 but I still don't trust it enough to use my computer when we're on generator. Generators provide really dirty power compared to what normally comes from the grid.

But that's just me.

that is what the UPS is for, it cleans up the AC. some will not run off generators at all they stay on battery!

i run a $100,000.00 paging terminal and network off a honda 5kw generator. we had to change out 1 ups becuase it did not like the AC the genny was doing and it would switch to battery and never try to run off power. the smarter the UPS, the more likely this is to happen!

local E-911 put in a new tower site a few years back and bought a 4 cabinet UPS with 84 batteries in it. it will run the site for 4 hours or more by itself. the old genny they installed has a mechanical governor and the output changes from 58hz to 61 hz regularly. the UPS would think the commercial power was too dirty and switch straight to battery! they ended up havign to put an electronic governor on the genny to solve the issue.

davidg said:

that is what the UPS is for, it cleans up the AC. some will not run off generators at all they stay on battery!

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That's pretty much what mine does, it either wants to run on battery, or run on the generator and beep while flashing lights due to any minor fluctuation in voltage. All in all, a disappointing surfing experience.

I can usually go for a day or two without the computer by keeping busy with all the other 'power outage' issues. My daughter? Not so much...

Excellent "Heads Up" on the need for a UPS, actually I knew that but hadn't quite got there yet in evolving this thing.

Thanks Sock Puppet, your input of 196cc/6.5HP also provides 30.15 cc/HP so the previously mentioned 27-30cc/HP does seem viable.

Thanks for your time,

Be well,

Ev

Since this has expanded beyond my original question into more of a "dos and don'ts" on emergency generator use, I'll add something. Specifically, it's absoloutely critical to GROUND THE GENERATOR!!!! When you're deciding on where to put it, etc. etc. make sure you have a ground available for safety. I plan on running a separate ground back to the "driven ground" at my service entrance just for this unit. This is a small unit, 3750 surge capacity, but that is big enough to fry a lot of eggs-or people...:-(

Lack of grounding could also play into some of the frequency issues discussed. I've emailed Generac to see what the frequency variation on this unit is. I do know that all the EMS tower sites around here have Generac generators installed usually without a UPS. However, those are the 8KW, auto transfer, propane units and comparing that to their (almost) bottom of the line (KW) unit wouldn't be smart.

Generally home computers are "speced out" to survive wherever people do in terms of temp, humidity, and have a pretty wide window when it comes to voltage and frequency. The problem comes in when you go under the minimum voltage levels, throw in some frequency variation and you're definitely looking for trouble. e.g. you're keeping the frig running, a few lights, and decide to check email. Most "charts" which tell you how much power you'll need indicate arund 800W for a frig or freezer. I would suggest that these numbers go back to some previous age. My house and appliances are almost 5 years old and both the frig and freezer run at 200w and 140w respectively. However, their starting loads are 1680 and 1200 respectively, so the 800W does cover their normal power consumption, but is way under their starting requirements. If you've sized your generator based on 800w, and then a 1680 load comes on, output voltage and frequency will be significantly diminished. If you're checking mail or something, this diminished voltage and frequency would go below the "specs" for the computer and damage could result.

In sizing this unit, I took actual measurements at the load center with a clamp on ammeter to determine what MY system loads are, not a chart on someone's website. Suggest you do the same.

Sequencing also comes into play, Power has been off for a while, time to start the generator, plug in the frig, it starts, generator picks up 1680W load and then settles to 200W load, add in freezer, 1200W load, then diminishes to 120W. Now wer're running at a steady state 320W-from a 3250W source so can plug in computer, tv and continue with life as we know it. The frig and freezer, having been off for a while, should just continue to run bringing temps back to where they should be. One other area to be very wary of is microwave ovens, they can easily run at 800-1000W and that's steady state which will definitely drag down your generators voltage.

Sorry, I got wordy on this one,

Ev

davidg said:

that is what the UPS is for, it cleans up the AC. some will not run off generators at all they stay on battery!

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Ja, seen that lots of times. I manage a few network and server farms and have two natural gas stand-by generators. I use top-of-the-line Eaton online UPSs that run on inverter all the time. Even the Eaton won't tolerate your average portable power plant. I doubt a cheap home UPS would ever work.

Hi Folks,

More on the "original" purpose of this post. Was just out in the barn topping off the hydro unit in a 27 year old Roper (20-50W) and remembered to look at the 2HP Sears cultivator. It was also marked 49cc, so that works out to 24.5 cc/HP however, it's a two cycle engine so that may account for the lesser cc/HP ratio.

I'm really starting to think that the "HP" rating on these engines is a function of what the marketing department wants on the decals for the engines. Depending on the size of the deck, these units are way overpowered-IMHO.

Be well,

Ev