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Old 07-13-2011, 08:07 PM   #1
lake_harley OP
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Altitude Gain - H.P. Loss

It seems I've read somewhere about how much HP reduction can be expected as altitude increases. Something like "X"% of HP per 1,000 feet of elevation. Anyone know the "formula"?

I'm considering a CO trip later this Summer with at least one possible 14K elevation in mind along the way. Potential bike for the trip is a Kawasaki 250 Super Sherpa (just to make it an adventure). It does okay here in MO at 4-500' ASL but I don't want to push it up any mountains.

Thanks, in advance!

Lynn
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Old 07-13-2011, 08:13 PM   #2
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I have always heard it is 10% per 1000 feet. Thus a 100hp engine at sea level would be 90hp at 1000ft. 81hp at 2000ft, 72.9 at 3000ft, and so on.

Not sure if this is entirely accurate but it seems close to me.

Marc
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Old 07-13-2011, 08:28 PM   #3
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I was afraid it was something like that. If I counted correctly as I hit the buttons on the calculator, I was already down to <10 HP by 10,000' ASL, on the 'lil Super Sherpa. I think I'd be down to 3rd gear soon. I did read a ride report of someone going to the top of Mt. Evans (14,000+) on a 250 "something", so I guess it could be done. I wonder if the altitude increase is gradual near the top of Mt. Evans, or relatively steep, in terms of elevation increase per mile of road?

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Old 07-13-2011, 08:31 PM   #4
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The top of Evans it fairly steep, HOWEVER I have seen scooters up there so your Sherpa should have no problems. That is why there is a lower gear.

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Old 07-13-2011, 09:30 PM   #5
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it's still a motorized vehicle...downshift and rev it.
it'll make it.
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Old 07-13-2011, 10:21 PM   #6
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Everything I read indicates roughly a 3% per 1000 feet power loss...if you can believe the stuff you find on the internet.

For the OP, if I recall, your Sherpa has a CV carb, right? All of my CV carbed DS bikes were less impacted than the cable pull slide carbed bikes at high elevation. I go to some of the highest off road passes each year in CO and don't even rejet from my home altitude of 1700' with my CV carbed bike...currently a KLX250S with a 300 cylinder. I have no real problems or even idle issues, even on Imogene pass at over 13,000'. The engine loses a bit of power, but nothing that causes a problem. Unless you're jetted a bit rich now, I'd bet you won't have an issue.
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Old 07-14-2011, 03:30 AM   #7
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Density, oxygen, burnt fuel...

Assuming the percentage of oxygen in air doesn't vary with altitude (in the ranges of human habitation) - which I believe is true, then you will have a 30% reduction in oxygen at 3000m over sea level (not far off the previous post of 3% per 1000ft) (1.2kg/m^3 vs 0.85kg/m^3).

A DRZ400 jetted for sealevel will be able to ride the Pamir Highway and cross the highest pass at 4,500m. It won't be fun but it is do-able. I know because my mate just did it and I was there on my WR250R which suffered the same massive loss in power. The WR returned excellent fuel economy in the high altitude sections with it's fuel injection. The same can't be said for the DRZ.
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Old 07-14-2011, 05:21 AM   #8
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I have a wr250X but it is w/ knobbies on both ends. My X suffered some when in NM & then I cleaned the caked air filter & it felt like it grew another cylinder.
Only asking if the air filter was dirty or clean on your buds wr250r? Now I clean it daily when out in the dusty west.

I understood that the FI system would make adjustments for elevation & even temp., unless your guy stripped off some of the little sniffers. My GS showed no slug infection. For the record I live at sea level normally but when not normal I love being in NM, AZ & CO.
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Old 07-14-2011, 05:28 AM   #9
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Originally Posted by Lomax View Post
The top of Evans it fairly steep, HOWEVER I have seen scooters up there so your Sherpa should have no problems. That is why there is a lower gear.

Marc
THAT'S the answer I WANTED....so I'll go with that one

I liked TNC's formula of 3%/1000' better than the 10%/1000' HP loss too. My problem now is that I'm running out of excuses to buy a bigger bike. All-in-all it probably boils down to not over-thinking it and just doing it.

TNC....you're right, the Sherpa has a CV carb and I'd agree it makes sense that they would be somewhat "altitude compensating". I don't think Sherpas could be considered to be jetted overly rich, even with the Dr. Jekel re-jetting that seems to be done on most of them, mine included.

Thanks for the input, everyone.

Lynn

lake_harley screwed with this post 07-14-2011 at 05:34 AM
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Old 07-14-2011, 05:39 AM   #10
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People ride bicycles up Mt. Evans, power only matters if you are in a hurry Note saying it'll make it is a whole lot different than saying it'll be ok. Ok depends on your preference for power. I thought my 919 and Bandit 1250 were a touch underpowered for colorado mountains. Off road I primarily use a CR250R in Colorado and find it well suited other than the lack of FI. I've ridden a DRZ400 that was uncorked and properly jetted and found it competent and capable at altitude, but not exactly huge fun. It got the job done just fine, it just didn't do with with much panache. I've also done colorado on an XR650R and found its motor was suitable as well - not surprising as it makes similar power to the CR250. In a way the XR was improved - its a bit of a bull at sea level in technical stuff and the altitude softend it just enough that it became quite rideable. all this to say, your sherpa will do it, but if you are looking for an excuse to get something faster this is as good a one you'll find An injected TE610 or something would make a lovely CO dual sport.

Power loss will be due to three major factors. First there's less oxygen, so you get less combustion - exact same thing as not having the throttle open all the way (i.e., lower manifold pressure - assuming a N/A motor here). Second is reduced engine efficiency due to effectively lower compression ratio - since you aren't pulling in as much oxygen, it gets squished up less. The third possible source is carburation/fuel mixture - carb'd bikes will be rich as they go up unless you re-jet constantly. FI should adjust to keep the mixture correct. It isn't always perfect, but its usually pretty darn good.

Also note that power loss might not be a perfect shift downward of your power curve by x% - it may well be non-linear depending on your bike's tuning (i.e., you may keep a larger percentage of power @ 10,000rpm than at 3,000rpm or vice versa - most bikes I've ridden felt like the top end pull was still pretty decent, but really lost their bottom end snap).

You get one big benefit at altitude though - that thin air takes a lot less power to punch through, so despite your reduced power you might not suffer any loss of cruise speed on the highway. I noted dramatically reduced fuel burn on my bikes at altitude.
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Old 07-14-2011, 12:54 PM   #11
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Quote:
Originally Posted by edteamslr View Post
Assuming the percentage of oxygen in air doesn't vary with altitude (in the ranges of human habitation) - which I believe is true, then you will have a 30% reduction in oxygen at 3000m over sea level (not far off the previous post of 3% per 1000ft) (1.2kg/m^3 vs 0.85kg/m^3).
On a positive note: Your engine is going to need the same oxygen/fuel ratio, so less oxygen requires less gas. You'll love your fuel mileage at higher altitudes!
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Old 07-14-2011, 01:34 PM   #12
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Quote:
Originally Posted by BikePilot View Post
Second is reduced engine efficiency due to effectively lower compression ratio - since you aren't pulling in as much oxygen, it gets squished up less.
BikePilot, are you certain of this?

Compression ratio is just a ratio, so it doesn't change as a function of altitude. What will change is the final (compressed) pressure, as that is a function of the ratio and the initial pressure (which, obviously, is lower). The question is, which of these two is the driver?

If altitude decreased efficiency, the same would occur during partial throttle at any altitude, right (as a result of the significant drop in intake manifold pressure due to the pressure drop across the throttle plate)? That would seem to "suggest" that engines are horribly inefficient at light (cruising) throttle, which I think is false (there are, of course, pumping losses as a function of intake vacuum, but I don't think that's the type of efficiency you're talking about here. I believe we're talking thermodynamic efficiency, not net power output or BSFC).
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Old 07-14-2011, 01:37 PM   #13
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3% is the figure I've always used. It's an estimate of course, and I believe that it assumes that jetting is corrected for altitude. 10% might be closer for bikes that aren't re jetted for altitude.
A CV carb should help keep you closer to the 3%. You can expect your bike to feel pretty wheezy but it's still easier than pedaling a bicycle. You might consider lower gearing.
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Old 07-14-2011, 01:58 PM   #14
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Get a super-charger or turbo.
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Old 07-14-2011, 02:23 PM   #15
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Originally Posted by kpt4321 View Post
Compression ratio is just a ratio, so it doesn't change as a function of altitude. What will change is the final (compressed) pressure, as that is a function of the ratio and the initial pressure (which, obviously, is lower). The question is, which of these two is the driver?
I appreciate all the input on the question(s). I'm actually feeling pretty good about the Sherpa getting to the summit(s). I think I'll continue planning to take the Sherpa, unless of course, a cooler bike comes along in the meantime!


kpt4321....I think I'd have to agree that the compression ratio remains the same, but the compressed pressure would be lower because of the reduced atmospheric pressure at altitude. Start with less...end up with less. I'd imagine that was the underlying intent/thought of BikePilot too. I would say the "driver" is the actual compression pressure, since that's what it really boils down to in making power. That's where the turbo or supercharger would come in
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