Altitude and two stroke performance??

R

Rcannon

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• Apr 22, 2004

• #1

How badly does altitude lower the performance of our bikes? The supercross is in town this week. A walk through the pits discloses a lot of information. I snuck in on "official service business"...lol

Anyway, a common theme was jetting issues and lack of power. My bike is a slug....not enough for the double..etc.

The altitude of SLC ut is about 4000 ft. The outside temp is around 60 degrees. Probably closer to 70 for the race.

How much power are the bikes down compared to a seal level environment? Are the 4 strokes hurt as badly? Is there anything a person can do to help get some of this power back?

 

T

TheJunkMan

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• Apr 22, 2004

• #2

I know Super Street and Super stock drag racing they hate running at Bandamier for the mile high nationals

 

bclapham

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i think its because of the new fuel regs more than anything- they heighten the gap between 2 and 4 strokes

 

R

Rcannon

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• #4

bclapham said:

i think its because of the new fuel regs more than anything- they heighten the gap between 2 and 4 strokes

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It should be fun to see what happens. I have not seen the 4 strokes do very well in Supercross here in SLC. The race might be better than usual. It is raining like crazy. The track will not be finished in tome to allow for Friday practice.

 

Rich Rohrich

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• Apr 22, 2004

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4000 ft altitude corrected to sea level is about a 12% power loss, or you can look at it as a 4.22 psi drop in pressure. It's sort of like giving up four pounds of boost on a turbo vehicle. :) So that 40 horsepower 125 is making closer to 35 now. :(

 

Last edited: Apr 22, 2004

SFO

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• #6

40hp 125?

 

Rich Rohrich

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SFO said:

40hp 125?

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I was being kind. :cool:

 

M

mxer842

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• #8

17%power loss on 2and 4stroke motorcyles is the rule of thumb here in CO.

 

SFO

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• #9

Rich Rohrich said:

I was being kind. :cool:

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Kindness killed the cat. :)
The subtleties are endless.
Or, the subtitles :thumb:

 

C

cujet

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• Apr 25, 2004

• #10

While Rich is correct in his HP numbers, some 2 stroke aircraft engines outperform (by a good bit) the 4 stroke competition at altitude. Of course the altitudes I am talking about are about 25000 feet. For instance, a typical Rotax 53HP ultralight engine will out perform a Continental 60HP at any altitude above 8000 feet.

This probably does not apply to such low altitudes as 4000 feet. And certainly the motocross 2 strokes are in a much higher state of tune to begin with.

Chris

Chris

 

bedell99

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• #11

This is more a seat of the pant's dyno. I live and race in Denver CO, but grew up in NY riding at sea level. In October, 2002 I drove home to visit my parents and took my beloved CR250 to race KROC(Kawasaki race of Champions) I rode 2 practice tracks(Island Motocross and Jack Frost) before the event to get the jetting sorted out. The bike then was stock. I noticed a very big difference in riding. Not so much as speed wise, but on jumps out of the corners and tricky rythem sections the bike accelarated much better. I can't explain it, the bike just had more oomfph. My father and I actually ended up bumping the main up by 4 sizes to a 390 and still felt like it was just a tad lean on top. So for Professional Supercross ridiners who are in complete harmony with there race bike, I owuld give the nod that losing a couple of horsepower is a big deal to them. Anyway here is a cool link for people who need facts and how much power is actually lost.

http://wahiduddin.net/calc/calc_hp_dp.htm

Erik

 

kdxtodd

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• Apr 26, 2004

• #12

SFO said:

40hp 125?

Click to expand...

KTM baby!

 

Rich Rohrich

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• Apr 26, 2004

• #13

kdxtodd said:

KTM baby!

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A 40 hp KTM ? :cool:

Maybe if the Tooth Fairy is the one doing the dyno pull . :laugh:

 

Last edited: Apr 26, 2004

marcusgunby

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• #14

LOL i have taught them nothing :ohmy: 40hp 125s dont exist other that in bragging competitions at the pubs. :whiner:

 

bclapham

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marcusgunby said:

40hp 125s dont exist

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yes they do, you just need to get a different magazine subscription! :thumb:

 

Rich Rohrich

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They exist on Dynojets running in chicken sheds all over the world. Just ask the shed owners. :)

 

marcusgunby

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• Apr 26, 2004

• #17

Im still confused why american dynos show a std ktm making 35hp? even when set to din(giving about 1 hp more) we get about 32-33,if there are both dynojets and calibrated then what makes the difference?