Altitude and Power

[Fergus]

Is it the lower oxygen level or lower atmospheric pressure that causes a reduction in power at altitude? I live at ~5k ft and would really like to install pods and do boom tube tips on my little 620. With stock tuning, will it be as lean as it seems? Or could it be already running a little rich because of the lower pressure/lower o2? Do I have it  backward?

Thanks

[He Man]

its a density thing. air at atitude begins to be less and less dense. less oxygen = less fuel = less power.

[DarkStaR]

less air (same fuel) = richer mix

[Speeddog]

620's are fuel injected, and adjust the mixture based on air pressure (altitude).

Less power at higher altitude due to less oxygen, and lower cylinder pressure.

[RUFKM]

Just to answer your first question the oxygen "level" is not affected by altitude as fresh air will be about 20.8% oxygen regardless of where you are on the planet. 

The lower atmospheric pressure results in a lower number of oxygen molecules in a given space (for example 620 cc's) or as He Man said "it's a density thing".  You can't get as much HP out of your 620 but your tee shots are to die for.

[Fergus]

Thanks guys.

I didn't realize the ECU adjusted the fuel mixture due to air pressure. It does this, but doesn't do it for air flow?

[Speeddog]

The ECU doesn't directly measure airflow.

It 'calculates' it from engine rpm, throttle opening, air pressure, and air temperature.

[WetDuc]

Boom tubes on a 620?  Now that's something I'd like to see.  How do they look with the DSS?

[Punx Clever]

GOOD fuel injection would have a mass airflow sensor and the pressure temperature sensor behind the throttle body.

Ducati's solution utilizes a comparison between atmospheric pressure & temperature and the throttle body opening to estimate mass flow of air into the engine.  I suppose this cuts down on the number of sensors needed because fuel injection systems typically utilize a TPS regardless.  It's an inferior system that is primarily reactive (in closed loop modes using the O2 to adjust) and predicted (in open loop following a set formula) as opposed to a proactive system in all cases (using as MAS primarily an the O2 sensor in the exhaust for fine tuning).

But, back to the original question, lower air pressure at high altitudes means less dense air which means less O2 (mass) per given volume of air.  More O2 means you can burn more fuel at the proper ratios.  Burning more fuel means more power.

To put it in a feel perspective... in Indiana at about 600ft gassing the throttle on my s2r in first always results in a massive wheelie... In north west New Mexico at about 6000ft it takes some skill to wheelie... at 10-12k at and around Ouray Colorado, it's almost impossible to wheelie.

[NorDog]

Just to answer your first question the oxygen "level" is not affected by altitude as fresh air will be about 20.8% oxygen regardless of where you are on the planet. 

This is not true.  Mountain climbers carry oxygen tanks to combat hypoxia because the percentage of oxygen in the atmosphere at high altitudes is in fact lower.

But as others here have illustrated, the issue is a bit more complicated than that; carb vs efi, etc.

A naturally aspirated engine tuned correctly for sea level will run rich at high altitudes.  That was one of the problems associated with the early manually adjustable fuel injection systems that came out years and years ago before electronic systems.

[Fergus]

Boom tubes on a 620?  Now that's something I'd like to see.  How do they look with the DSS?

It won't be boomtubes (unfortunately they aren't made by motocreations for the 620), but a similar concept with tips and baffles.

[Goat_Herder]

This is not true.  Mountain climbers carry oxygen tanks to combat hypoxia because the percentage of oxygen in the atmosphere at high altitudes is in fact lower.

I think the percentage of oxygen in the air remains the same at different altitude.  But the air density gets lower the higher you climb.  Lower air density = proportionally less oxygen molecures per breathe. 

[NorDog]

I think the percentage of oxygen in the air remains the same at different altitude.  But the air density gets lower the higher you climb.  Lower air density = proportionally less oxygen molecures per breathe.  

Yeah, I think you're right.

[UPDATE]  Let me rephrase: You ARE right; I'm a dunderhead.   

[Punx Clever]

Atmosphere content does change with altitude... but not in the realm of human existence.  The shuttle operates above the level where the atmosphere is no longer well mixed.

Everything you want to know about the atmosphere:
http://en.wikipedia.org/wiki/Earth_atmosphere

[Fergus]

Thanks for all of the comments. I feel smarter already. 

If ECU makes adjustments for pressure and gets an indirect sense of airflow, does it just not adjust for airflow? Is that the reason it runs lean with less back pressure (aftermarket pipes) and increased airflow (cut or removed air box) and requires ECU adjustments, or PCIII, etc.?