Air Density Effect on Power

[Bruce Bohannon]

Another Question: This is my first FI bike. ISTM the difference in temperature affects the power output moreso on this bike than a cv carb bike. ?????
I've only ridden this bike about 500 miles; and mostly below 60F and about 400 feet above sea level. At colder temps - the bike not only raises in 1st (180 lb rider) but breaks loose in 2nd at times. Only 1100 miles on busa. But the day when I was riding above 70F the bike was quite predictable and was not raising as abruptly in 1st gear. 2nd seemed a little (boring? - NO, need a word?) more forgiving: relatively speaking.
Is it my imagination? Does temp have a bigger effect on a FI bike?
Or is a FI bike supposed to be able to adjust better to air density? TIA

 

[OB_valklex]

Adjusting to air density (altitude/temp/humidity) means that the air/fuel mixture will be the same, as the FI on the busa does. You always lose power when the air loses density, but the FI bike won't run overly rich at high altitudes like a carbureted would (reducing the power even further). You shoudn't feel the difference with small variations. Check your sensors' parameters listed on the service manual.

 

[OB_DanaT]

Whoa guys,

Air density can make a HUGE differnce. What would you think of a Busa turning out 117RWHP? Well thats what you get when you hit a mile high.

On another note, with that kind of power loss in Johannisburg (like the MCN test) thay are absolutely full of **** that the ZX12 felt about as fast as a Busa. Are they trying to tell us that a bike that has alitude problems (5000ft is about a 23% hp loss) feels as strong as a Busa at sea level. Its total BS. If anyone has ridden in Colorado, they will know that a busa feels like a 750 (you dont want to know how bad a 600 is....)

-Dana

 

[OB_Jamie]

As you climb in altitude the engine has to work harder to pull the same amount of air that it pulls in. The reason for this is air pressure. At sea level there is 14.7psi pushing on everything. Since this is a constant we on Earth call this 0 pressure. But 0 pressure is achieved in space. The Earth acts like a natural turbo that is always adding 14.7psi of boost at sea level. As you climb up into the mountains the "boost" gets less. Thus the power output gets less. Think of it as swimming under water. At 1 foot deep the weight of the water is only pushing on your ears a little bit at 20ft deep the weight of the water is really pushing on you. The deeper you go the more pressure. The same holds true with the atmosphere. On top of the mountains there is less air pushing down on you so the engine loses power due to pumping losses. Every engine regardless of FI or not makes less power on top of a mountain. This is why drag strips have correction factors for their times.

You also lose power because there is less oxygen available at higher altitudes. Since you need oxygen to make power and now up high you have less of it, you'll make less power.
What makes FI cool is the as you climb the mountain the sensors "see" less oxygen so they add less fuel. That's were carbs lose their appeal. If I jet my carbs to run great at sea level then I ride up a mountain the thinner air means my air fuel ratio gets richer the higher I get. As you know too much fuel will drown out HP. Carbs can't compensate for thinner air so carb guys lose power. That's why they make carb kits for high altitude bikes. When those high alt. guys come down off the mountain they'll be too lean.


Now your night time vs. day time issue kind of works the same way.
At night the air is cooler. Cooler air is smaller than hot air. That means that you can fit more air in your engine at night than during the day. More air = more power. Since you have FI, your computer can measure air temps. The computer "knows" that when it sees cooler air to add more fuel to keep the air/fuel ratio correct. Thus again more power.
That's why if you are a carb guy you shouldn't jet your bike in the winter or at a shop that is at a much higher or lower altitude. I jet my bike to run a touch rich (on the dyno) during the day. That way at night, when the roads are clear, and when I need the power, the A/F ratio is correct because of the cooler air.


Big Post,
Does it make sense?
Jamie

 

[OB_Jamie]

An engine at sea level that makes 160hp won't make 160hp at 5000ft. It doesn't matter if it has FI or not.

Now if it had a turbo or a blower that would be different. That's why drag strip correction factors aren't used (accurately) on forced engines.

Jamie

 

[OB_vortech347]

Well Jamie, you are partly right. The only engines that are not affected by altitude and air density are the ones running oxygenated fuel such as nitromethane. I am not talking about the MTBE that they put in regular gasoline to make it burn cleaner. That is a very small amount of o2 being added. Every other gasoline burning vehicle whether naturally aspirated or not will be affected by air density. Remember the blower has a smaller volume of air to draw from at higher altitudes so therefore it will not be able to feed the motor as much. You can compensate on blown vehicles by turning up the wick on the blower but that requires you make an adjustment (i.e. variable wastegate) and your blower has the extra capacity to pump at a faster rate.

 

[OB_GSXRTURBO1]

Jamie, excellent posts.
Vortech, good point, but I believe that a force fed engine is LESS affected by air density than a N/A engine. Usually, especially in a turbo application, you have more boost available than you can use. When running in thin air you just turn the screw on the wastegate to make more power. The power may still be limited, but it hurts power output a lot less than the N/A engine.

 

[OB_DanaT]

Also,

As I drive a turbo car at high altitude I can make a few seta of the pants comparision bewteen the car at low altitude.

The max power is close to the same. Where you really notice a differnce is off-bbost performance. Remember that off boost low compression engines aint screemers and with altitude loss they are even worse. What does this mean? Turbo lag is much worse at high altitudes because you dont have a good air charge when off boost to get that turbo spooled up. Once they do sppol up though the turbo rush is much more noticable.

A 175hp Mustang GT turbo here rountinely will stomp the same vintage 225hp 5.0l mustang. But off the line the 5.0 really annianlates it. But ince boost is bult..watch out for turbo cars at altitude.

For a while, Mustangs had to hide in fear of Talons here.....

-Dana

 

[OB_vortech347]

GSXRTURBO1, Yes a blower car is less susceptible to air density but only by a small margin. I have a blown 347 and there is a very noticeable difference in the power output on a cool day as compared to a hot day. The only real way to combat the thinner air is increase the output from the blower which will make up for the difference in air density. Late model turbo cars have a boost sensor that will allow the wastegate to not open up as much so the turbo can move enough lighter air to keep the boost at the level set by the computer.

 

[OB_Jamie]

Vortech,
Turbos will make the same PSI regardless of altitude or air temperature. A turbo's wastegate is powered by boost thus it will only open when it gets the amount of boost it needs. So, it self corrects for altitude. If it made 7psi at sea level, when it goes into the mountains the waste gate will just keep itself shut until it gets it's 7psi again.
As far as nitro,....... um, what can I say? I thought we were talking about the real world. I think this post was started by a guy who wanted to know if his bike made more power in cool weather. I don't remember him asking if he should configure his engine to run a different fuel that's why I didn't mention it. I don't know anything about nitro so I don't go around telling everyone about it (don't get all upset I was talking about someone else on this site). That being said it seems to me that any engine that consumes air would be affected by altitude and air temp. Unless this nitro engine you are talking about only has fuel lines hooked to the head and no source for air to enter, then I would venture to guess that it is affected too.
Forced engines aren't as effected as N/A engines. That's why drag strip corrections don't work on these engines. I never said forced engines were exempt. Don't put words in my mouth and then tell me that I said something wrong.

Jamie

 

[OB_DanaT]

Jamie,

You are correct in saying that a turbo will keep the same boost pressure at all altitudes this does not make more power. The key to power is quantity of oxygen that you get into the engine and not the pressure. From high school chemistry you will remember the ideal gas law (PV=NRT). Well you want to increase N. As you can see increasing T (temperature) also increase presure. As you compress air with a turbo it is heated thereby increasing pressure without adding more oxygen. Although the power is much better at high altitudes with a turb it is still not the same as at sea level. There is more than just boost pressure to keep in mind.

-Dana

 

[OB_CanuckBusa]

Now you have my attention. Since I live near the mountains I ride at around 3500 feet elevation and above. While my Hayabusa is fast, it didn't feel as fast as what all the bike magazines were reporting when I first got it. I was expecting my arms to get ripped out. Sport Rider said to expect massive wheelies in first and second, but it doesn't wheelie as easy as I was expecting (I didn't mess around with wheelies much last year, but when I was I was rolling off and 'snapping on' the throttle around 4000-6000 rpm, not 8000 so that may explain the front wheel's reluctance to leave earth.) So my question is: How much HP/torque do I lose at 3500 feet? Is there an approximate formula? Does this, apart from my lack of revs, explain why my front wheel sticks more than it lifts? Or is it my fuel filter...

Too bad the bike doesn't get appreciably lighter as I lose HP.

 

[OB_Jamie]

Dana,

WHAT THE FUC*! I didn't say the power levels would be the same as at sea level I said boost pressure would be the same. Don't preach the turbo theroy to me, I know how it works! Christ, go pick a fight with someone else, I was only trying to help explain to the guy who wanted to know. I didn't realize I'd have to defend internal combustion engines in general.

Jamie

 

[OB_Jamie]

Dana,
You said, "From high school chemistry you will remember the ideal gas law (PV=NRT)."

I third grade we had a lesson, let me see if I can relay it to you.

We were given a work sheet. It had instructions on it.
1. Draw a big X in the top right hand corner.
2. Draw a Z in the bottom left hand corner
3. Flip the paper over and write your name on the back
4. Put the total of 27.4 and 34.1 in the bottom right corner.
5. Only follow #3 and #5's instructions.


The lesson here is read before you act.

I already said air temp has to do with power output. I don't need you to "teach" me what I already said. Why don't you try to belittle someone else?

Jamie

 

[OB_Bill Koz]

An engine’s power comes from air packed into the cylinders by atmospheric pressure. Basically air by volume is 78 percent nitrogen and 21 percent oxygen. It is the oxygen that promotes fuel combustion, so as density decreases, so does power. But because air temperature and barometric and water-vapor pressures must be considered as variables, the SAE uses a baseline of nominal values for a power/altitude formula.

Let me give you a example. If you had a 100 bhp engine at sea level, it would produce 95 bhp at 1500 ft., a loss of 5 percent. At 3000 ft. it would drop to 91 bhp, a 9 percent loss, at 5000 ft. its down to 85 bhp., and at 7000 ft. its 80 bhp. Approx. 3 percent per 1000 ft. Supercharged and turbocharged engines suffer but to varying degrees.

Engines suffer from more than a loss of power, they have to work harder with less oxygen, which will effect the engine temperature, and the boiling point of the cooling system.

Koz

 

[OB_DanaT]

Jamie,

First I was't belittling you. I was mearly pointing out a techinical flaw in boost your boost pressure theory. You stated:

"Turbos will make the same PSI regardless of altitude or air temperature. A turbo's wastegate is powered by boost thus it will only open when it gets the amount of boost it needs. So, it self corrects for altitude."

My response is while yes they will make the same pressure, the amount of air molecules is not necassarily the only factor that increases the boost. You are putting more work (ie heat) into compressing air at 10000ft than you are at 0 ft. Therefore you have created articifical heat in the air charge that to most people when they see 10psi of boost at 10000ft they assume it is the same as 10psi at 0 ft. This just isn't so.

-Dana

 

[Bruce Bohannon]

Yes, we both are saying density makes a huge difference. The question is - does FI compensate for it more than a CV Carb can / does. This is a relative question.

I think the first answer is - yes, FI can compensate more for external conditions.
Agree?

For one, I need more seat time! Posing questions in my spare time.

 

[OB_GASABUSA]

Earlier somebody mentioned that dragstrips compensate for altitude.What other compensation factors are used and how do they calculate the differences?Does anybody out there know?Is it like the Dyno where you use a number of factors and end up with DIN OR SAE etc?
Gasabusa

 

[OB_CanuckBusa]

Koz, thanks for the formula. Looks like I'm down on power 10%. That could explain why my front wheel is sometimes reluctant to leave earth (though on a cool night it comes up easier).

 

[OB_GSXRTURBO1]

Gasabusa, the times are the actual times, but there is a correction factor that CAN be applied to the times to equate perfect sea level conditions. I think it's a bunch of crap when magazines use these correction factors. Just tell me where the vehicle ran (elevation) at what the temp and baro were, I'll then draw my own conclusions. Besides, I think sometimes the numbers are "adjusted" to compete with other publications. Bottom line: do what you want with your timeslip, but the numbers on the timeslip are "real", not corrected. The correction part is up to you.