riding at altitude - carburetor or fuel injector?
riding at altitude - carburetor or fuel injector?
I've heard that fuel injected (FI) bikes perform much better at altitude, is that true? I live at 5400', and plan to commute at that altitude when it's above freezing. On the weekends I plan to ride at 5000' - 10000', sometimes higher. Will I be happy with a carbureted bike? How is the throttle response at altitude? Does it vary a lot from model to model? My local dealership says FI is the way to go, of course they could be biased. Should I pay more and buy a FI bike, or save money and get one with carbs? What are some of the pros/cons for each?
Last edited by CO_Tim on Mon Aug 21, 2006 6:00 pm, edited 1 time in total.
All bikes will be down on HP when running at those altitudes.
I'm going to cut and paste something I wrote in another thread, with minor edits...
FI is better because it is more likely to hold the proper fuel:air mixture over a range of altitudes. However, HP decreases by altitude even with FI... because the limit on HP is based on the density of the air going into the engine. The density varies based on altitude, air temp, humidity, and a few other factors... if you are at 5000' but it the air temp is 72(f), the "effective" altitude is higher than if the temp was 49(f). In aviation this difference between real and effective altitudes is encapsulated in a concept called "Density Altitude" It is critically important to pilots because it impacts not only engine HP but how well the wings lift.
If you were riding at 5000', and the air temp was 80 degrees and humid all that adds up to a density altitude (the altitude your engine thinks it is) of close to 7800 feet! And, since you lose HP as your density altitude goes up, even a fuel injected engine would only put out about 80% of the sea level HP.
The reason that is true is that the air at that altitude/temperature/humidity is about 79% as dense as "standard" air.
That's point one... now we get to the mixture being off. FI uses a computer and one or more sensors (exhaust gas, intake pressure, mass air flow, etc) to determine the amount of air flowing into the engine and add the right amount of fuel. Feedback from the sensors tunes the fuel flow second by second for a near-perfect mixture. Carbs on the other hand are set up to mix in a certain amount of fuel per unit of air... but they do this using air pressure differences in the carb itself. If the density of air flowing through the carb change the mixture changes and there is typcially no feedback to correct the mixture. I say typically because there are many carbs that do compensate for altitude... aircraft carbs typically have a pilot-operated mixture control which the pilot uses to tune the carb as the plane gains altitude. Other carbs use diaphrams to sense the altitude and automatically compensate (VW used this on some old bugs, some aircraft use them, as well as a bunch of engines that are expected to change altitude frequently). There were even some carb setups that used oxygen sensors (just like fuel injection) and electronic controls to change the mixture.
However, cheap simple carbs don't have this. They are set up (commonly called "jetted") to provide a best mixture at a certain altitude. If you go up they become rich. If you go down they become lean. Most engines are set up to run right at sea level... but if you know you will be running at a certain altitude rainge, you can jet the carbs for that altitude. At least some motorcycle makers (Kawi is the one I know of) have "high altitude jetting" recommended for bikes that live at or near 5000'.
So how much impact does the mixture have?
Ideal mixture is around 14.7:1 air:fuel mixture. In other words, for every 14.7 pounds of air one pound (sixth of a gallon roughly) of gas is mixed in. That's what your carb is set up for (approximately). Now, this is simplistic... but if we assume the carb is creating an ideal mixture at sea level but is running in 80% density at our actual altitude, the mixture is going to be roughly 11.7:1 (this is wrong, but it isn't as wrong as it could be). That means there is too much fuel for the amount of oxygen... net result is unclean combustion, carbon deposits, reduced combustion temperatures, inefficiency, and HP loss. If the mixture is too rich the engine will start to stumble and run rough. If it is even richer it will stop running altogether.
When does an engine stop running right because of too much fuel? As it works out, 8:1 air:fuel is about the richest mixture that will fire in most engines. 12.8:1 is considered a "reasonable" mixture... good enough to intentionally choose in many cases. Not ideal but no short term harm. Frankly I'm not sure off the top of my head what the percentage loss as the mixture goes up but it's not huge until the engine starts running rough.
What's all that mean? It means that most of the loss you are seeing is the air density not the bad mixture. For a 100RWHP FI or carbed bike set up for the altitude bike that means you've got maybe 79-80HP available at 5000' on a warm day. Replace the FI wth carbs that are set up for sea level and you've got maybe 70-75HP.
FI is an advantage (especially if you want optimum fuel economy while crossing mountains but still need to operate at sea level) but it isn't the huge advantage compared to just having more HP to start with.
I'm going to cut and paste something I wrote in another thread, with minor edits...
FI is better because it is more likely to hold the proper fuel:air mixture over a range of altitudes. However, HP decreases by altitude even with FI... because the limit on HP is based on the density of the air going into the engine. The density varies based on altitude, air temp, humidity, and a few other factors... if you are at 5000' but it the air temp is 72(f), the "effective" altitude is higher than if the temp was 49(f). In aviation this difference between real and effective altitudes is encapsulated in a concept called "Density Altitude" It is critically important to pilots because it impacts not only engine HP but how well the wings lift.
If you were riding at 5000', and the air temp was 80 degrees and humid all that adds up to a density altitude (the altitude your engine thinks it is) of close to 7800 feet! And, since you lose HP as your density altitude goes up, even a fuel injected engine would only put out about 80% of the sea level HP.
The reason that is true is that the air at that altitude/temperature/humidity is about 79% as dense as "standard" air.
That's point one... now we get to the mixture being off. FI uses a computer and one or more sensors (exhaust gas, intake pressure, mass air flow, etc) to determine the amount of air flowing into the engine and add the right amount of fuel. Feedback from the sensors tunes the fuel flow second by second for a near-perfect mixture. Carbs on the other hand are set up to mix in a certain amount of fuel per unit of air... but they do this using air pressure differences in the carb itself. If the density of air flowing through the carb change the mixture changes and there is typcially no feedback to correct the mixture. I say typically because there are many carbs that do compensate for altitude... aircraft carbs typically have a pilot-operated mixture control which the pilot uses to tune the carb as the plane gains altitude. Other carbs use diaphrams to sense the altitude and automatically compensate (VW used this on some old bugs, some aircraft use them, as well as a bunch of engines that are expected to change altitude frequently). There were even some carb setups that used oxygen sensors (just like fuel injection) and electronic controls to change the mixture.
However, cheap simple carbs don't have this. They are set up (commonly called "jetted") to provide a best mixture at a certain altitude. If you go up they become rich. If you go down they become lean. Most engines are set up to run right at sea level... but if you know you will be running at a certain altitude rainge, you can jet the carbs for that altitude. At least some motorcycle makers (Kawi is the one I know of) have "high altitude jetting" recommended for bikes that live at or near 5000'.
So how much impact does the mixture have?
Ideal mixture is around 14.7:1 air:fuel mixture. In other words, for every 14.7 pounds of air one pound (sixth of a gallon roughly) of gas is mixed in. That's what your carb is set up for (approximately). Now, this is simplistic... but if we assume the carb is creating an ideal mixture at sea level but is running in 80% density at our actual altitude, the mixture is going to be roughly 11.7:1 (this is wrong, but it isn't as wrong as it could be). That means there is too much fuel for the amount of oxygen... net result is unclean combustion, carbon deposits, reduced combustion temperatures, inefficiency, and HP loss. If the mixture is too rich the engine will start to stumble and run rough. If it is even richer it will stop running altogether.
When does an engine stop running right because of too much fuel? As it works out, 8:1 air:fuel is about the richest mixture that will fire in most engines. 12.8:1 is considered a "reasonable" mixture... good enough to intentionally choose in many cases. Not ideal but no short term harm. Frankly I'm not sure off the top of my head what the percentage loss as the mixture goes up but it's not huge until the engine starts running rough.
What's all that mean? It means that most of the loss you are seeing is the air density not the bad mixture. For a 100RWHP FI or carbed bike set up for the altitude bike that means you've got maybe 79-80HP available at 5000' on a warm day. Replace the FI wth carbs that are set up for sea level and you've got maybe 70-75HP.
FI is an advantage (especially if you want optimum fuel economy while crossing mountains but still need to operate at sea level) but it isn't the huge advantage compared to just having more HP to start with.
Ride it like you think owning it matters.
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jmillheiser
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FI is the way to go up here. you will still lose some power, but your bike wont run super rich like a carbed bike will.
my bandit has carbs and it does not like our 10000' passes.
nice to see another TMW member in this part of the world.
so what style of bike are you looking for? Is this going to be your first bike?
my bandit has carbs and it does not like our 10000' passes.
nice to see another TMW member in this part of the world.
so what style of bike are you looking for? Is this going to be your first bike?
Wow, thanks for the thorough write-up dieziege! As an engineer I knew about air density, but definitely learned a lot.
Yep, first bike. I'm debating whether to buy a bike that I can keep for a while, vs a starter bike that I would sell next spring. For a "keeper" my top choices are sport tourers like 2003+ Suzuki SV650 or 1997+ BMW F650CS or ST. 2003 is first year the SV650 came with FI. I plan to add heated grips and side cases so I can use it in 3+ seasons. If I just get a starter then I want a 250-500cc standard or sport bike that I can sell very easily and without losing much money. In CO what models hold their value the best and are in high demand?jmillheiser wrote: so what style of bike are you looking for? Is this going to be your first bike?
Carb's up high
Great explaination. Making a trip out through Yellowstone and BearTooth Pass. Five FI Haleys and one Star 1700 carbed bike. Anyone have any experience with the 1700?
Do you think he will be able to make the mountains? Hope so, it will sure be a major problem if he can't run at those elevations.
Do you think he will be able to make the mountains? Hope so, it will sure be a major problem if he can't run at those elevations.
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High_Side
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Re: Carb's up high
I took my VTR through there a couple of years ago, and it ran like absolute junk at 11,000' . 48mm carbs are challenged by big changes...sportshot wrote:Great explaination. Making a trip out through Yellowstone and BearTooth Pass. Five FI Haleys and one Star 1700 carbed bike. Anyone have any experience with the 1700?
Do you think he will be able to make the mountains? Hope so, it will sure be a major problem if he can't run at those elevations.
While it ran poorly (my buddies SV650 ran great), it is really not that much of a concern. It's not like your bike won't run at all: it will backfire, idle poorly, and have a lack of power but it IS just one mountain pass. No big deal.
got an SV
I ended up getting a carbureted 2001 SV650, because it was the cheapest reliable bike that put a big grin on my face. The 2003 SV bikes with FI are more than I wanted to spend. And since I commute on it most of my miles are at 5000', no problems after 500 miles. I'll be curious to see how it performs at 11000'.
2001 Suzuki SV650 (red)