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Discussion Starter #1
Hi,

Has anybody used these bikes with low grade fuel? I understand the engine runs at quite a high compression ratio. Will this cause problems if the bike is used with low grade/low octane fuel?

Also, at what altitude might I start to find problems with the bike?

Cheers,

Gabriel
 

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The vn750 has a 10.3:1 compession ratio, and is designed to run on regular grade, 87 octane gas. This seems to me, to be quite high compression for regular gas, but I have seen reports here of the vn750 running worse on higher octane mid grade or premium gas.

What grade/octane level do you expect to find on your adventure in Asia? Lower octane will cause pre-ignition (pinging) at some point.

What altitude will you reach too? If your bike is tuned to run near sea level, the thinner air at altitude will have it running rich I think.

Sorry I can`t offer any any more specific answers.
 

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I ride up to 4,300 feet in my area. The bike runs and starts ok, but there is a loss of power. Higher octane may help?

DT
 

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I ride up to 4,300 feet in my area. The bike runs and starts ok, but there is a loss of power. Higher octane may help?

DT
Higher octane would make it worse, because higher octane gas is harder to ignite.

Use regular gas at all times. I think once you get above 7000 ft you might have some issues.

Do a google search for " Dial a Jet " ...a system that lets you increase or lower your carbs fuel to air ratio with a turn of a knob.

KM
 

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I ride here 7,200 ft in Mexico city and... uhmmm, how can I put it?, here the gas is really very bad... piss quality bad. Not much running issues, some hesitation going full throttle but not much more so I think you will be ok
 

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Discussion Starter #6
Thanks for the info All. I guess its looking good if I have to use low grade fuel....and if I have to go up high. These will be things I wont be doing often.....and wont be pushing the bike either. Just glad to know the bike can handle it if it needs to.
 

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The low octane and the altitude go together. Since the ambient air pressure is lower at altitude, for a given engine, the combustion chamber pressures are also lower, so pre-detonation is less likely and a lower-octane fuel can be used.
 

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I use 85 octane (regular) and ride at 5280 ft. (Denver) and have no problems.
:motorcycl
 

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I am currently reading Jupiter's travels by Ted Simon...in that book he rides a Triumph 500 with "low compression pistons" that run much smoother than stock and work well for poor fuel encountered in his travel. Any applicability to a Vulcan 750?
 

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Higher octane would make it worse, because higher octane gas is harder to ignite.
Use regular gas at all times. I think once you get above 7000 ft you might have some issues.
Do a google search for " Dial a Jet " ...a system that lets you increase or lower your carbs fuel to air ratio with a turn of a knob.

KM
Actually, higher octane has a longer burn time. The loss of power may be compensated with 92+ octane. Same principle as piston airplane engines (108+ fuel)

DT
 

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Actually, higher octane has a longer burn time. The loss of power may be compensated with 92+ octane. Same principle as piston airplane engines (108+ fuel)

DT
Not sure how that matters here, but I am not an aviation engineer. My thinking would be as you gain altitude , the fuel mixture would become richer, and a rich mixture is more apt to burn if the fuel is more cumbustable. Thus, you would get a better chance of cumbustion with regular gas as opposed to higher octane.

If it "burns longer" you would have more fuel going unburned , which is a problem of a rich mixture already.

Perhaps some one can explain this to me.


KM
 

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Not sure how that matters here, but I am not an aviation engineer. My thinking would be as you gain altitude , the fuel mixture would become richer, and a rich mixture is more apt to burn if the fuel is more cumbustable. Thus, you would get a better chance of cumbustion with regular gas as opposed to higher octane.

If it "burns longer" you would have more fuel going unburned , which is a problem of a rich mixture already.

Perhaps some one can explain this to me.


KM
The are a few things to consider (beyond emissions) when you tune the mixture. The stoichiometric fuel-air mix is 1-14.7, this is where all the hydrocarbon from the fuel burn and mix with the oxygen (nice for emissions and fuel consumption) but max power actually uses a bit more fuel so the mixture sits at about 1-13 or so. Also to consider it's the combustion temperature lean mixtures ( above 1 to 15) produce hotter exhaust gases so if you go very lean you end up burning a hole on the pistons, valves, valve seats, etc. Richer mixtures produce a lower temp exhaust gases but you can only go so rich before you foul a plug and it stops igniting the mixture.
Also the is the fact that for the gas to actually ignite in the combustion chamber it must be almost fully vaporized to get a good burn so again too rich it's not that good
The CV carbs on our bikes balance out fairly good at a good range of altitudes without too much fuzz as a result of their design but still they are limited. Here fuel injection is head and shoulders above our carbs.

As for higher octane fuel, as it was stated before, the number only makes reference to the fuel (in actual air-fuel mix) to resist autoignition due to heat from the compression of said mixture. It's actual power producing properties are basically the same as the lower octane fuel (different octane same manufacturer) as octane rating is modified by additives that are about 5% of the total fuel volume. So on an engine where it's expected to see high compression/high intake temp (turbo charge/supercharged engines/racing engines). they use high octane. Now it's possible to make an engine that takes advantage of this characteristic of the fuel to actually make more power from X amount of displacement but this is in the net result of the fuel + engine design.
There are SAE papers that talk about this and you will find one piece of data that will clear up this. This is the flame front speed, this is a number that helps understand how efficient an engine design is and how all comes together to make power in an engine.

Here's some more reading about it: http://en.wikipedia.org/wiki/Air-fuel_ratio
 
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