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Join Date: Jan 2011
Location: Smack in the middle of Illinois
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When I was a kid, I had the misconception that 'more fuel meant more power'. I could not buy bigger jets for my Sears mo-ped, so I drilled the jets with the smallest drills in my dad's drill index. The engine would start, but it had no power and would not run above half throttle. This was my first lesson in carburation and air/fuel ratios. My dad had to buy a new carb for me. $35 isn't much today, but in 1963 it would buy 150 gallons of gasoline.
As my education in engines progressed, I learned that more fuel was not more power, but just the opposite - more air meant more power! Power comes from the heat and speed of the burn. Low octane fuels burn faster and the heat produced is higher, so the lower the octane the more power the engine would produce, but too much heat will damage the engine and too fast of a burn will damage the engine also, but it takes longer.
High octane fuels were formulated for long stroke high compression engines. Ideally the fuel is ignited by the spark plug, the expanding gas pushes against the piston and the flame goes out when the piston reaches the end of it's stroke. If the fuel ignites before the spark (pre-ignition), it may be caused by too low of an octane fuel, something in the combustion chamber that is hot enough to ignite the fuel vapor (glowing carbon, a hot electrode on a spark plug or a sharp, slightly jagged edge at the top of the cylinder) or a too lean mixture coupled with high compression.
An over rich mixture ignites easier when the engine is cold because only the vapor burns. When fuel is cold, it doesn't vaporize as easily as it does when it is warm - therefore a cold engine needs a richer mixture, but a warm engine performs better with a leaner mixture.
The power generated by a fuel is dependent upon the BTU's of the fuel in question. Ethanol has a higher octane (burns more slowly) than gasoline and when added to gasoline, will raise the overall octane rating of the gasoline it is added to, but alcohol does not vaporize as readily as gasoline and therefore is harder to ignite resulting in a lower temperature flame and less BTU's per pound. Since you have to burn more alcohol to get the same amount of BTU's, fuel consumption increases (less miles per gallon) as the % of alcohol increases so does consumption and mileage decreases. E100 has 76,100 Btu per gallon, compared to 114,000 Btu per gallon of gasoline, therefore it takes approximately 1/3 more pure ethanol power a vehicle the same distance as gasoline.
Both the 2003 & 2007 TRX250EX, for all intents and purposes, are the same vehicle. That being said; take 10 2003 or 2007 models and put the same driver on each one and test them. There is always one fastest and one slowest vehicle of the lot. Attribute that to manufacturing variations of assembly.