i got gas



on my way to work i noticed that the arco had 87 gas for 1.97/gallon. WOWIEE under 2.00/gallon for gas, would i ever see it again? i wasn't sure... till today!



now as a mechanical engineering student, i took two thermodynamics classes in college, and i learned about the (R + M)/2 equation (R stands for Research Octane Number , and M stands for Motor Octane Number) that determines the octane of gasoline (it's on the yellow sticker that says (87,89, or 91) the octane rating on the pump at your local gas station.

my professor (Dr. Samuelson) was the guy who they asked to come up with the equation. (yea he came up with that easy equation) but how you get R and M is the hard part i swear!!! ok ok not impressive. regardless, i've concluded that gasoline octane rating does not affect my car, only my pocket. and i've concluded that it's approx $172.00 more a year to run premium 91 gas. that's based on the assumption that there's a $0.16 difference between 87 and 91 and that i get an avg of 23 mpg. (btw i'm going to get gas today at lunchtime)



according to handymanUSA there is no difference between the two grades of gas for a normal car. i've read tons of posts on different msg boards as well, but those are contradicting, the problem is that nobody references anything when posting to those sites. i can't believe what some people say on msg boards simply because their computer name is 97mustangman. this person could be a 16 year old girl who plays with barbies, or a seasoned car mechanic. who knows?



what i do know is what's going on here. and i'm breaking it down easy style:



-the history of octane rating goes back to AKI (Anti Knock Index). what the heck does Knock mean? it's when fuel explodes instead of burns and ends up pushing the pistons (the parts that move in the engine) in the wrong direction, and that's bad for your engine. so the AKI is the number used to find out how much resistance that specific gas has to knocking.



-octane is what governs the burning of fuel, more octane, slower burn. most people think more octane means it burns hotter. that's wrong.



-slower burning fuel prevents "engine knock" which sounds like something is knocking inside your engine and wants to come out.



-most cars will not have engine knock with 87 octane gas, and if they do... a tune up will most probably fix it. (this doesn't apply to old cars, 25years or so old)



-high performance cars have a higher compression (turbos, superchargers, or just high performance naturally aspirated engines aka non-turbo)



-the reason it costs more is because it has to go thru more refining than 87 to get the octane in the gas, more work = more cost.



-it's not better for the environment to use 91.



-your car won't know a diff if you change 91 to 87 and back again, you're not locked in if you only put 91 in and then suddenly change to 87.



-lastly, there are tolerances on octanes. Hypothetical example: that means 87 is really 87 +/- 3 so it could be 90 or could be 84, meaning that 87 or 91 could be the same thing depending on the batch you get!



quick questions (FAQ): so then, why does my lexus/mercedes say use premium unleaded fuel only? AAAAnd My daddy says I'll get better gas mileage with premium gas, is he wrong?



your car says premium fuel only and coincidentally gasoline companies put their most expencive stuff as "premium". isn't 87 premium? when crude oil is processed it comes out to about 70. then they process it more to get it up to about 90. after that they add stuff to it to make it higher than that. (some fuel is rated well over 100)



better mileage and better for your car seems to be the grey area. one site said approx 3% more power is attained with high octane gas, other sites say that no gas mileage is gained with higher octane. one site said that you'll get MORE gas mileage with 87 cause it burns faster and hotter. (heat equals energy) so that means more gas mileage as long as your car doesn't knock. carbon build up with 87 or 91 was also up in the air. some sites said yes and others said no.



the variable isn't the gas. it's the engine. if you're driving a car that that is not a super machine, and isn't 25 years old, you should most likely be putting 87 octane gas. save the 170.00 a month so you can buy drinks for me or something, that will get you further than higher octane gas.



p.s. i've read and looked and tried to find info about turbocharged cars, with no luck. if anyone has a credible source of information confirming the benefits of 91 gas and turbocharged engines, please feel free to let me know! please.