i posted this in another topic
, but thought some of you may find some value in this. (oh, and as far as my credibility is concerned - i'm a chemical engineer by degree...)
they will burn the same. no difference in carbon buildup within the samebrands. however, compare shell or chevron to an off brand and you can and will have more buildup w/ the off brand b/c they tend to have less detergents and poorer quality detergents in them.
now as far as power goes, you probably won't feel the difference. if the engine was meant to run at it's optimum on 93 octane and you put 87 in it, then it will simply retard the timing enough to prevent detonation. in doing so, you loose some power, but typically less than 3 or 4%. that's about 10-12hp and for the everyday driver, you'll never feel the difference. however, you will more than likely have a decrease in your efficiency (MPG's) if you drop from a recommended 93 down to 87 due to the engine retarding the timing. this is more noticeable on automatics than on manual transmission equipped vehicles due to the downshifting at cruising speeds from the loss of the top end power.
for fun, let's walk through a quick & dirty calculation with some assumptions:
15 gallon use of fuel. 93 octane is $2.50 and 87 is $2.30 per gallon. you normally get 27MPG with 93, but only 25MPG with 87. which grade is more economical to use?
for 93 octane:
15 gallons * 27 MPG = 405 miles, 15 gallons * $2.50 = $37.50 to fill up
405 miles / $37.50 = 10.8 miles per dollar -OR- 9.26 cents per mile
now for 87 octane:
15 gallons * 25 MPG = 375 miles, 15 gallons * $2.30 = $34.50 to fill up
375 miles / $34.50 = 10.87 miles per dollar -OR- 9.2 cents per mile
as you can see, if you gain a simple 2 MPG by using 93 octane, then it's a wash in cost even though it's $0.20 per gallon more to purchase. if you get MORE than 2 MPG better efficiency by using the higher octane fuel (or you can get it for LESS THAN $0.20 per gallon more than the 87 octane fuel), then you are acutally MORE EFFICIENT and CHEAPER to use the higher grade and higher cost fuel. this is totally independent of gas prices overall, and totally dependent on the difference in cost between the two grades and the difference in your MPG's between the two.
the reason that they require 93 is due to the high compression ratio of the engine. higher ratios means that during the compression cycle, the piston moves in the upward direction so far that the air inside is compressed. compression causes heat, and that heat can cause premature detonation of the fuel without the spark plug ever firing. this is detonation. the engine has a knock sensor and will detect pre-detonation conditions and will adjust engine timing accordingly to compensate.
also, if you have an older engine, you're more likely to get pinging (detonation). why is that? it's because older engines typically have some carbon buildup on the valves and on the piston head. those carbon deposits can get heated to very high temperatures while the engine is running and will actually glow (think of charcoal in a grill). those glowing embers will then ignite the fuel and cause detonation. using something like BG44K or Seafoam on a regular basis will remove the carbon deposits and therefore will reduce your chances of pinging.
why does 93 not ping when 87 octane will? it's in the octane rating. 93 is "harder to ignite" than 87. it doesn't burn any cleaner, or have more energy than 87 octane does. however, it requires more energy to initiate combustion, and therefore is less prone to detonation during the compression cycle or due to hot embers in the combustion chamber. also, like metioned before, 93 octane allows the engine to run at peak performance by allowing the timing to be as far advanced as possible.
on the contrary, for an engine designed to run on 87 and then you put 93 in it, you are simply wasting your money. the engine will still run the same and you won't notice a bit of difference in power or performance. absolutely ZERO difference. as long as it is well maintained w/ low amounts of carbon deposits that would normally cause detonation and lead to a retarted timing, then using 93 octane won't do anything but make you poorer.
oh, and one last thing... "the Saudis" don't get that extra $0.20 per gallon for the premium fuel. that goes to the refinery who cracked and distilled and blended the fuel and to the pockets of the station owner - all in the US. we don't get one drop of fuel in the US that is "made" outside of our borders.
as far as Mid-Grades go. it's really a waste. most older stations only have two tanks anyway - used to be for leaded and unleaded and now for low and high octane. when you buy mid, you are getting 89 octane, but that's also what you get when you pump the low-grade gas. look at the rating - it's a MINIMUM octane rating system. the two in-ground tanks are for 89 and 93. this is also very typical for lots of newer stations as well.
ok class. back to work...