i posted this in another forum that i frequent, but thought some of you may find some value in this since your 100's "require" 93, but can run on 87. (oh, and as far as my credibility is concerned - i'm a chemical engineer by degree...)
Lesson in Octane
they will burn the same. no assertainable difference in carbon buildup within the same brands. 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 6-8hp 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 17MPG with 93, but only 16MPG with 87. which grade is more economical to use?
for 93 octane:
15 gallons * 17 MPG = 255 miles, 15 gallons * $2.50 = $37.50 to fill up
255 miles / $37.50 = 6.8 miles per dollar -OR- 14.71 cents per mile
now for 87 octane:
15 gallons * 16 MPG = 240 miles, 15 gallons * $2.30 = $34.50 to fill up
240 miles / $34.50 = 6.96 miles per dollar -OR- 14.38 cents per mile
add that up for somebody who drives 20k miles per year, and the difference is an whopping $66 per YEAR...
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.
as far as Mid-Grades go. it's really a waste.
ok class. back to work...
Lesson in Octane
they will burn the same. no assertainable difference in carbon buildup within the same brands. 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 6-8hp 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 17MPG with 93, but only 16MPG with 87. which grade is more economical to use?
for 93 octane:
15 gallons * 17 MPG = 255 miles, 15 gallons * $2.50 = $37.50 to fill up
255 miles / $37.50 = 6.8 miles per dollar -OR- 14.71 cents per mile
now for 87 octane:
15 gallons * 16 MPG = 240 miles, 15 gallons * $2.30 = $34.50 to fill up
240 miles / $34.50 = 6.96 miles per dollar -OR- 14.38 cents per mile
add that up for somebody who drives 20k miles per year, and the difference is an whopping $66 per YEAR...
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.
as far as Mid-Grades go. it's really a waste.
ok class. back to work...
Last edited:
yup. got me there! 
