Truth about running higher octane than require

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When discussions come up about running gasoline that's higher octane than what's recommended in the owners' manual, it seems like some people are of the opinion that it's fine, but some say it can cause problems. What's the real scoop on this when it comes to late model cars with EFI, knock sensors, and computer controlled spark timing with coil-on-plug ignition? My car's manual recommends 87 octane, but it's got a relatively small engine with a 10.5:1 compression ratio. I'm curious as to whether I'd see any benefits in power or smoothness running higher octane for a while, but I don't want to experiment too much unless I know it's safe.
 
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Running a higher octane than is required should have no effect or positive effect depending on your computer setup and whether your engine is equipped with a Knock sensor to detect engine knocking and tell the ECU to retard timing based on knock and advance the timing to a certain degree if there is a lack of knocking (using premium). Running a lower octane than is required should have drastic effects on mileage as the ECU and engine are generally tailored for specific octanes to achieve the best performance/mileage but in most cases running a lower octane should just result in mileage loss as the ECU retards the timing to make up for the knocking caused by pre-ignition due to the lower octane. Some cars which have poorly written logic/code IMO will knock like crazy even though they shouldn't, when using a lower octane than is required because the ECU isn't compensating at all or isn't compensating enough for the lower octane and this I think is just poor programming on the MFG's part or their "we told them to use XX octane so it's not our problem" attitude/arrogance. ;\)
 
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If it has a knock sensor, it MAY add a little bit of timing. Since they started putting in knock sensors it allows them to be fairly aggressive with their timing curves for better power numbers. So in theory, when you run a higher octane it may add a bit. Otherwise, no issue except a lighter wallet.
 
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In the manual of the Camry, it says to run 87 octane but in parentheses it says the research octane was 91. So maybe a higher octane may be advantageous if the manufacturer programs the ecu to take advantage of it like Onmo says.
 
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 Originally Posted By: asiancivicmaniac
In the manual of the Camry, it says to run 87 octane but in parentheses it says the research octane was 91. So maybe a higher octane may be advantageous if the manufacturer programs the ecu to take advantage of it like Onmo says.
87 AKI = 91 RON
 
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Also, there is some evidence that running an octane higher than required can lead to exhaust valve damage over time due to the slightly slower burn characteristics of the higher octane fuel. Some car makers, Saturn and Buick I believe, have warnings about this in the owners manual. Generally, you want to use the lowest octane that does not result in pinging for the greatest economy. With newer engines with knock sensors and VVT, higher octane in engines that are designed for it (Nissan VQ40, for example) will give higher power and AOMETIMES better mileage.
 
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 Originally Posted By: crinkles
87 AKI = 91 RON
^^ This. In the US, octane is given as AKI (Anti-Knock Index). "Research Octane", or RON, is used in other parts of the world; the numbers are obtained by a slightly different method and higher than AKI.
 
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I am using 93 octane in my 99SL2 during the summer. At Saturnsfansforums a retired engineer had convincing information that it helped gas mileage since it would help keep the knock sensor from reducing timing. All I know is that the car runs great and gas mileage is where it should be. I would not expect to see benefit with a more powerful motor.
 
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My manual says: Your Mazda will perform best with fuel listed in the table. Regular unleaded fuel 87 [ (R+M)/2 method] or above (91 RON or above) Fuel with a rating lower than 87 octane (91 RON) could cause the emission control system to lose effectiveness. It could also cause engine knocking and serious engine damage. I've run higher than 87 (I think it was 91) for a few tanks and did not notice any improvements in gas mileage or performance. I guess the reason for no performance gain is that I only use credit card at gas stations, therefore my wallet did not get lighter, so no weight savings
 
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Some pumps / gas stations show that 87 Octane has up to E10 and 89 Octane has up to E5 in it...Super or 91 + shows E0.....Ethanol is [censored] for gas mileage. I fill up with 91 or higher and I don't think the extra power is that noticable but I do notice the the extra gas mileage......Now I know it cost more however based on extra price per tank I still make out better with 91 + .
 
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 Originally Posted By: rationull
When discussions come up about running gasoline that's higher octane than what's recommended in the owners' manual, it seems like some people are of the opinion that it's fine, but some say it can cause problems. What's the real scoop on this when it comes to late model cars with EFI, knock sensors, and computer controlled spark timing with coil-on-plug ignition? My car's manual recommends 87 octane, but it's got a relatively small engine with a 10.5:1 compression ratio. I'm curious as to whether I'd see any benefits in power or smoothness running higher octane for a while, but I don't want to experiment too much unless I know it's safe.
I will not hurt a thing. run a few tanks and see if there is more power or improved M.P.Gs. What gives you the idea that it would be unsafe?
 
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If the engine can't or won't lean out the AFR enough or run enough timing advance, you'd end up with slightly more combustion byproducts. Unlikely in a modern car, though. When it comes to gas, my feeling is this: Unless your owner's manual specifically says to avoid a particular type, it will be fine as long as your engine is in good shape.
 
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 Originally Posted By: KrisZ
In Canada, most pumps (that I've seen) just have a sticker that says "Gas contains 10% ethanol",
Not sure if it matters but I usually see the phrase "contains at most 10% ethanol"
 

rationull

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 Originally Posted By: Steve S
What gives you the idea that it would be unsafe?
Comments like (emphasis mine):
 Originally Posted By: d00df00d
If the engine can't or won't lean out the AFR enough or run enough timing advance, you'd end up with slightly more combustion byproducts. Unlikely in a modern car, though.
d00df00d qualified it above though by saying it's unlikely in a modern car, and that was really the question I was looking for an answer to. If I can overcome my laziness, I'll hook up my SG and see if I can notice a difference in spark timing with 87 vs 89 or 91 gas.
 
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Its safe but there will be no measurable benefits except that the premium <i>may</i> contain more detergent.
 
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When I first bought my truck, and gas was around $1.00 a gallon, I used 91 octane. I got better mileage with the higher octane. But since my truck only requires 87, and gas is over $3.00 a gallon, I no longer run it. There seems to be a feature in my trucks computer that will reset the spark curve a little if higher octane fuel is used. On long runs with a heavy load, I can get over 1 mpg better fuel mileage with higher octane fuel. There is a point of diminishing returns if you use higher octane fuel, such as 110 octane aircraft fuel. With 110 octane you will most likely have poor mileage and some increased engine wear.
 
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I believe the btu content is the same ..so there's no power difference. There may be a performance (drivability) "depending". The whole anti-premium thing was a carry over from when only Euros were fuel injected and spec'd premium. The premiums had the detergents and cleanliness standards that regular did not. That was long over history, but the oil companies passively promoted the continued belief. EXXON had to pay big fines and engage in corrective advertising over it. The rating is merely it's resistance to fracturing the smooth flame front flash. You're at peak efficiency somewhere just shy of light knocking. You're managing the ever expanding flame front into an ever decreasing volume well in a narrow +/- range of that event. Once it's lit ..it's lit. What happens less with higher rated fuel is scattered multiple flame fronts that collide with each other. That's the knock you hear under load. There are alterations done to gas for seasonal/regional differences. I'm not well versed on them ..so I don't know about burning valves these days. In the past you could burn valves due to too high an octane since lead was used as the anti-knock compound of choice. Someone using AV gas so they could run 12.5+ pistons in a street car would have a very dirty exhaust ..lead foul plugs ..and whatnot. You were already passing fuel through the cylinder and just catching the "poof" part of the passage through as it got ignited. You were breathing fire ..and it was dirty fire.
 
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My Hyundai runs more efficient on 89 than 87 by 1-1.2mpg. My crackhead dealer service manager gave me the idea saying that 2.0L engine runs better on premium. I didn't believe him but went half-way to see if there was a difference. I've been at it for 60k miles and it does make a difference. The owner's manual uses the "87 octane or higher" terminology as well.
 
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