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#4228183 - 10/18/16 08:21 AM Mazda Skyactive octane/mpg
NateDN10 Offline


Registered: 01/10/10
Posts: 1735
Loc: Rochester, NY
This summer I got a new car, a 2016 Mazda3, stickshift, 2.0L. In the US the owner's manual recommends 87 octane and the engine has a 13:1 compression ratio, but in other countries it recommends higher octane and has a 14:1 compression ratio. I was interested in whether higher octane would have any benefit in the US version of the engine.

I didn't do a super scientific test, just did 2 fillups of 87 octane followed by 2 of 93, then 2 more of 87 and 2 more of 93.
Here's the raw data:
Code:
fuelup_date	model	  miles	 gallons mpg	octane
10/17/2016	Mazda 3	  309.9	 8.295	 37.36	93
10/11/2016	Mazda 3	  306.6	 7.672	 39.96	93
9/22/2016	Mazda 3	  344.1	 7.846	 43.86	87
9/9/2016	Mazda 3	  334.6	 9.286	 36.03	87
8/29/2016	Mazda 3	  362.1	 7.806	 46.39	93
8/23/2016	Mazda 3	  362.7	 9.105	 39.84	93
7/28/2016	Mazda 3	  331.4	 9.206	 36	87
7/17/2016	Mazda 3	  302.7	 8.315	 36.4	87


And here are the statistics. I tried to do a box-and-whisker plot, but I can't spare the time to figure out how to make Excel give me what I want!
Code:
	87 octane mpg	93 octane mpg
median	36.215	        39.9
q1	36.0225	        39.22
min	36	        37.36
max	43.86	        46.39
q3	38.265	        41.5675


So it seems I get a little better mpg statistically with 93 octane, but it's certainly not worth the extra cost!
_________________________
2016 Mazda3 - 15,000 mi.
2011 Toyota Sienna - 91,000 mi.

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#4228196 - 10/18/16 08:36 AM Re: Mazda Skyactive octane/mpg [Re: NateDN10]
mazdamonky Offline


Registered: 12/08/15
Posts: 556
Loc: Arkansas
I think this has been discussed before on Mazda forums and tends to come to a similar conclusion. Though a lot of them decide to either run midgrade or 93 because they claim it seems to have a little better acceleration. In theory this makes since as it is a high compression engine and higher octane fuel would help substantially with keeping this engine from pinging which would let the engine run at full power more often.
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#4228204 - 10/18/16 08:47 AM Re: Mazda Skyactive octane/mpg [Re: NateDN10]
ARCOgraphite Offline


Registered: 05/17/09
Posts: 11969
Loc: N.H, U.S.A.
Its not all or nothing with octane. Its finding the sweet spot for optimum combustion timing. And sometime you might optimize a low rpm luggability to the detriment of high rpm. On my car with lowly 9.X comp ratio, it usually wants a couple to 3 gallons of 93 on top of the 87 tankfull in "dry" weather. In High humidity it does fine on swill.
Anymore octane and it gets sluggish. Also it appears to take more run time than you would expect to write over LT fuel trims and timing maps. I still am disappointed with typicall OBDII engine performance and mapping and snowballing/ detuning. Lambda control with safeguards aint what it could be. MAybe it is in UHP cars. I hear the monster corvette still has problems due to failsafe nannies where the engine choked at (possibly) bad fuel and would not build advertised boost/power. Think the was RodentRack?
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#4228209 - 10/18/16 08:51 AM Re: Mazda Skyactive octane/mpg [Re: NateDN10]
Bottom_Feeder Offline


Registered: 12/07/08
Posts: 3010
Loc: Occupied Virginia
Not sure where or why you're making a connection between octane and fuel economy. Higher octane is to fight pinging and pre-detonation, not for increasing fuel mileage although that may be the end result if the engine runs better on it (even if the manual says it doesn't 'require' it).

Running 87 octane in a 13:1 engine just does not seem right to me, but it's the 21st century so who knows.

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#4228214 - 10/18/16 09:01 AM Re: Mazda Skyactive octane/mpg [Re: Bottom_Feeder]
NateDN10 Offline


Registered: 01/10/10
Posts: 1735
Loc: Rochester, NY
Originally Posted By: Bottom_Feeder
Not sure where or why you're making a connection between octane and fuel economy. Higher octane is to fight pinging and pre-detonation, not for increasing fuel mileage although that may be the end result if the engine runs better on it (even if the manual says it doesn't 'require' it).

Running 87 octane in a 13:1 engine just does not seem right to me, but it's the 21st century so who knows.

The idea is that the engine may have to retard timing to get pinging in check, and retarded timing reduces efficiency.
_________________________
2016 Mazda3 - 15,000 mi.
2011 Toyota Sienna - 91,000 mi.

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#4228241 - 10/18/16 09:29 AM Re: Mazda Skyactive octane/mpg [Re: NateDN10]
Rand Offline


Registered: 08/20/03
Posts: 12641
Loc: NE,Ohio
The problem is gas mileage is dependent on conditions as well.

I always get better gas mileage in fall and spring.. or even with certain weather conditions.

Last night driving to Grove city I got [email protected] on the way there.

coming home I got [email protected]

Now If I had filled up @grove city would I say the gas did it?
but I didnt fill up it was merely atmospheric ie headwind.

Throw in A/C use traffic.. even stopping at a drivethrough.. and there is no direct comparison. In my Case it was a straight 90mile drive with no A/C use.

I do feel there might be a slight bump in MPG with 93 on my vehicle as well.. but its definitely not worth the 25% price difference for premium here
2.09 vs 2.59

Now If I had a turbo that is almost guaranteed to run better on 93 vs 87.

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#4228242 - 10/18/16 09:33 AM Re: Mazda Skyactive octane/mpg [Re: NateDN10]
Bottom_Feeder Offline


Registered: 12/07/08
Posts: 3010
Loc: Occupied Virginia
Originally Posted By: NateDN10
Originally Posted By: Bottom_Feeder
Not sure where or why you're making a connection between octane and fuel economy. Higher octane is to fight pinging and pre-detonation, not for increasing fuel mileage although that may be the end result if the engine runs better on it (even if the manual says it doesn't 'require' it).

Running 87 octane in a 13:1 engine just does not seem right to me, but it's the 21st century so who knows.

The idea is that the engine may have to retard timing to get pinging in check, and retarded timing reduces efficiency.

I get that, hence my '... if the engine runs better on it' statement.

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#4228273 - 10/18/16 10:21 AM Re: Mazda Skyactive octane/mpg [Re: NateDN10]
Miller88 Offline


Registered: 04/09/08
Posts: 12703
Loc: Onondaga County
My parents GTDI Escape gets enough of a mileage increase going from 87 to 91 that the 91 ends up being something like $.50 cheaper per 1000 miles.

We calculated that a few years back when gas prices were higher and there was a larger price difference between 91 and 87. They went from 23 to 26/27 with it in daily commuting.
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#4228295 - 10/18/16 10:39 AM Re: Mazda Skyactive octane/mpg [Re: Bottom_Feeder]
Tman220 Offline


Registered: 09/20/16
Posts: 144
Loc: MN
Originally Posted By: Bottom_Feeder
Originally Posted By: NateDN10
Originally Posted By: Bottom_Feeder
Not sure where or why you're making a connection between octane and fuel economy. Higher octane is to fight pinging and pre-detonation, not for increasing fuel mileage although that may be the end result if the engine runs better on it (even if the manual says it doesn't 'require' it).

Running 87 octane in a 13:1 engine just does not seem right to me, but it's the 21st century so who knows.

The idea is that the engine may have to retard timing to get pinging in check, and retarded timing reduces efficiency.

I get that, hence my '... if the engine runs better on it' statement.


So what is it you do not understand? or were you just making a statement?

I would say most cars these days with anything approaching 10:1 compression and above would benefit mileage-wise and power by increasing the octane.

This is due to the higher efficiency attained by keeping the timing advanced closer to peak efficiency rather than the point where the knock sensor gets angry. (as has been pointed out already) I have proven this with my own vehicles as well.


Edited by Tman220 (10/18/16 10:41 AM)
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#4228310 - 10/18/16 11:04 AM Re: Mazda Skyactive octane/mpg [Re: NateDN10]
DBMaster Offline


Registered: 09/18/12
Posts: 5323
Loc: DFW
Too many uncontrolled variables involved in any fuel economy testing described on BITOG. Also, Mazda states 87 octane is to be used for Skyactiv engines in the U.S. That is why the U.S. version has lower compression than other countries' versions. If the car would "run better" or get better fuel economy on premium gas what would be the drawback of Mazda saying so in the owner's manual? I have seen no convincing evidence that using higher octane improves anything. I ran a few tanks of premium in mine and realized that if there was a difference I would need some sort of precision equipment to tell because I didn't "feel" anything.

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#4228317 - 10/18/16 11:08 AM Re: Mazda Skyactive octane/mpg [Re: Miller88]
Rand Offline


Registered: 08/20/03
Posts: 12641
Loc: NE,Ohio
Originally Posted By: Miller88
My parents GTDI Escape gets enough of a mileage increase going from 87 to 91 that the 91 ends up being something like $.50 cheaper per 1000 miles.

We calculated that a few years back when gas prices were higher and there was a larger price difference between 91 and 87. They went from 23 to 26/27 with it in daily commuting.


Right that is a turbo car. Almost always its worth it to run at least 91 octane in those.

the FA20DIT in some subaru foresters has a 10.6:1 ratio and boost(psi) in the mid-high teens. giving it a very high effective compression ratio. .. it will run "crippled" on 87 but 91+ is really a must.

The FB25 NA has a 10:1 and I can tell with 93 some difference over 87 however my gas mileage would have to go up near 7mpg to cover the cost difference so unless I know I'll be towing in hilly terrain I usually roll 87 octane.

Now back when it was 10cents extra for 89 and another 10cents for 93 (20c total)
I would just run 93 because I hate pinging.

Right now the premium is more 50-60C total for 93 vs 87. or around 25%
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#4228332 - 10/18/16 11:32 AM Re: Mazda Skyactive octane/mpg [Re: NateDN10]
danthaman1980 Offline


Registered: 03/12/10
Posts: 1001
Loc: SE Wisconsin
In my '14 Mazda 6, I get better mileage on premium too.

The high-compression Skyactiv engines pull all sorts of tricks in order to avoid predetonation with lower octane gasoline. One particular trick I heard somewhere is that when you use regular gas, the engine runs slightly rich (maybe only under load) to keep the pistons cooler. Not sure if this is accurate, but given that it has been widely reported that Skyactiv engines have avoided fuel dilution by using premium gasoline, it stands to reason that the reason for the fuel dilution could be that the engine is running rich on 87.
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2003 Chevy Silverado 1500 5.3 LS Quadrasteer ~230,000 miles; M1 0W-30

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#4228346 - 10/18/16 11:53 AM Re: Mazda Skyactive octane/mpg [Re: NateDN10]
09_GXP Offline


Registered: 12/14/10
Posts: 430
Loc: Indianapolis, IN
Here's a boxplot and 2 sample t-test showing that the MPGs with 87 is not statistically less than the MPGs with 93.


image upload

upload foto
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#4228365 - 10/18/16 12:26 PM Re: Mazda Skyactive octane/mpg [Re: danthaman1980]
DBMaster Offline


Registered: 09/18/12
Posts: 5323
Loc: DFW
Originally Posted By: danthaman1980
...it has been widely reported that Skyactiv engines have avoided fuel dilution by using premium gasoline, it stands to reason that the reason for the fuel dilution could be that the engine is running rich on 87.


I have seen it reported by one member on BITOG. Do you have other references to this phenomenon other than badtlc's posts? I ran 30,000 miles on the same oil using microGreen filters and had no signs or symptoms of fuel dilution in the UOA.

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#4228379 - 10/18/16 12:41 PM Re: Mazda Skyactive octane/mpg [Re: 09_GXP]
NateDN10 Offline


Registered: 01/10/10
Posts: 1735
Loc: Rochester, NY
Originally Posted By: 09_GXP
Here's a boxplot and 2 sample t-test showing that the MPGs with 87 is not statistically less than the MPGs with 93.


image upload

upload foto

Thanks!
_________________________
2016 Mazda3 - 15,000 mi.
2011 Toyota Sienna - 91,000 mi.

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