Poll: Fuel Octane Choice (CX-9 2016+)

What octane do you put in your CX 9?

  • 87

    Votes: 19 57.6%
  • 91

    Votes: 7 21.2%
  • Other

    Votes: 7 21.2%

  • Total voters
    33
The manual says that you want to put at least 87 octane into the tank. 91 is optional if you want the increase in horsepower.

Mazda has stated that there will be no difference in longevity putting 87 or 91, and 91 is only for the horsepower.

The engine has a compression ratio of 10.5:1. For comparable cars, the RDX has a compression ratio of 9.8:1, the x5 has a compression ratio of 10.2:1. Both recommend 91 octane. How is Mazda able to recommend 87? Additionally, back then, weren't all turbo engines supposed to be 91 octane and above?

So what do you guys put in it?
 
There is more to the story than the compression ratio. The mazda skyactiv engines are all high compression ratio engines. The normally aspirated skyactiv engines runs at 13:1 compression ratio. Yes higher compression typically leads to higher temperatures which increases the risk of premature ignition (hence the requirement for higher octane fuel), but mazda has used a few tricks to help manage cylinder temperature such has improved exhaust flow, etc.

When using 87 octane, the engine changes a few of its settings to prevent knocking, these changes results in a lower horsepower output. So really its: 91 octane allows the engine to run at its full potential, 87 octane the engine reduces its output to protect itself from knock.

I don't think there is anything to worry about with using 87 octane in term of longevity. It used to be that all turbo engine where performance engine, tuned to get the most out of the engine. Now turbos are used to design engine for good fuel economy, which is less stringent on horsepower and less prone to heat management challenges (for example, ford ecoboost, honda turbo engines, etc). All of them run on 87.
 
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If you are just puttering around town, you'll be more than fine with 87 and you likely wont notice the drop in power at all. If you look at the power curves for the 2.5T, the engine doesn't "de-tune" itself to prevent knock until around 4000 RPM. If you don't mash the throttle you wont run the engine in this range anyway.

Power Curve can be found here:
Torque Curves for 2.5L Mazda engines

Also note that there isn't an instant switch between 227 and 250 hp the second you fill up with premium. The car doesn't know what button you press at the fuel station... it takes a while for the ECU to learn what the capabilities of the fuel are so it may take a full tank (or more) before you notice the change in performance. YMMV
 
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2010 CX-9 GT
The manual says that you want to put at least 87 octane into the tank. 91 is optional if you want the increase in horsepower.

Mazda has stated that there will be no difference in longevity putting 87 or 91, and 91 is only for the horsepower.

The engine has a compression ratio of 10.5:1. For comparable cars, the RDX has a compression ratio of 9.8:1, the x5 has a compression ratio of 10.2:1. Both recommend 91 octane. How is Mazda able to recommend 87? Additionally, back then, weren't all turbo engines supposed to be 91 octane and above?

So what do you guys put in it?
Prior to the adoption of direct injection, turbo engines typically were around 8:1 compression. In a traditional fuel injected engine, the fuel is injected into the intake manifold behind the intake valve. The fuel evaporates in the intake as it is pulled into the cylinder. With DI, the fuel evaporates in the cylinder, which tends to better cool the fuel/air charge and raises the knock resistance. This allows higher compression. There are other direct injected 4 cylinder engines with even higher compression ratios than the Mazda. I wouldn't get hung up on that.

The Mazda can get away with 87 octane because the engine control unit has a lot of authority to adjust the spark advance on the engine. In general, in order to make power at higher rpm, the spark timing needs to be advanced. However, this raises cylinder pressure and temperature and can cause knocking. The Mazda ECU can pull back the timing enough to not break anything, but this reduces the engine output. Mazda decided that most people wouldn't miss the reduced power at high rpm and tuned the engine accordingly.

If I was driving it, I'd put in 93. If my wife was, I'd put in 87. She never revs the engine past 3000 rpm. I do.
 
Alright - who put the other. Yes, I should have put 93 instead of 91, but overwhelming response is 87 octane thus far.
 

singlemalt_18

My Way IS the Highway
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2018 CX-9 Signature Silver & 2007 CX-7 GT AWD Liq Platinum
Alright - who put the other. Yes, I should have put 93 instead of 91, but overwhelming response is 87 octane thus far.
Count me as the "other" other... 93 almost exclusively with an occasional 91 when refilling on long roadtrips; never 87.

Full disclosure: I still drive my original 2007 CX-7 GT AWD with the original DISI Turbo 4. Throughout the past (nearly)14 years, I almost always did the half 89 / half 93 schtick when I filled up. It doesn't get great mileage compared to new 9s, but the "smart" transmission with that engine make it a 4,000 lb pocket rocket with unbelievable breaking... I still LOVE driving this vehicle! Overall acceleration & breaking response is awesome.

I know longer use it on the long roadtrips as I am willing to give up some adrenaline for the quiet elegance & refinement of my 2018 Sig 9
 
Count me as the "other" other... 93 almost exclusively with an occasional 91 when refilling on long roadtrips; never 87.

Full disclosure: I still drive my original 2007 CX-7 GT AWD with the original DISI Turbo 4. Throughout the past (nearly)14 years, I almost always did the half 89 / half 93 schtick when I filled up. It doesn't get great mileage compared to new 9s, but the "smart" transmission with that engine make it a 4,000 lb pocket rocket with unbelievable breaking... I still LOVE driving this vehicle! Overall acceleration & breaking response is awesome.

I know longer use it on the long roadtrips as I am willing to give up some adrenaline for the quiet elegance & refinement of my 2018 Sig 9
I'm not particularly concerned about performance. I'm more concerned about longevity, which I've heard the higher octanes should provide longer longevity as it is less prone to knock.
 
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Mazda CX-9 Grand Touring 2016
Vertigo, the terminology is 93 or 91 or 87 AKI...anti knock index. Yes, the higher AKI gasolines are, by definition, less prone to knock, but..... The car's control systems, as noted above, are smart. They take care of it for you. High AKI gas isn't needed at high rpms, it just gives more giddy-up when you're at those high revs. I've tried long freeway trips with both 87 and 93. The 93 was a bit smoother, a bit brisker, but not worth the cost to me. You can safely try both and see which is your preference.
 
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2010 CX-9 GT
I'm not particularly concerned about performance. I'm more concerned about longevity, which I've heard the higher octanes should provide longer longevity as it is less prone to knock.
This isn't going to buy you anything at all except a lighter wallet. The ECU will push the engine tune until it senses knock and then back off from there. It constantly adjusts the tune to be right on the threshold of knocking. Of course, you could source 100 octane gas if you really wanted the safety margin as the ECU is only designed to work with 93 AKI maximum.
 
I'm not particularly concerned about performance. I'm more concerned about longevity, which I've heard the higher octanes should provide longer longevity as it is less prone to knock.
Basically, if you put 91 in an engine designed for 87 you can run into knocking issue. If you run an engine designed for 87 with 87 you won't have any issues. This engine was designed with 87 in mind, so it will be fine with 87. In addition you driving habits also factor in to how much knock you may get, if you drive it full throttle, full load and high revs, you are more likely to encounter knock. If you are mostly at half throttle and in the low revs then you are less likely to have knock. There is also a significant difference between high revs at light throttle application (5000 rpm with half throttle) compared to high revs at full throttle.

I believe this engine is fine longevity wise with 87 octane. An additional argument is that Mazda in Canada offers an unlimited mileage warranty on the powertrain, regardless of the type of fuel used. It doesn't mean the engine is fail proof, but it shows they have some confidence that the failure rates won't be very high on 87 fuel. Of course time will tell...
 
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Mazda CX-9 Grand Touring 2016
Just a reminder to readers around the world...The U.S. and Canada measure gasoline's resistance to ignition knock using the average of research octane number (RON) and motor octane number (MON) and call it the Anti Knock Index (AKI). Other parts of the world use just RON and show numbers about 4 or 5 points higher than AKI for the same gasoline.
 
Does anyone know how the ECU determines how to adjust the tuning for different fuels? The Mazda sales people seem confident that you can run 87 all day without any issues, but I'm just worried about relying on a system (such as a knock sensor) that could fail and result in detonation.
 
Does anyone know how the ECU determines how to adjust the tuning for different fuels? The Mazda sales people seem confident that you can run 87 all day without any issues, but I'm just worried about relying on a system (such as a knock sensor) that could fail and result in detonation.
The ECU uses the O2 and knock sensors to adjust the timing of the engine. It is monitoring this constantly while it's running. Every modern vehicle works this way, even the fancy cars that require 91 wont have knock when you fill em with 87. You will be just fine running 87 all day because this engine is designed for it.

Don't waste your time worrying over the small probability of a sensor failing because there are literally hundreds of sensors on the vehicle. If it does fail, the car wont run properly even if it was filled with 91.
 
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Mazda CX-9 Grand Touring 2016
" Every modern vehicle works this way, even the fancy cars that require 91 wont have knock when you fill em with 87. " Even not-so-modern cars. I put over 200,000 miles on my 1996 Volvo turbo 850, most with 87 AKI. I bought 93 for mountain freeway driving for the fun of it. the car ran fine with nothing except normal maintenance all those miles.
 
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