Poor Fuel Economy solved!? Maybe

I'm confused by the statement that one would never notice any difference between 91 and 87 octane fuel unless you are running the engine about 4,000 RPM.

Isn't the torque at 2,000 RPM for 91 octane almost three times that of 87 octane? And about double at 3,000 RPM?

How can one not notice more torque?
 
I do recall reading the ratings were higher with premium yes. Does it get a more complete burn as well I assume? Something from the past makes me recall something to that effect which would indicate that there would potentially be less chances or deposits being formed?
I had a DI engine before this one and it certainly ran better on premium but man that was a spendy fill up -F150 5.0l engine.
It would be nice to see some scope comparo photos for actual evidence one way or another over the speculation... would save some bux for sure to use Regular 87 but if Premium reduces deposits (potentially expensive repairs/cleaning?) and gives good power it may be worth the cost?
I would like to not have to care (unlimited funding) but where is the fun in that right?
 
I'm confused by the statement that one would never notice any difference between 91 and 87 octane fuel unless you are running the engine about 4,000 RPM.

Isn't the torque at 2,000 RPM for 91 octane almost three times that of 87 octane? And about double at 3,000 RPM?

How can one not notice more torque?
For actual data rather than “feeling” the thread below shows the difference in acceleration between 87 and 91 with the 2.5 turbo. Again, if you don’t explore the engine above half throttle then you may not see any difference. As far as the NA 2.5, I’d expect very little noticeable difference between 87 and 91, if any.

 
I'm confused by the statement that one would never notice any difference between 91 and 87 octane fuel unless you are running the engine about 4,000 RPM.

Isn't the torque at 2,000 RPM for 91 octane almost three times that of 87 octane? And about double at 3,000 RPM?

How can one not notice more torque?
Read the power curves carefully. Red line is using 87 AKI octane and gray line is using 93 AKI octane on the 2.5T. Both fuels get 310 lb-ft maximum torque @ 2,000 rpm with two lines overlapping up until 4,000 rpm. 87 AKI fuel gets 227 hp maximum horsepower @ 5,000 rpm showing by the red line; whereas 93 AKI fuel gets 250 hp maximum horsepower @ 5,000 rpm showing by the gray line. The overlapping power curves on both horsepower and torque with 2 different octanes up until 4,000 rpm indicate the engine performance will be the same on horsepower and torque up until 4,000 rpm between AKI 87 regular and AKI 93 premium gas.

By releasing this official power curve on the 2.5T, Mazda wants to show “additional high-rpm performance is possible with higher octane”, and “to prevent unnecessary spending on premium fuel, both power ratings are published”.

BA9095A1-09E2-41FC-877B-6B8C0A0D90E3.png
 
If I understand what you are saying, the upper red line shows Torque and the lower red line shows HP. Yes?
 
If I understand what you are saying, the upper red line shows Torque and the lower red line shows HP. Yes?
The upper red / gray lines (@1,000 rpm) shows torque with the scale to the left; and the lower red / gray lines (@1,000 rpm) shows horsepower. Unfortunately Mazda forgot to put an hp scale to the right of the graph like most people do, which may cause the confusion.
 
If I understand what you are saying, the upper red line shows Torque and the lower red line shows HP. Yes?
Correct. The difference in actual performance data (not dyno charts) shows a real increase in performance when running 91 octane versus 87, but it is nowhere near three times the torque. The dyno chart shows the difference before and after a tune, not just a change in octane.
 
Whatever fuel you use, the intake valves see none of it.

Octane is a burn retardant. It makes to fuel burn weaker. Sure, you can advance timing, but in these engines I'd never run anything above 87. They all feel slower with premium Subjective, I know.

Quote myself to correct. "No difference on NA engines"
 
So an Update for everyone. Intake valves were filthy, not quite VW bad, but after 60k they had their fair share of build up. That being said, I cleaned them and saw a bit of an uptick in mileage but it soon fell again.

Checked my ATF fluid and it looked rather dark so I changed that, (I had already changed the diff and transfer case fluid) drove the car 100+ miles yesterday and I was able to get 27mpgs not trying which is a 4mpg increase! going to do another change since I want to get all the old fluid out, but the car shifts much better, and rolls more freely at speed.

So in the end I think after 60k miles and alot of towing I scorched the ATF fluid and all that was needed was a quick drain and fill. Go figure. I replaced with FZ fluid from Mazda, but didn't change the strainer since its not recommended from the manufacturer. LMK what you guys think, but I'm happy to be getting an extra 50 miles per tank especially with increased gas prices.
 
I personally run 91+ on any engine with an effective compression ratio of 10:1 or higher out of habit. I've had high CR engines knock since the change from 92 to 91 in CA. The euro spec 2.5L with 14:0 compression and 91 octane gets 192hp, so not much of a difference with both higher compression and premium fuel. I'm guessing you might get 1-2hp extra by using 91, numbers not worth publishing.
 
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Consider Top Tier gasoline.


There is a lot in there, but here is an excerpt.

Does TOP TIER™ gasoline help in GDI (Gasoline Direct Injection) engines?


Although the technology and mechanical equipment for direct fuel injection differs from carbureted systems, the short answer is that the “keep clean” effect in the two different fuel systems may not differ greatly. The data and information that the sponsoring engine manufacturers have is not available in the public domain, but AAA conducted an independent investigation comparing the minimum EPA detergent levels to TOP TIER™ Detergent Gasoline. The full report is available for free download here. AAA did a great job showing where in the engine you can benefit from gasoline meeting the TOP TIER™ performance standard and even commented on GDI engines (see section 2 of their report on pages 9 and 10; and the sub-section on emissions found in section 5.3 – page 19).
 
Only if you rev the 2.5T above 4,000 rpm.

View attachment 298118

Continued posting of this graph without supplemental explanation only causes confusion because, quite frankly, it is a terrible exercise in graphing for reasons other than omitting the HP scale on the right.

I gather you're saying the lower red line is HP and the upper red line is torque for both fuels up until divergence at around 4000 RPM with the gray lines continuing on for the higher octane and the red lines continuing on for the lower octane If so, Mazda should say so to prevent the first confusion encountered.

Further, at a minimum, Mazda should have changed the color for low octane to a third color at the divergence. Better yet there should be 2 colors prior to divergence and 4 colors after the divergence. Then they could provide an explicit color key at the base of the chart. One color meaning two or four different things prevents that.

Did Mazda provide explanatory verbiage to go along with this graph? If so, you should produce that when posting this chart. If not, you should provide your own. Every time this graph is posted the same confusions ensue.
 
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The “chart” is from a Mazda presentation made by one of their engine designers. In the video he explained the graph in detail. Unfortunately I cannot find that Youtube video, I’ll keep looking.
 
The “chart” is from a Mazda presentation made by one of their engine designers. In the video he explained the graph in detail. Unfortunately I cannot find that Youtube video, I’ll keep looking.
If that video is the one from the second post in the following thread then Mazda pulled it off youtube.


Anyway, evidently graphic design is not something they teach Mazda engine designers.
 
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Continued posting of this graph without supplemental explanation only causes confusion because, quite frankly, it is a terrible exercise in graphing for reasons other than omitting the HP scale on the right.
Mazda does put the comments with this power curve as additional high-rpm performance is possible with higher octane, and to prevent unnecessary spending on premium fuel, both power ratings are published. And you don’t think the intention from this graph by Mazda is you don’t waste your money on premium fuel unless you push the engine over 4,000 rpm for higher performance?

I gather you're saying the lower red line is HP and the upper red line is torque for both fuels up until divergence at around 4000 RPM with the gray lines continuing on for the higher octane and the red lines continuing on for the lower octane If so, Mazda should say so to prevent the first confusion encountered.

Further, at a minimum, Mazda should have changed the color for low octane to a third color at the divergence. Better yet there should be 2 colors prior to divergence and 4 colors after the divergence. Then they could provide an explicit color key at the base of the chart. One color meaning two or four different things prevents that.

Did Mazda provide explanatory verbiage to go along with this graph? If so, you should produce that when posting this chart. If not, you should provide your own. Every time this graph is posted the same confusions ensue.
I believe this power curve graph is obvious at least for me, even though it lacks a second HP scale. The red lines for both torque and hp are for AKI 87 fuel and the gray lines are for AKI 93 fuel starting from 1,000 rpm. Red and gray lines are overlapping under 4,000 rpm because the torque and hp performance are the same between 2 fuels, and red is covering gray, hence all you can see are red lines under 4,000 rpm. The graph also pointed out the peak performance with numbers on both torque and hp by 2 different fuels.
 
To me the graph is very easy to understand. The red line less than 4K rpm for both hp and torque is actually red and grey, meaning that the performance is identical with both octanes. Past 4K the maximum benefit is 23 hp and even less torque, at 5K rpms. Many people might like that extra 23 hp when they give their car the beans, merging into traffic or joy riding.
 
What the graph doesn’t show is any improvement in part-throttle response. Even below 4000 rpm, quick throttle inputs could result in pulled timing if the knock sensors pick up any detonation. Anecdotally, when I filled up with 87 I noticed a softer part-throttle response below 3500 rpm. Ran the tank very low, filled back up with 91 and response improved. Hardly scientific but that’s my experience.
 
And you don’t think the intention from this graph by Mazda is you don’t waste your money on premium fuel unless you push the engine over 4,000 rpm for higher performance?
Clearly I made no comment on Mazda's intention. My point related to presentation in the graph and how to read it. I know what it says and got it one of the precious times you presented it. Have you not observed the confusion that ensues for many looking at it for the first time without an accompanying explanation? That confusion is evident in multiple threads including this one.
Red and gray lines are overlapping under 4,000 rpm....
What a confusing way to explain it for somebody who is encountering it for the first time. since only a red line is visible. What you should say is the red lines indicate identical performance until divergence where the gray lines begin.

My primary point is if the intent is to communicate then the explanation, however you do it, along the lines of your post #24, should accompany the chart the next time you initially present it unless the object is to confuse and then come to the rescue.
 
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The red and gray lines do overlap indicating identical performance. Like you said, the red covers the gray (mostly) until there is a difference in performance. If it weren’t for yrwei sharing this graph over and over again, we’d never be able to squash the “does higher octane yield X?” questions that keep cropping up. Yrwei to the rescue! 😄
 
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