Different Octane, Different Engine Tone

T

ToNeZ90

Member
:
2010 Mazda 3 Base Sedan
Hey boys and girls!

Here is my little story:

About 5 weeks ago i started to use 91 octane instead of 87 since the car is breaking 90k miles. in week 1 of filling her up, the car was idleing a little funny, but i thought nothing of it.

Week 2 i filled her up with 91 again and the idling improved.

In week 3, i filled the tank with 91 octane like i planned. about half way into the 3rd week i noticed that the engine was sounding a little unusual. The tone of the engine at 3k rpm was sounding deeper and more agressive. (of course no performance increase) I told my friend to listen to the engine tone (without telling him what to look for) and he told me "the engine sounded more throaty".

I switched back to 87 octane last week (4th week) and the sound went away.

NOW i added 91 octane this week (week 5) and the car is acting a little weird at idle again. It's like the engine is getting used to the octane.

Anyone else ever experience this? I just thought it was a little funny.
 
Well the higher octane is going to have a more efficient burn during combustion so the more aggressive sound makes sense, and yes Ive heard a few times that switching between octanes will "confuse" (for lack of a better word) the engine before it acclimates to the new octane.
 
engine needs to "calibrate" to the new octane? is that better? lol bigger word(2thumbs)
 
Haha yea something like that, im sure someone out there has a technical explanation but you understand the gist lol.
Also Ive never heard of upgrading octane as a no-no but my mom used to own a Mitsu Galant (GTX? I think? Not sure) and the owners manual said that using anything less than premium fuel would significantly reduce the life of the engine. So dif. engine obv perform better with certain octane fuels.
 
Sleezer1533 said:
Haha yea something like that, im sure someone out there has a technical explanation but you understand the gist lol.
Also Ive never heard of upgrading octane as a no-no but my mom used to own a Mitsu Galant (GTX? I think? Not sure) and the owners manual said that using anything less than premium fuel would significantly reduce the life of the engine. So dif. engine obv perform better with certain octane fuels.
Click to expand...

see that engine needs 91 or 93 octane. ours is just 87 (not turbo'ed). thats why it caught me soo off guard. it makes sense though. I knew it couldnt be a placebo effect, since it took so long to notice
 
Iv'e mentioned in the past that when I use 93 non-ethenol fuel, the engine seems a bit louder & the car seems to accelerate slightly better. Also get a couple of MPG better. W/ our lower compression ratio, it shouldn't make a difference, but the above explanation makes sense.
 
Sleezer1533 said:
Well the higher octane is going to have a more efficient burn
Click to expand...

Wrong!

ToNeZ90 said:
engine needs to "calibrate" to the new octane? is that better? lol bigger word(2thumbs)
Click to expand...

The P5 ECU is "calibrated" for 87 octane. It will not "search", for lack of a better term, for a more advanced spark angle in hopes that a higher octane fuel is used.
 
I am guessing that the engine is advancing your timing a bit once it adjusts to the higher octane. When you put regular in the knock sensor tells the engine that the timing is to far advanced so it pulls it back a bit. That could explain the noise and acceleration difference. I haven't data logged any of this so it is just a theory. The ECU will pull timing due to detonation so it may be possible that it periodically tries to advance it some to keep the engine as efficient as possible.......or not.......
 
Last edited:
kcbhiw said:
Wrong!



The P5 ECU is "calibrated" for 87 octane. It will not "search", for lack of a better term, for a more advanced spark angle in hopes that a higher octane fuel is used.
Click to expand...

a little more than just the word "wrong" would be appreciated. like an explination ;)
 
the ONLY thing a higher octane rating does is prevent premature ignition from compression or "hot-spots". fyi: diesel engines burn the fuel from compression, not spark. gas engines dont want to ignite from compression, so you have to add octane to prevent it.

the stock pro5/es ecu is turned for 87. anything higher than that is throwing away money, UNLESS you are getting ping from some other conditon. (like dirty cylinders, or the worng spark plugs)
 
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