87 or 91+ octane?

[Zandcwhite]

Funny how most questions are answered in that little book!!

And yet we have 40 threads showing a complete lacking of reading comprehension. “Optimal performance“ says it all. No 87 won’t harm the engine, but we spent $50k+ on a hobby vehicle, another $15k+ on mods (being conservative here, probably closer to $25k but I’ll never add it all up), and push this little 2.0t pretty hard off road/on road/ and on road trips spending 10+ hours rolling 85mph. If I was worried about saving a few bucks in gas, I wouldn’t drive a lifted Jeep on 38’s. On the other hand, the 5.38 gears and tuner are clear evidence that I am concerned with optimal performance. Much like the gearing debate that is over played by the massive gearing range in the 8 speed auto, fuel choice comes down to personal preference. If you bought your Jeep to cruise around the mall or the beach and never bury the skinny pedal or push the legal speed limits, you might as well run 87 as you’ll never need or use optimal performance anyway.

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[Chile1]

It’s a utilitarian spartan vehicle so mine only gets 87. I have a 3.6L 6sp MT.

 

[LuvHydro]

87

 

[Pinky Tuscadero]

The manual does not say anything about getting better mpg with higher octane
Currently averaging 25.3 mpg on the last 3 tanks all city all 87 octane
I've asked twice for real performance 1/4 mile tests on the octanes by someone who actually wants to have some real proof but all I see is old man coffee shop or drunk tavern talk with zero actual information so good luck with this mess.

 

[Zandcwhite]

The manual does not say anything about getting better mpg with higher octane
Currently averaging 25.3 mpg on the last 3 tanks all city all 87 octane
I've asked twice for real performance 1/4 mile tests on the octanes by someone who actually wants to have some real proof but all I see is old man coffee shop or drunk tavern talk with zero actual information so good luck with this mess.

I could run it to empty, take the 93 octane tune off, drive around on 87 for a day or 2, test 0-60 and 1/4 mile times, put it all back and prove what I already know by having driven it stock vs the mods, but why would I bother? Do you really expect someone else to put in the time and effort to prove to you what Jeep clearly tells you in the manual? A tazer let's you test 0-60, as does a draggy. If you doubt what Jeep is telling you, test it or just be happy with your less than optimal performance. Mazda actually shows 2 different power ratings for their turbo 4 cylinder and the higher octane fuel equates to 23 extra hp and 10 extra ftlbs. Will you feel a nearly 10% increase in horsepower? If you drive it hard enough you should. If you mostly cruise around like grandma going to church, you never hit the power peak at any octane.

 

[Pinky Tuscadero]

I could run it to empty, take the 93 octane tune off, drive around on 87 for a day or 2, test 0-60 and 1/4 mile times, put it all back and prove what I already know by having driven it stock vs the mods, but why would I bother? Do you really expect someone else to put in the time and effort to prove to you what Jeep clearly tells you in the manual? A tazer let's you test 0-60, as does a draggy. If you doubt what Jeep is telling you, test it or just be happy with your less than optimal performance. Mazda actually shows 2 different power ratings for their turbo 4 cylinder and the higher octane fuel equates to 23 extra hp and 10 extra ftlbs. Will you feel a nearly 10% increase in horsepower? If you drive it hard enough you should. If you mostly cruise around like grandma going to church, you never hit the power peak at any octane.

Well, off to church with granny then,
Seriously though, I hadn't taken into account anyones custom mods on this octane issue at all.
Sounds like you have a heavily modded Jeep with 38" tires on it and 5.38 gears so, yeah, you are exactly what the manual calls for "heavier load conditions" and 93 octane for sure year around or "hot weather" is also where unmodded Jeeps will most notice an increase in performance if using premium vs regular fuel according to the manual
I do apologize for any confusion on my part as I simply believed we were looking for the difference between 2 octanes on one specific engine on a factory Jeep and a real world test would be the only way to get such information.
I'm old school and 1/4 mile times are what we use as seen in "Hot Rod", 0-60 times were what they'd use for grandpas Oldsmobile as seen in "Car and Driver"
It's all good on a quest for information none of us really needs ?

 

[Zandcwhite]

Well, off to church with granny then,
Seriously though, I hadn't taken into account anyones custom mods on this octane issue at all.
Sounds like you have a heavily modded Jeep with 38" tires on it and 5.38 gears so, yeah, you are exactly what the manual calls for "heavier load conditions" and 93 octane for sure year around or "hot weather" is also where unmodded Jeeps will most notice an increase in performance if using premium vs regular fuel according to the manual
I do apologize for any confusion on my part as I simply believed we were looking for the difference between 2 octanes on one specific engine on a factory Jeep and a real world test would be the only way to get such information.
I'm old school and 1/4 mile times are what we use as seen in "Hot Rod", 0-60 times were what they'd use for grandpas Oldsmobile as seen in "Car and Driver"
It's all good on a quest for information none of us really needs ?

I'd like to see a real world test myself, but it's far more trouble than it's worth for me. I appreciate that Mazda releases ratings for both, and wish Jeep did the same.

 

[Pinky Tuscadero]

I'd like to see a real world test myself, but it's far more trouble than it's worth for me. I appreciate that Mazda releases ratings for both, and wish Jeep did the same.

Agreed

 

[BigGreen]

ITT: people that don't understand how octane impacts performance.

Just because you don't notice detonation, doesn't mean it's not happening. Here's hoping pistons/rings are up to the added heat.

 

[Initial-Jeep]

KNOCK KNOCK!

Who's there?

Spoiler

87 OCTANE, SILLY! ?

 

[jjvincent]

ITT: people that don't understand how octane impacts performance.

Just because you don't notice detonation, doesn't mean it's not happening. Here's hoping pistons/rings are up to the added heat.

Knock sensors were designed to detect detonation and have been in use since the early 80's. Thus, the ECU adjusts things like timing, fuel and in a turbo motor, boost levels if the knock sensor detects detonation. Thus most motors run off the knock sensor and that explains why some can make more power with higher octane. More octane, more aggressive engine map and vice versa.

 

[BigGreen]

Knock sensors were designed to detect detonation and have been in use since the early 80's. Thus, the ECU adjusts things like timing, fuel and in a turbo motor, boost levels if the knock sensor detects detonation. Thus most motors run off the knock sensor and that explains why some can make more power with higher octane. More octane, more aggressive engine map and vice versa.

Indubitably fine chap.
However, how does an ECU detect knock prior to knock existing? When knock happens, the pistons and rings are already way to hot and the damage is done, even if its just temporary.

 

[Heimkehr]

Knock sensors were designed to detect detonation and have been in use since the early 80's. Thus, the ECU adjusts things like timing, fuel and in a turbo motor, boost levels if the knock sensor detects detonation. Thus most motors run off the knock sensor and that explains why some can make more power with higher octane. More octane, more aggressive engine map and vice versa.

...how does an ECU detect knock prior to knock existing?

It doesn't, and John's post doesn't suggest that the ECU can prospectively identify knock prior to the actual event. Its role is reactive, and not precisely preventative. It observes and responds.

 

[jjvincent]

It doesn't, and John's post doesn't suggest that the ECU can prospectively identify knock prior to the actual event. Its role is reactive, and not precisely preventative. It observes and responds.

Exactly. You can actually take a knock sensor and hit it with a tool (i.e. screwdriver) and it'll think it's a knock and the ECU will respond. Plus, if you have a Porsche Cayman GT4 (the race version) you can actually get Porsche Motorsport to flash the ECU so it'll run pump gas as opposed to race gas (i.e. 91 octane vs 98). In a nutshell, it just presents a new range of engine maps that are according to the parameters that 91 octane brings. Yes, it will not make as much power but if you are running in a series with endurance races, it makes sense to do this as $12/gal Sunoco GTX260 VS $4/gal 91 octane will add up the savings when you are burning 15 gal/hr and the races are twin 8hrs (total of 16). Plus, that doesn't count practice or qualifying. So people usually will forgo the loss of a few hp (at least $2K savings per event)

On most ECU's, they have a knock counter (just like a misfire counter). If it goes too high, it throws an error code. So if you are going to detonate the engine to death, it'll warn you far in advance and probably go into limp mode.

 

[J1337P]

#nondieselfolkproblems

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