higher performance with 91/94 octane on a gm lq4 engine or no?

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i have a 2003 hummer h2 with a 6.0 LQ4 v8, the manual says to fill 87 and doesnt say anything else about higher octane. my vehicle is very heavy and imo the engine is underpowered for the weight because if you hit small hills and are trying to go 80mph, the truck always has to downshift and it gets annoying cuz its always revving high and then making excessive noise. so for my work im always taking the same highway about once every few weeks for a 6 hour road trip and i used to always fill 87 and would always be annoyed and would basically have to drive slower just to not overly shift and raise transmission temps and rev the engine extra - it just bothers me and the noise gets annoying. so a few weeks ago, i emptied my tank and filled 94 and then unplugged the battery and drove in my city a few days, i then hit the highway and did my normal road trip and i no joke felt like the truck barely freaked out over the small hills compared to what it did on 87 octane. like i genuinlely felt it was making more power and made it unnceccesary to downshift every hill. i then just filled 94 octane for the rest of the week and when i went back home and took the same highway back a few days later, i noticed the exact same thing. way better driveability, rarely would downshift like how it did before and was a really good ride.

now heres the thing, i always hear people saying the gm lq4 engine is a low compression engine and on a stock tune (which my vehicle has), higher octane wont increase performance. but for me, i feel like i honestly noticed a diff. i am not that technical with engines but could it maybe be that 87 was pulling my timings before and then with 94 the timing got increased and which made more power? idk i just wanna hear what yall think

or basiclaly i could have hit perfect road conditions (colder temps, no wind) which exerted less stress on the truck and made it easier for it to drive rather then downshifting all the time. i feel like this would be such a coincidence as i didnt notice a diff in temp or wind etc

i also wanted to ask lets say if this is true and that i am experiencing a performance advantage then would 91 octane be able to give me the same performance advantage as 94 or no? because i read and heard that a vehicle has to actually be tuned to take advantage of 94 octane as most vehicles can only take the benefit of 91 octane and that in this situation i would be wasting $$ on 94. and or that the diff in performance for a stock tune between 91 and 94 is negligible because the stock tune cant take advantage of higher then 91-92

thanks!
 
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You are kidding, right?

with all the troubles you have with that Hummer, running premium fuel is a total waste.

🎹🎹
 

jalikaria24

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You are kidding, right?

with all the troubles you have with that Hummer, running premium fuel is a total waste.

🎹🎹
haha it actually drives fine and ive been replacing whatever needs replacing since it never really had maitnenace befor before.

if this is actually imporoving my highway driving then i dont mind paying the extra $10
 

OVERKILL

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Should be easy enough with a scan tool to confirm if it pulls timing on that run, and how much, on 87, then compare to both 91 and 94. Why guess? figure it out.
 
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Should be easy enough with a scan tool to confirm if it pulls timing on that run, and how much, on 87, then compare to both 91 and 94. Why guess? figure it out.
This. For most vehicles, there won’t be any difference or only a very slight difference that doesn’t warrant running higher octane than what it’s tuned for.
 
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What about the octane creep thing? Deposits build up on piston tops, combustion chambers causing compression ratio to go up, which could require the need for higher octane fuel. Being 17yrs old. . . . . . .Might be the reason.
 
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What about the octane creep thing? Deposits build up on piston tops, combustion chambers causing compression ratio to go up, which could require the need for higher octane fuel. Being 17yrs old. . . . . . .Might be the reason.
That is what I was going to type........
 
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Your PCM calibration has Low Octane & High Octane timing tables, If too much pre-ignition is occurring.....It uses the Low Octane table. But it always tries to use the High Octane table first.
Watching Knock Retard doesn't always tell you the whole story as you don't know which Base Timing Table you're running on.
 

jalikaria24

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Should be easy enough with a scan tool to confirm if it pulls timing on that run, and how much, on 87, then compare to both 91 and 94. Why guess? figure it out.

thanks so much for this, i have a android phone and i used to use the torque app just to clear codes and stuff, but i guess i just use a tank of 91, cehck the timing and compare it to a few tanks of 94? and see whats higher?
 
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Try premium without alcohol followed by premium with alcohol and see if there is a difference. Racers run alcohol to get to higher horsepower that gasoline won't allow. If you had an E85 emgine I would try that first and maybe some Nitromethane to spice things up. j/k about the nitro.
 

OVERKILL

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thanks so much for this, i have a android phone and i used to use the torque app just to clear codes and stuff, but i guess i just use a tank of 91, cehck the timing and compare it to a few tanks of 94? and see whats higher?

Basically, or if it shows timing retard from knock, or knock events, you can use that.
 
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