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Octane And Efficiency


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High performance vehicles or engines require a higher octane. this is due to a higher engine compression, FI, or the like. IMO, there should be a low grade and high grade. The middle or mix is a waste.

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Let me rephrase that.

 

Does octane in itself have any effect on fuel efficiency(not preignition or detonation suppression) in an engine that is running perceivably proper?

 

NO

 

Your engine will be most edfcient running the proper octane for its respective compression ratio. (87 for most of GMs engines)

 

Your engine may not run 'bad' with a higher octane, and depending on the ability of the ecm to adjust timing you may see a slight performance improvement (but not much, and could just as easily decrease performance), but you are likely to decease efficiency.

 

In most cases, a performance tune will increase the dynamic compression ratio which requires a higher octane to prevent predetonation but will also improve performance and efficiency

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Hey Matt-

First NO, then you say ..."with a higher octane... you are likely to decrease efficiency".

So do you mean that IF the ECM can NOT compensate with a change in timing for an octane change, THEN you will see a derease in efficiency? Now that makes sense.

 

So on a followup question, say one detonates lightly on 89 but notices decreased efficiency on 91, and assuming good quality fuel, what would that mean? I only have about 5 tanks on my tune, but I think that's what I'm seeing. Probably need a couple more tanks to confirm.

 

Oh yea- I got my shackles on this weekend, and yes, it wasn't gonna happen without bed lifting, but I did try!! 6 more bolts, but not a big deal at all. :P

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Lol engines are complicated and its hard to type on my cell. What you interpreted from my post was basically the point... The octane does not determine the efficiency, the engine and computers parameters are what matters, but running the wrong octane will hurt efficiency ... Is that a long confusing run on sentence or what??

 

Key factor here- you have an aftermarket tune! You need to talk to your tuner of you're getting predetonation! Something probably needs to be tweaked in the tune, mixing grades of gas is not the correct resolution imo.

 

Your tuner should be able to explain the octane and efficiency stuff too.... Its much easier to explain in person

 

Edit: if its pinging with 89 and that's what you're tuned for, it could also be a bad blend. If you're tuned for 91 then I can see it pinging on 89... But if you're losing performance with 91, sounds like a tuning issue

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I think what Matt is getting at is if a vehicles components (compression, cam, timing, etc) function properly with a lower grade fuel, then going to a higher grade will have NO effect on efficiency.

 

But adjusting the above mentioned components can have an effect on efficiency, but those adjustments may require a higher octane fuel.

 

Given your scenario of all things being equal and you have a motor that does not require premium fuel, any improvement you see in mileage is due to some sort of placebo.

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66nova- actually it was just the oposite. When I went from 89 to 91 to get away from slight detonation, my ping stopped but efficiency went down about 10%.

 

 

Very interesting. Is the gas from the same station?

 

Also, one question that pops into my mind is...Since it was detonating with the 89, are you sure you do not have a tendency to accelerate a little harder with the 91? I would assume that you back off the throttle when you had a ping. Is it possible that with the 91 you may be on the gas slightly harder, not necessarily hot rodding, but you get what I am going at.

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Not the same gas station, but I have been staying away from questionable quicky stores etc. I do get very repeatable results with the ping on 89 from any source, and on the first tank of 91 no detonation at all. I drive pretty conservatively when I'm thinking about it and the trafic dictates such, and am concious of when it happens- always 1300-1600 RPM, 45-55MPH, slight grade, steady throttle (or on cruise-easiest way to elicit it). My mileage went from 16-16.5 on 89(and ping) to 15 on 91( no ping). Neither the cost per gallon nor the 1-1.5 MPG alone are huge, but the combination of both kinda sux... or about $9 per tankfull...

But again, probably need to run a few more 91 tanks to be sure that that is really whats going on.

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It takes a few tanks (generally speaking) for your computer to adjust all of its parameters to the new octane... it doesn't automatically change on the first tank of 91. So maybe try running a few tanks of 91 to see if your efficiency bumps back up.

 

Also, pinging can result in engine damage... so although you may be losing money by running 91 octane, you'll actually save money by not pinging

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All of ther parameters? Sounds like only one to me.

I'm half temped to :lol: on this one, but I don't really know anything on this self adjusting ECM thing?

 

Preign.- yea I had a boss who's 70's LTD sounded like a diesel, I (the go-cart "mechanic") told him it was his valves :cheers:

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All of ther parameters? Sounds like only one to me.

I'm half temped to :lol: on this one, but I don't really know anything on this self adjusting ECM thing?

 

So you have no clue how octane affects an engine, you don't know how tuning works, and you dont know how the ECM makes adjustments to timing and LTFTs as it adjusts to different gas mixtures, and so you come here and ask for help, and then try to call BS when we try to help you.....ok :cheers:

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Matt-I'm sorry. Apparently I misread.... I thought your previous post said "... the parameters of.." not ".. it's parameters to...". The first implies there are multiple parameters within the fuel with an octane change, whereas the later refers correctly to all of the computers tuning parameters. Maybe it's both?

 

My bad; a "the-of" and "it's-to" misunderstanding... :cheers:

 

So I did just read some about STLTs and LTFTs elsewhere and have a cursory understanding now.

 

The question is, for anyone who cares to answer, do I stick with 91 and let it self adjust and see if I get my efficiency back, or go back to 89 and see if it takes more than 4-5 tanks to stop detonating?

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My philosophy on knocking (detonation) is to either increase octane or decrease timing. So, if you're getting detonation with 89 octane, *I* would run 91 for a while and see what happens. It may take a while for the computers to sort things out and realize they can increase timing, without getting more knocking/detonation. More timing generally leads to more power and... as you've been saying "more efficiency."

 

Good luck! Let us know how it goes! :cheers:

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