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  #13  
Old 05-14-2012, 01:17 PM
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Quote:
Originally Posted by CIEGOS View Post
I always find it ironic that people who get the 5.7L don't worry about how much gas it consumes but rather what octane to put in their gas guzzler in order to save money. If you can't afford the extra 5% more in cost maybe the v6 is more up your alley... just saying
I agree, and some people seem to be victims of a bit of buyer's remorse. I went V8 knowing that it meant a little more gas consumption, on slightly higher octane fuel, but superior towing capacity. I don't have any immediate towing need, nor did I care since the cost difference between 89 and 87 is minimal and I only commute to the office (48 miles) once a week.

If I had a 48 mile round trip every day AND noted that I'd have to do it on premium at higher fuel consumption rates with the continued lack of a justification on the towing requirement, I may have chosen differently.

I think a lot of people also want to min/Max as well. They want more bang for their buck - minimal cost, maximum benefit.

I'd never opt for a V8 over the 0-60 speed and 1/4 mile times, because this isn't a sports car... It's a utility vehicle.

To each their own.



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  #14  
Old 05-14-2012, 02:23 PM
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Re: 89 octane for 5.7 Hemi recommended?

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Originally Posted by Yadkin View Post
I've had a vehicle before where the OM recommended 89 and had very poor luck with fuel consistency. Apparently stations get 87 and 91 delivered in separate tanks then mix the two at the pump to make 89. So I usually ended up buying 91. I hated having to toss that money out the window so traded in the car.

wow, I didn't realize thats how we get 89 octane.
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Old 05-14-2012, 02:47 PM
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Re: 89 octane for 5.7 Hemi recommended?

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Originally Posted by Yadkin View Post
I've had a vehicle before where the OM recommended 89 and had very poor luck with fuel consistency. Apparently stations get 87 and 91 delivered in separate tanks then mix the two at the pump to make 89. So I usually ended up buying 91. I hated having to toss that money out the window so traded in the car.
If that is the case, they only sell 93 in Massachusetts, so do they mix 2/3rd 87 with 93 to get 89
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Old 05-14-2012, 03:37 PM
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Re: 89 octane for 5.7 Hemi recommended?

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Originally Posted by dizz1979 View Post
If that is the case, they only sell 93 in Massachusetts, so do they mix 2/3rd 87 with 93 to get 89
I honestly don't know. I do know though, that stations typically have only two tanks.

Quote:
It is called "blending". Not all midgrades are a blend though. Originally, each product had a dedicated storage tank and pumped a specific grade or octane. Currently, some of these systems still exist and are termed "straight" meters.
A majority of the fuel dispensers now only have two octane storage tanks, typically 87 & 92/93. When you select 89 for example the pump electronically determines the proper mix or "blend", (if its working correctly). It may mix 70% 87 and 30 % 93 for example to arrive at 89. However, this is a progamable function and this ratio is adjustable.
http://wiki.answers.com/Q/Do_gas_sta...make_89_octane

Apparently this "programmable function" is abused at some stations, which is why I had variable success buying mid-grade when I drove a Mustang GT that preferred it.
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Old 05-14-2012, 06:08 PM
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Re: 89 octane for 5.7 Hemi recommended?

I've always heard that changing octane was going to mess with the learned timing of the car. However, a mechanic told me that any new car (last 8 years at least) has an anti-knock sensor and will retard timing if it notices anything wrong. So the vehicle may ride rough if you are pushing its performance, but under normal driving conditions it would be hard to notice the difference. That being said, he recommended sticking with the same octane. He also told me running 87 would be perfectly fine in the Hemi, even with the 89 being recommended. But this is just a his words- nothing I can prove, but I trust his judgement and experience!
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Old 05-14-2012, 08:28 PM
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Re: 89 octane for 5.7 Hemi recommended?

Short form: That is sooo last century.

Long form: Back in the day, all the computer could modulate was mixture and advance but with FBW the computer can modulate the throttle opening and control the peak chamber pressure as well as a lot of other emissions and performance related things as well.

True, octane relates to flame propagation rate which relates to optimum spark advance so you want to back off the timing slightly for a lower octane to maintain peak power.

The key is that everything relating to timing is a kludge back to the days of poorly controlled leaks (carbs). All engines have a "sweet spot" usually somewhere between 7 and 14 degrees after top dead center. Everything in tuning is about reaching maximum chamber pressure at that point. How long before that point you fire the plug(s) depends on the flame propagation rate which is a function of fuel/air density in the chamber, octane, and how far the flame needs to travel for (near) complete combustion.

Detonation or pre-ignition (knock) OTOH is a function of octane and chamber temperature so when operating near the limit you want as cool a charge temperature as possible. Water or alcohol injection has been used in the past to supercool the mixture and avoid detonation particularly with boost. (personally do not care for boost in a street SI engine but live at sea level. Said I do not care for it, not...)

Consequently with a higher octane gas, you need more advance than lower under the same conditions to hit the "sweet spot". At the same time a higher octane will be more resistant to detonation under heavy load and temperature conditions.

The thing to remember is that until you reach detonation, 89 with more advance will generate almost exactly the same power as 87 with less advance. A really good computer will be able to sense where the sweet spot is and tune accordingly.

Further to manage efficiently on E85, the computer needs to be able to sense a much greater range becuse unlike octane, as you add ethanol the total energy per pound drops (but the effective octane increases).

Bottom line without bothering with the boring math, they probably use about 84 octane gas mixed with 10% ethanol to get 87 PON.

Things get really complicated here but the computer really does a really good job of figuring out where to fire the spark but if you really want to test it you will need to make some identical test runs on 87 and 89 while monitoring Inst MPG, short and long term fuel trim, IAT, IMAP, advance (needs a notebook PC and about $50 of hardware and plugs).

I suspect that for identical conditions the 87 would run 4-8 degrees less advance but the cruise fuel trims and MPG would be the same. Delieve it or don't.

Now remember back near the top where I mentioned that one of the main factors in detonation is heat ? Since I live in a hot climate this is why I want to keep the coolant under 190F even at 100F ambient and the IAT within 10F of ambient but at the same time I need to avoid a P0128 code.

Since I do not know the thresholds, am going to need to experiment a bit.
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Old 05-14-2012, 08:32 PM
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Re: 89 octane for 5.7 Hemi recommended?

I was driving BMW's before coming back to the JGC. Higher grade was suggested but I only ever used 87. I never has an issue and had 140k miles on both I had.

All that said, coming up on my first fill up tomorrow so I need to decide. Interesting, the dealer pumped 87 on their free tank they gave me.
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Old 05-15-2012, 06:50 AM
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Re: 89 octane for 5.7 Hemi recommended?

How much are you guys paying for 89 octane? I filled up at $3.82 this morning in Southern Illinois (near St Louis).
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Old 05-15-2012, 07:53 AM
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Re: 89 octane for 5.7 Hemi recommended?

Quote:
Originally Posted by ColinIllinois View Post
How much are you guys paying for 89 octane? I filled up at $3.82 this morning in Southern Illinois (near St Louis).
$3.41 for 89 w/ 5% cash back, so currently, $3.24...
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  #22  
Old 05-15-2012, 08:24 AM
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Re: 89 octane for 5.7 Hemi recommended?

Quote:
Originally Posted by padgett View Post
Short form: That is sooo last century.

Long form: Back in the day, all the computer could modulate was mixture and advance but with FBW the computer can modulate the throttle opening and control the peak chamber pressure as well as a lot of other emissions and performance related things as well.

True, octane relates to flame propagation rate which relates to optimum spark advance so you want to back off the timing slightly for a lower octane to maintain peak power.

The key is that everything relating to timing is a kludge back to the days of poorly controlled leaks (carbs). All engines have a "sweet spot" usually somewhere between 7 and 14 degrees after top dead center. Everything in tuning is about reaching maximum chamber pressure at that point. How long before that point you fire the plug(s) depends on the flame propagation rate which is a function of fuel/air density in the chamber, octane, and how far the flame needs to travel for (near) complete combustion.

Detonation or pre-ignition (knock) OTOH is a function of octane and chamber temperature so when operating near the limit you want as cool a charge temperature as possible. Water or alcohol injection has been used in the past to supercool the mixture and avoid detonation particularly with boost. (personally do not care for boost in a street SI engine but live at sea level. Said I do not care for it, not...)

Consequently with a higher octane gas, you need more advance than lower under the same conditions to hit the "sweet spot". At the same time a higher octane will be more resistant to detonation under heavy load and temperature conditions.

The thing to remember is that until you reach detonation, 89 with more advance will generate almost exactly the same power as 87 with less advance. A really good computer will be able to sense where the sweet spot is and tune accordingly.

Further to manage efficiently on E85, the computer needs to be able to sense a much greater range becuse unlike octane, as you add ethanol the total energy per pound drops (but the effective octane increases).

Bottom line without bothering with the boring math, they probably use about 84 octane gas mixed with 10% ethanol to get 87 PON.

Things get really complicated here but the computer really does a really good job of figuring out where to fire the spark but if you really want to test it you will need to make some identical test runs on 87 and 89 while monitoring Inst MPG, short and long term fuel trim, IAT, IMAP, advance (needs a notebook PC and about $50 of hardware and plugs).

I suspect that for identical conditions the 87 would run 4-8 degrees less advance but the cruise fuel trims and MPG would be the same. Delieve it or don't.

Now remember back near the top where I mentioned that one of the main factors in detonation is heat ? Since I live in a hot climate this is why I want to keep the coolant under 190F even at 100F ambient and the IAT within 10F of ambient but at the same time I need to avoid a P0128 code.

Since I do not know the thresholds, am going to need to experiment a bit.

Do you know who killed Kennedy?
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  #23  
Old 05-15-2012, 10:16 AM
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Re: 89 octane for 5.7 Hemi recommended?

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Old 05-15-2012, 07:29 PM
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Re: 89 octane for 5.7 Hemi recommended?

Just paid $3.49 for 89 today.
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