What fuel for 5.7

[AnthonyD1978]

Just wondering if the reverse is true, Do low altitudes require higher octane to maintain stability?

Technically yes, lower elevation = more O2 = leaner fuel mixture. Not really a concern with any modern engine. Run at least the minimum requirement (87oct for us) and the engine will be able to compensate as needed.

 

[Gman]

Octane also matters depending on the type of engine. Turbo/Supercharged engines will likely benefit with higher octane (and usually there's a recommendation for "Premium Fuel" in the owner's manual). More O2 in the charged air volume, more likelihood of preignition under compression.

Naturally aspirated engines, like the 5.7l Hemi, don't usually have a problem with an overabundance of O2 in the intake cycle and can usually get by with lower octane levels.

When I make trips across country, I will buy the equivalent *grade* fuel for what I buy at home. The formulations are typically suited to the local area.

Premium at octanes varying from 89 to 93 for the twin turbo Mercedes GL 450. Regular from 85 to 89 (if I remember correctly) for the naturally aspirated Hyundai Azera GDI V6. Once I get my RAM in the next few weeks, it'll probably be 87 octane unless I notice any issues. I may even try 85 to see how the engine performs.

This is from the manual for the 5.7l Hemi;

5.7L Engine

Do not use E-85 flex fuel or ethanol blends greater than 15% in this engine. This engine is designed to meet all emissions regulations and provide satisfactory fuel economy and performance when using high-quality unleaded gasoline having an octane range of 87 to 89 as specified by the (R+M)/2 method. The use of 89 octane “Plus” gasoline is recommended for optimum performance and fuel economy.

While operating on gasoline with an octane number of 87, hearing a light knocking sound from the engine is not a cause for concern. However, if the engine is heard making a heavy knocking sound, see your dealer immediately. Use of gasoline with an octane number lower than 87 can cause engine failure and may void or not be covered by the New Vehicle Limited Warranty.

"Plus" or mid-grade is 87 octane here.

 

[ExcursionDiesel]

4 factors affect pre-ignition for a given octane.

1. Dynamic Compression (not static). This is the actual compression during operation and affected by cam timing and head flow.

2. Altitude

3. Temperature (overall combination of head temperature and ambiant)

4. Timing

Worst case is low elevation and high temperatures. Best case is high elevation and lower temps. Most of us live somewhere in between and 87 octane works fine because the engine is designed for 89.

Hook up a max load trailer and run 80mph in Texas with 100°F temps to really heat up the engine and it might ping under acceleration with 87 octane. Pulling timing isn't the fix-all. Significant power is lost with enough timing pulled.

I learned a lot about engine tuning while dealing with an engine build that needed 96+ octane due to high compression and poor cam selection.

Ram engineers did an excellent job with the Hemi making power on low octane fuel.

 

[surfingcrna]

4 factors affect pre-ignition for a given octane.

1. Dynamic Compression (not static). This is the actual compression during operation and affected by cam timing and head flow.

2. Altitude

3. Temperature (overall combination of head temperature and ambiant)

4. Timing

Worst case is low elevation and high temperatures. Best case is high elevation and lower temps. Most of us live somewhere in between and 87 octane works fine because the engine is designed for 89.

Hook up a max load trailer and run 80mph in Texas with 100°F temps to really heat up the engine and it might ping under acceleration with 87 octane. Pulling timing isn't the fix-all. Significant power is lost with enough timing pulled.

I learned a lot about engine tuning while dealing with an engine build that needed 96+ octane due to high compression and poor cam selection.

Ram engineers did an excellent job with the Hemi making power on low octane fuel.

Thanks as well. Being in Florida it looks like I am both hot and low. I'll keep that in mind when I tow.

 

[Gman]

The 5.7l Hemi is very 'old school' in the manufacturer's chase for better mileage. I'm actually surprised that its efficiency is as good as it is without Gasoline Direct Injection. This tells me that the MDS is a pretty effective system and that the engine should hold up for a very long time when properly maintained.

 

[ExcursionDiesel]

The 5.7 Hemi at 395hp makes 69.3 hp per liter. The LS3 6.2 in my SS Camaro makes 68.8 hp per liter in stock form. The Camaro has a very low aero drag yet only gets 22mpg at 75mph and it's a 6 speed manual. Its also gutless at low rpm. The 3.21 equipped 5.7 Ram 2WD gets close to that with poor aero drag and an auto transmission in comparison. This is a testament to the Hemi's efficiency. More power AND efficiency. A good tune and intake/exhaust on the Hemi is good for 425 hp on 93 octane.

 

[meltedcheez]

I would like to get in on this discussion beings from California and all. We have 87, 89, and 91 at the station here. All have at least 10% ethanol in them due to State regulations. Of course the owner's manual recommends 89 and i have been running 89 since I bought it, but with price jumps upwards of 30-40 cents throughout the year it hurts at the pump. Sometimes gas can get close to $4.25/g in SoCal.

I'd like to think running 89 increases its efficiency and power...but does it? A lot of what I have been reading lately is in the boat 89 is a scam by refineries and big oil anyway. Is the FCA in collusion with Big Oil to push higher cost fuel on a vehicle that doesn't need it?
I might run a few tanks with 87 and see if I notice any changes in MPG. I know I didn't buy this truck for fuel economy but every little cent counts when I am saving to get the mods I want.

 

[surfingcrna]

I would like to get in on this discussion beings from California and all. We have 87, 89, and 91 at the station here. All have at least 10% ethanol in them due to State regulations. Of course the owner's manual recommends 89 and i have been running 89 since I bought it, but with price jumps upwards of 30-40 cents throughout the year it hurts at the pump. Sometimes gas can get close to $4.25/g in SoCal.

I'd like to think running 89 increases its efficiency and power...but does it? A lot of what I have been reading lately is in the boat 89 is a scam by refineries and big oil anyway. Is the FCA in collusion with Big Oil to push higher cost fuel on a vehicle that doesn't need it?
I might run a few tanks with 87 and see if I notice any changes in MPG. I know I didn't buy this truck for fuel economy but every little cent counts when I am saving to get the mods I want.

Learning from what the others have replied, it would seem higher octane affects neither efficiency nor power, just stability. The ethanol mix does seem to decrease available energy, however, this isn't mitigated by using a higher octane. You just lose the power. But keep in mind, my mechanical theory experience is limited to hot wheels and reading.

 

[meltedcheez]

Learning from what the others have replied, it would seem higher octane affects neither efficiency nor power, just stability. The ethanol mix does seem to decrease available energy, however, this isn't mitigated by using a higher octane. You just lose the power. But keep in mind, my mechanical theory experience is limited to hot wheels and reading.

Will it negatively affect longevity of the engine by using 87?

 

[surfingcrna]

Will it negatively affect longevity of the engine by using 87?

Not from what I've read. The engine is made to adjust. If the engine needs more stability, as noted by knocking, then you should use a higher octane. A few posts up someone wrote a good piece about the hemi

 

[MrHankbot]

Learning from what the others have replied, it would seem higher octane affects neither efficiency nor power, just stability. The ethanol mix does seem to decrease available energy, however, this isn't mitigated by using a higher octane. You just lose the power. But keep in mind, my mechanical theory experience is limited to hot wheels and reading.

You've got it right. The only reason engines can make more power on higher octane is because they can be run more efficiently (hot or high pressure or both) than on a lower octane which can allow for a higher utilization of the available power in the fuel, but the amount of available power doesn't change with octane. The dirty secret is you can run an engine leaner on higher octane gas, it doesn't always have to just be used for extra power, it can be used for the same power with lower fuel consumption if an engine is tuned for it.

Ethanol does change the available power in the fuel, for the worse, but it has a high effective octane rating (approximately 113) because it resists predetination better than gasoline. For this reason if you can force enough of it into the motor with enough air under enough pressure you can make more power burning ethanol. This confuses people. Don't let it. Gasoline is approximately 122,000 btu/gallon, 100% ethanol is 84,530 btu/gallon, roughly 70% of the energy density of gasoline, so you have to use more ethanol to do the same amount of work at the same efficiency.

E10 has 96.7% of the energy of 100% gasoline. So while it would seem that we're getting a good deal because we're getting gas that's a bit cheaper, usually 5-10%, we're losing some of that savings to lost energy density. Despite being 96.7% of the energy density of regular gas I find E10 to yield 10% less fuel efficiency in the real world.

 

[mikemcfarland]

alright ... all you gasoline wizards. just out of curiosity - does "shelf life" effect mileage or engine wear?
what im wondering is.... im assuming that there is definitely a much greater sale of lower octane due to $ that would mean a "fresher" grade of gas (less stagnate) if the mid grade and higher grade has been sitting at the gas station for a longer extended period of time, how does this effect the gasoline performance.. if any?

 

[AnthonyD1978]

Some of you are over thinking it. Run 87 and you'll be fine. If you're questioning the quality of the gas and/or putting the truck through some rough times (towing, high temps, etc) then go with 89+ if you want.

I'm not a fueling expert, but I've done 100's of logs and tunes on my cars using pump gas, race gas, E85 mixes (I love e85), etc

 

[MrHankbot]

alright ... all you gasoline wizards. just out of curiosity - does "shelf life" effect mileage or engine wear?
what im wondering is.... im assuming that there is definitely a much greater sale of lower octane due to $ that would mean a "fresher" grade of gas (less stagnate) if the mid grade and higher grade has been sitting at the gas station for a longer extended period of time, how does this effect the gasoline performance.. if any?

Short version of the answer. Avoid gas that's too old as the volitile compounds evaporate relatively quickly, and as they do the gas starts to change into something with higher amounts of gums and varnish that is harder to combust and burn efficiently.

That said.

As long as it's not older than a month or two at the station, don't worry about it. I'd worry more about how long it sits in your tank than in the gas stations tanks. Your tank goes through higher temperature swings (causing condensation) and higher overall temperatures in a vented environment (increased volatile compound loss).

Best thing to do, top off at least once a month even if you don't need to, from a station you trust that is conveniently located for you.

 

[Grunkster]

so here in MN E15 is the big thing now...(AND .25 per gal cheaper!!) ...thoughts on running this in the 19' RAM?

 

[MrHankbot]

so here in MN E88 is the big thing now...(AND .25 per gal cheaper!!) ...thoughts on running this in the 19' RAM?

What you're talking about is 15% ethanol I presume?

Up to you, they say it's fine for vehicles 2001 and newer. I, with no reason other than probably flawed personal beliefs, will probably not run E15 unless I don't have a choice. If I do and find that it's the same fuel economy as E10, I may use it when it's cheap enough.

If 5 years from now people are not having any wear issues with long term E15 use, I'll use it just like I use E 10 now.

 

[AnthonyD1978]

so here in MN E88 is the big thing now...(AND .25 per gal cheaper!!) ...thoughts on running this in the 19' RAM?

Do not run E88 in the truck! On my 5.7 it specifically says no E85 on the fuel door. E88 has more ethanol than E85 (3% more).

The truck might be able to handle E30-E40 before the fuel pump(s) can't keep up. In old cars ethanol would hurt the fuel lines/pumps. In new cars that's not an issue with the more modern materials used in the fuel systems (ethanol rated). The issue is that our fuel pumps can't keep up since ethanol is ~30% less dense the fuel pumps need to pump at +30% rate.

 

[Grunkster]

that is incorrect. E85 is 85% ethanol, 15% fuel.....E15 is 15% ethanol, the rest fuel.....they are completely different but i know it's confusing

 

[SpeedyV]

E85 is 85% ethanol, 15% fuel. 88 octane is called E15, not E88, and is 15% ethanol, 85% fuel.

 

[AnthonyD1978]

that is incorrect. E85 is 70% ethanol, 15% fuel.....E88 is 15% ethanol, the rest fuel.....they are completely different but i know it's confusing

Wrong. The 88 after the E is literally the ethanol content.