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What fuel for 14 5.7

3K views 17 replies 9 participants last post by  bill_de 
#1 ·
hey guys what fuel are you all using for 2014 5.7?

As I posted in another thread..I just got my 5.7 limited first tank was super...but now im curious what others are using...ty
 
#7 ·
I only run 87 in my MY12 hemi with great results...including good fuel economy. It's the same engine in the MY14. I haven't tried 89...Costco doesn't sell it. :)
 
#8 ·
I always put 89 in mine (91 when at Costco) and my wife has put 87 in the first couple times. I don't feel a difference in performance, but for peace of mind, I just put what the manufacture recommends. They build the engine, so they know best right? It's only a couple more bucks and keeps me from going in and buying junk food to make me fatter.
 
#12 · (Edited)
Since I live at 8500FT elevation, there is certainly less pressure and thus slower burning fuel mixture. My concern is that modern vehicles already adjust their timing (and fuel amounts) for the higher altitude. So the engine management system is still looking for 89 octane. I know it will "back-off" timing if it detects pre-ignition, but I would rather just feed it the 89 it expects. (Years ago vehicle manufactures sold cars with milled "Denver Heads" to increase internal pressures. Also the carbs came with leaner "Denver Jets." That would not work today because we all drive back down to sea level, and the EPA would not allow those mods.)

Also, several previous vehicles owned were turbo charged. So the altitude excuse to sell lower octane fuel really did not apply for turbo/supercharged engines. For those engines, 93 octane would be nice "up here," but they only sell 91.
 
#13 ·
A car doesn't know what octane fuel is in the tank. The recommended rating (89 for example) is what the vehicle was designed for during use at or near sea level. As altitude increases, the need for a fuel to resist pre detonation decreases by about 1 ON per 1000 feet. Roughly. With that in mind, 85-86 (R+M/2) is equivalent to 89 (R+M/2) at 6000 feet.
On turbo engines, this still applies. Boost ratio remains the same regardless of air density.
( atmosphere pressure + required boost) / atmosphere = p2/p1 ratio
Lower air density at the intake would result in less pressure on the exit of the spool. Of course, we are talking absolute pressures here.
Lower octane fuels generally deliver more energy per gallon. Ethanol is a common way to cheaply increase the octane rating of a fuel. Although ethanol contains less energy per unit of measure than does petrol.
I usually use 87 unless towing or it is really hot around here. FWIW, I am damn near at sea level.
 
#14 ·
My point is that modern vehicles measure atmospheric pressure (including temperature and humidity) and take that into consideration when determining ignition and fuel mapping. The mapping also includes assumptions as to fuel being used. For the 5.7L Hemi, Jeep probably assumes 89 octane. So the initial mappings are appropriate for that fuel. Once the engine fires, sensors provide feedback to the engine management system so that the initial mappings can be refined to the actual properties of combustion.

Turbocharged engines, with variable compressor vanes, compensate for most of the lost of atmospheric pressures at altitude.
 
#15 ·
My point is that modern vehicles measure atmospheric pressure (including temperature and humidity) and take that into consideration when determining ignition and fuel mapping. The mapping also includes assumptions as to fuel being used. For the 5.7L Hemi, Jeep probably assumes 89 octane. So the initial mappings are appropriate for that fuel. Once the engine fires, sensors provide feedback to the engine management system so that the initial mappings can be refined to the actual properties of combustion.

Turbocharged engines, with variable compressor vanes, compensate for most of the lost of atmospheric pressures at altitude.
You might be making a lot of assumptions. I think it's a lot simpler than that. When the system senses pre-detonation it retards the timing. It doesn't assume what gas you chose.



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#18 ·
At the higher altitude it won't detect pre-detonation with lower octane gas, so it will not adjust the timing to correct for it. It's the pressure in the combustion chamber that counts. That is lower at high altitude because the air is 'thinner'.
 
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