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Old 09-14-2011, 07:34 AM
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garner garner is offline
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Join Date: Nov 2010
Location: Harker Heights, TX
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Vehicle: 2012 Dodge Ram lonestar
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Engine: 2009-20?? 345ci (5.7L) Hemi V8 390hp 407lb/ft
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Quote:
Originally Posted by GTyankee View Post
I usually use the 89, but when prices climb, i may use some 87 between tanks of 89, so it may be more like 88 octane.
Now when i cross into Arizona, i only use 89, prices are 20 to 30 cents cheaper a gallon

The reason that they suggest using 89 octane is because of the Compression ratio
Displacement 5.7L (345ci) Compression Ratio 9.6:1
Bore 3.92 in. (99.5mm) Torque 375 @ 4400 RPM
Stroke 3.58 in. (90.9mm) Horsepower 345 @ 5600 RPM
Fuel Type Gasoline 87 Octane Net Weight 92.9 kg (205 lbs.)
Fuel Induction Sequential Multi‐port Fuel Valve gear OHV MDS 2v/cyl

The general rule is
87 octane up to a compression of 9 to 1
89 octane up to a compression of 10 to 1
91 octane up to a compression of 11 to 1

103 can be used up to 16 to 1


worthless stuff
there are many other variables & 2 different compression types, 1 is DYNAMIC, which i think is the one that engine manufactures use

Compression Ration =
Cylinder vol. + clearance vol. + piston Comp. vol. + gasket vol. + chamber vol. divided by Clearance vol. + piston vol. + gasket vol. + chamber vol.

Read more: http://www.carcraft.com/techarticles...#ixzz1XshuZNpp
Compression ratio on the 4th gens is 10.5:1
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