5.7 HEMI & 87 octane gas

[henrymopar]

I reliize owners manual recommends 89. I have tried both and did not see any difference in performance or millage. I have installed the trailer hitch package avalable from Chrysler ( when you take the receiver out, you can't tell it has a hitch) and have towed a car trailer with no problems. Every one I know it this area all use the 87 octane gas in their 5.7s. The 37 Dodge was my first car, and I have had several other new Chrysler products I wish I had kept. I will try to post pictures later.

[Mike_Levy]

You may not be realizing it, but the knock sensors are pulling timing to compensate for you being cheap and saving 30 cents a tank. At least you better hope they are, other wise you'll be spending a helluva lot more then you "think" you're saving on a rebuild or replacement.

I don't understand the logic of not putting in good gas. You spent $30k on a car. Even if you filled up the tank every single day, you'd barely save a hundred dollars a year, using 87 over 89. Whats the point?

[chuckkent]

Well if you are driving a SRT8 I understand the extra octane. But if your like me with 5.7 that I didn't buy it to smoke my tires off .....87 octane is perfect. The other day I got 30.8 mpg on 87 octane. The car performs well doesn't knock so I would say you are wasting your money if you put 89 or higher in a 5.7..... I wouldn't even consider a 6.1 unless they put MDS on it.

[MikeHTally]

Even though I knew mine needed 89, I accidentally put about half-a-tank of 87 in not long after we bought her. She was NOT a happy girl. I do tend to lean on her a fair bit. My wife does as well. Every so often we'll add a half tank of 93 to the half tank of 89, to get something close to 91, which is the preferred diet of the Hemi.

[jeepgrady]

How are you only saving .30 per tank when 89 costs .10 per gallon more?

I've used 87 in both my C and WK for several years now. Performance and mileage are equal to when I used 89.

[kevenj]

Originally Posted by jeepgrady

How are you only saving .30 per tank when 89 costs .10 per gallon more?

I've used 87 in both my C and WK for several years now. Performance and mileage are equal to when I used 89.

Using your number of 10 cents a gallon difference in cost for 89 octane; if you drive 12,000 miles a year, and if you average only 16 MPG, you use 750 gallons of gas per year. So your extra cost to use the correct 89 octane gas is $75.00 PER YEAR or $6.25 PER MONTH! If you drive 24,000 miles a year it will cost you an extra $12.50 a month to use the right gas!

You may not be able to feel it, but the car is set up for 89, and if you use 87 the knock sensors WILL tell the computer to retard timing and your performance will be affected. The manual says it's "ok" to use 87 and it will not damage the engine due to the knock sensors, but you will not get optimum performance.

[kevenj]

Originally Posted by chuckkent

The other day I got 30.8 mpg on 87 octane.

You know, everyone's mileage will vary depending on many factors; driving style, road conditions, tempurature, etc......

... but c'mon man..... We're all 300 owners here, most with one of the versions of the Hemi..... and anyone who owns one knows that a claim of 30 mpg is not even remotely credible! Who are you trying to kid? yourself?

If you reset the MPG calculator in EVIC while you are driving, yeah you may see 30 MPG or even higher, for a very short time. Hell I've seen it display 37 or 38 MPG when I've reset it while cruising at 65 on a slight downgrade. But there is no way possible to AVERAGE 30 mpg with a 300C for more than a mile or 2. It just takes a certain amount of energy to move a 4200 lb brick down the road, no matter what grade gas you use. Technology is fantastic but it can't repeal the laws of physics.

[chuckkent]

In Ohio the land where I live is very flat. I averaged 30.8 for 15 miles by feathering the gas pedal....... I average 26 to 28 on the highway in cruise at 60 and 19 to 21 around town.....One jack rabbit start and you can for get it but the fun of acceleration off sets the mpg every now and then.......

[kevenj]

Originally Posted by chuckkent

....... I average 26 to 28 on the highway in cruise at 60

In cruise at 60 on a flat highway? My car won't even GO that slow on the open road

[MoparCanuck]

I see detonation in your future.....

The compression ratio is set and isn't alterable except by heavy mods to the guts of the motor. That being said, 89 octane is more resistant to detonation that 87. The reason the SRT's need 91 is because their compression ratios are set even higher than our 5.7s (among other things) and need an octane rating even MORE resistant to detonation than we do.

You can use 87 as was previously said as long as your computer is retarding the timing in order to prevent detonation (or pre ignition, which is what happens on the compression cycle before the plug has a chance to fire as the gas explodes due to it reaching it's flash point as it's compressed and the temperature of the charge increases). But why would you use a gas that wasn't planned as being required? That's like saying you're running 14" rims on your car simply because you can.