Chrysler 300C & SRT8 Forums banner

1 - 20 of 373 Posts

·
Registered
Joined
·
7 Posts
Discussion Starter #1
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.
 

·
Registered
Joined
·
534 Posts
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?:rolleyes:
 

·
Registered
Joined
·
544 Posts
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.
 

·
Registered
Joined
·
1,036 Posts
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.
 

·
Registered
Joined
·
731 Posts
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.
 

·
Premium Member
Joined
·
4,541 Posts
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.
 

·
Premium Member
Joined
·
4,541 Posts
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!:pat: 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.
 

·
Registered
Joined
·
544 Posts
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.......
 

·
Registered
Joined
·
774 Posts
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.
 

·
Registered
Joined
·
24 Posts
I have a 5.7 and always use premium 91 oct in my car. Am i wasting my money since the manual recommends 89? Is my stock compression too low for 91 oct fuel?
 

·
Registered
Joined
·
774 Posts
I have a 5.7 and always use premium 91 oct in my car. Am i wasting my money since the manual recommends 89? Is my stock compression too low for 91 oct fuel?
I'm using 91 as well, and mine is stock. I chose 91 because under certain loads and conditions, the engine may require a higher octane to avoid the timing from being retarded, and I feel better using the higher octane just in case.
As well, as an engine ages, it's octane requirements tend to increase. This is due to more deposits forming resulting in less space in the cylinder, thus increasing compression. I don't care about the $ difference, as it doesn't amount to more than 0.20 a gallon. Others will dissagree with me, but in the end, it's only MHO.
 

·
Registered
Joined
·
544 Posts
If your car doesn't spark knock under load then the octane is high enough. Putting higher octane in an engine that doesn't need it is wasteful. Most engineers say that a small amount of knock is acceptable. If you are using higher octane where it is not needed the detonation in the cylinder is occurring late after TDC. So the optimum situation is to have detonation slightly before TDC and not after. Many manuals say that running too high of octane gas will actually decrease performance. There is a lot of snake oil sales going on in the auto business....don't fall for it!!!!!!
 

·
Premium Member
Joined
·
4,541 Posts
I'm using 91 as well, and mine is stock. I chose 91 because under certain loads and conditions, the engine may require a higher octane to avoid the timing from being retarded, and I feel better using the higher octane just in case.
I don't believe that is correct; While the computer will retard timing as needed to compensate for 87 octane (resulting in less performance but no engine damage) I don't believe it can advance timing any further than normal to gain any advantage from using HIGHER octane gas, unless of course you have a programmer (flashpaq or predator) set to a 91 or 92 octane performance tune. Without a programmer the 300C is designed for 89 and putting a higher octane fuel in it will not result in a more aggressive (advanced) timing curve. So you get no benefit, and it actually CAN be counter productive. If you use substantially higher octane than is needed you can end up with unburned fuel going straight into your catalytic converters (as Rambit has pointed out already).


So Hey does anyone know what kind of OIL i should run ? :biggrin: :biggrin: :)
 

·
Registered
Joined
·
106 Posts
You are sooooo right. I work for Chrysler in the service world, and trying to explain fuel milage is the hardest thing due to so many different reasons that effects fuel milage. 30 mpg. PLEASE. Let the 6.1 get mds and I will look at it as well.
 

·
Registered
Joined
·
1,480 Posts
hmm

I posted this question a little while ago, i was putting 91 in my car for three weeks after i got it, the manual says 89, so i decided to ask the people on this forum what they would do.... here is the thread... basically what i learned is, go with what the manual says, chrysler probably knows best. According to the manual. 2.7=87 3.5's & 5.7's = 89, srt's = 91.

http://www.300cforums.com/forums/general-discussion/51729-gasoline-89-91-a.html
 

·
Registered
Joined
·
353 Posts
As I have told my wife on occasion, there is only one steering wheel in each vehicle. And at the time I have control of it. Likewise, everyone has to drive their own car. But for me, I will use the octane gas that the guys that made the car recommend. I don;'t like paying $3.00 plus for gas either. But if I want to get down the road I will buy it and not go cheap for the sake of the life of the motor.
 

·
Premium Member
Joined
·
2,691 Posts
The static CR on a 5.7L MDS Hemi is 9.6:1 and that is hardly 91 octane territory (or even 89). I'm not saying during the summer with "spirited" driving you won't see a performance difference between 87 and 89, you may be able to measure a small difference. No damage is going to occour at these compression ratios and during the winter I will run 87 all day long. Come summer I will experiment and see how midgrade and premium perform. I also plan to mod, after I mod it will be run on the higher octane fuel since I will be altering the PCM's programming.

As for milage... I've not seen 26MPG yet.... My average is 15.7 :)
 

·
Premium Member
Joined
·
4,541 Posts
Compression ratio is only ONE of many factors that determines what octane you need and whether an engine will detonate or not on a given octane rating. Ignition timing (controlled by the computer as previously stated), valve timing (not adjustable on the current Hemi) and Valve overlap all play a big part as well. One engine with 9.6:1 compression ratio may do just fine on 87 while another design engine with the same compression ratio could require 91 octane. Stick with what the manufacturer recommends since they know the whole equation.
 

·
Premium Member
Joined
·
2,691 Posts
Keven,

Well I didn't really care to go into all the details, of course if you want to we certainly can. I built my first 11 second on street tire Mopar when I was 16. I do know a little bit about engines (Hemi's and Wedges and a few others). There are many many more factors than even what you listed. But a general rule of thumb is that a sub 10:1 static compression engine (with alloy heads no less) is not likely to detonate. Add to that the fact that alloy heads do not hot spot like cast iron and then add to the fact that it's a hemi which has much larger combustion chambers with less sharp edges to retain hot spots than your typical wedge, less cause for detonation.

And then of course we can go into the whole PCM controlls the ignition timing, knock sensors and all the other technological whiz bang that makes the modern multipoint efi system work... It's not going to detonate on 87 and I'm not "cheap" for using it. There's a time and a place for premium and it ain't now...

Ya know.. This is my second "new" hemi (and certainly not the first "hemi" in the family considering pops was a two time Nat record holder with a 68 SS/AA Hemi Cuda one of 50 ever made) I tow a 9K boat with an 04 non MDS Hemi Ram on 87 and it's never suffered from detonation.

And I do agree, stick with what the manufacturer recommends and they say for the 5.7 that 87 is just fine.

And I think this is a great topic by the way! :)
 
1 - 20 of 373 Posts
Top