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  #1  
Old 12-05-2010, 10:56 AM
jimmiefan jimmiefan is offline
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Default 89 Octane - why does the manual indicate to use 89 Octane?

why does the Ram require 89 Octane?
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  #2  
Old 12-05-2010, 11:03 AM
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I believe that the ECM is factory tuned to use 89 and that if you use 87 the computer compensates for it. I have been using the more expensive 89 since I want to ensure my engine is performing at it's max. However I may be wrong and throwing my money away so perhaps others will chime in and clarify as well.
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  #3  
Old 12-05-2010, 11:15 AM
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Originally Posted by RansRAM View Post
I believe that the ECM is factory tuned to use 89 and that if you use 87 the computer compensates for it. I have been using the more expensive 89 since I want to ensure my engine is performing at it's max. However I may be wrong and throwing my money away so perhaps others will chime in and clarify as well.
You're not wrong. Using cheaper fuel is possible but the engine won't perform at its peak because the PCM will pull off ignition timing to prevent spark knock.
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Old 12-05-2010, 11:49 AM
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compression determines Octane rating,
regular fuel will run anything that is made to run below a compression of say 9 to 1
from around that point & up, like 9.5 to 1 compression would use mid grade
old Hudson cars were 6 cylinder & had 12 to 1 ratio & we used Sunoco 103 octane

goggling i found this

The octane level required by an engine is determined by its compression ratio: higher-compression engines require higher-octane fuel. For example, a basic sedan generally only requires the standard octane fuel offered at gas stations, while a high-performance sports car or race car may require a high octane fuel. The owner’s manual lists information on the type of fuel you should use in your car.

Gas stations typically offer three different octane levels of fuel: regular, mid-grade, and premium. The regular grade generally has the octane level required by most cars, and mid-grade and premium are each a step up. Then, of course, there is the super high octane fuel available at race tracks, which is designed for the extremely high compression ratios that race car engines have. Pretty much all of the fuel grades commercially available have cleaning additives in them, so you needn’t choose a high octane fuel with the idea that it will clean your engine better.

Basically, you’re wasting your money if you buy high octane fuel. You should start off with the octane level your car’s manual calls for; if you still hear knocking or pinging, you might try the next step up. If the problem persists, you’ll probably need to have a tune-up or some diagnostic work done. Don’t think that putting high octane fuel in your little Toyota will make it run like a race car – it just doesn’t work that way.

your Ram 5.7L has a Compression Ratio 9.6:1
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Old 12-05-2010, 12:00 PM
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Ignition timing plays a role in determining octane rating as well. Think about it - you can choose different tunes such as an 87, 89, or 91 octane tune with programmers. Compression obviously hasn't changed but the ignition timing curve has.
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Old 12-05-2010, 12:18 PM
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That must explain why no matter witch tune i use 87 or 91 it seems to run better with 89 octane.. Great info!
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  #7  
Old 12-05-2010, 01:26 PM
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Compression ratio, timing, knock settings all contribute to a octane rating requirement.

My truck will run on 87 but not well. It's always in knock retard, the engine stumbles, it pings and the mileage sucks. It will run on 89 but not that great. It's always on the verge of knock.

I run 92 octane on a 91 octane CAI DS tune and it runs great. No loss of power because there's no knock, no timing problems and I get a lot better mileage than I was when it was stock on 87 & 89 octane.
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Old 12-06-2010, 07:06 PM
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Originally Posted by RamTech View Post
You're not wrong. Using cheaper fuel is possible but the engine won't perform at its peak because the PCM will pull off ignition timing to prevent spark knock.
Thanks for the clarification RamTech. I just spent $108.00 to fill up my tank today $1.19 per liter for 89. I was tempted to get the 87 for $1.12 per liter but resisted.
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