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  #13  
Old 12-01-2011, 04:52 PM
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Re: 87 or 89 Octane in Hemi?

91 octane. With some cars, a little higher octane will get you a little more HP. Lower octane will work but the ECM will cut things back a bit. I have no idea if the same applies to my Hemi but I go for the 91 anyway. Maybe a waste but whatever. I have also read where sometimes you don't get a true 91 (or 89/87) so I feel like I'm hedging a bit.
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  #14  
Old 12-02-2011, 08:21 AM
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Re: 87 or 89 Octane in Hemi?

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Originally Posted by drwaldo69 View Post
91 octane. With some cars, a little higher octane will get you a little more HP. Lower octane will work but the ECM will cut things back a bit. I have no idea if the same applies to my Hemi but I go for the 91 anyway. Maybe a waste but whatever. I have also read where sometimes you don't get a true 91 (or 89/87) so I feel like I'm hedging a bit.

Actually octane is a anti-knock additive that does not increase HP or gas mileage.
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Old 12-02-2011, 08:30 AM
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Re: 87 or 89 Octane in Hemi?

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Originally Posted by peckmv View Post
Actually octane is a anti-knock additive that does not increase HP or gas mileage.
True. But, on high compression engines tuned for 93 octane, running 87 octane will cause the ECU to retard timing, resulting in a loss of hp/torque.
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Old 12-02-2011, 08:50 AM
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Re: 87 or 89 Octane in Hemi?

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Originally Posted by pwguy View Post
This statement is true for using gasoline with an octane rating higher than an engine is designed for. If you use lower than the recommended octane both power and mileage will be adversely affected. The change may not be noticeable to most people under normal conditions.
Octane rating doesn't affect mileage.

http://mn.gov/commerce/weights-and-m...ctaneFacts.pdf
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Old 12-02-2011, 08:53 AM
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Re: 87 or 89 Octane in Hemi?

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Originally Posted by JGCCAYENNE View Post
True. But, on high compression engines tuned for 93 octane, running 87 octane will cause the ECU to retard timing, resulting in a loss of hp/torque.
It will reduce what is known as "motive power" not HP or brake horsepower. What happens is that with a lower octane fuel, the engine's computer will retard the timing which results in a motive power reduction.


http://en.wikipedia.org/wiki/Motive_power
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Old 12-02-2011, 10:53 AM
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Re: 87 or 89 Octane in Hemi?

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Originally Posted by peckmv View Post
It will reduce what is known as "motive power" not HP or brake horsepower. What happens is that with a lower octane fuel, the engine's computer will retard the timing which results in a motive power reduction.


http://en.wikipedia.org/wiki/Motive_power
Motive power in this case is torque. If torque is reduced, hp is reduced as well since hp is just a mathematical calculation derived from torque, at any given rpm. That is why, on a dyno plot, hp and torque always cross at exactly 5252 rpm.

hp = (torque * rpm)/5252
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Old 12-02-2011, 11:09 AM
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Re: 87 or 89 Octane in Hemi?

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Originally Posted by JGCCAYENNE View Post
Motive power in this case is torque. If torque is reduced, hp is reduced as well since hp is just a mathematical calculation derived from torque, at any given rpm. That is why, on a dyno plot, hp and torque always cross at exactly 5252 rpm.

hp = (torque * rpm)/5252
I see what your saying but, my main point is that octane doesn't increase the btu's of fuel, therefore no increase in HP. I have heard people say that higher octane gives them more HP and better gas mileage and thats not really true. I usually refer to brake horsepower which is a power measurement based on an engines ability to maintain its output under load. I don't mean to be argumentive. I just enjoy this discussion.
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Old 12-03-2011, 07:09 PM
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Talking Re: 87 or 89 Octane in Hemi?

I picked up our new overland on weds. and I can only assume that they filled it with 87 because I noticed a little lag at tip in with auto mode. i was down to about 1/4 tank today and topped off with 93 and there was an immediate change. The reply about the timing being retarded if lesser fuel is used is correct, I believe that is exactly what happened the HEMI is a relatively high compression engine at 10.5:1 and the lower flash point can affect timing.
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Old 12-03-2011, 07:43 PM
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Re: 87 or 89 Octane in Hemi?

I thought premium fuel shouldn't be used in the hemi? Anyone know?
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Old 12-03-2011, 07:56 PM
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Re: 87 or 89 Octane in Hemi?

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Originally Posted by jgc4ever View Post
I thought premium fuel shouldn't be used in the hemi? Anyone know?
It is tuned to produce maximum power with 89 octane. Anything over that is a waste of money. If you use 87 octane, the ECU will very slightly retard timing. My guess is that you will lose about 5 lb-ft.

What he may have noticed is the electronic throttle and transmission shift map adapting to his driving style after about 100 miles. It is covered in the Owner's manual.
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Old 12-04-2011, 07:14 AM
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Re: 87 or 89 Octane in Hemi?

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Originally Posted by JGCCAYENNE View Post
It is tuned to produce maximum power with 89 octane. Anything over that is a waste of money. If you use 87 octane, the ECU will very slightly retard timing. My guess is that you will lose about 5 lb-ft.

What he may have noticed is the electronic throttle and transmission shift map adapting to his driving style after about 100 miles. It is covered in the Owner's manual.
I plan to use 89 but topped with 93 as there was about ten gallons left in the tank. The throttle and shift mapping have been making adjustments but we were right at 201 mi. when I filled and there was an additional difference. Maybe it was shitty fuel to boot.
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Old 12-04-2011, 08:06 AM
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Re: 87 or 89 Octane in Hemi?

A petroleum executive once told me 87 octane is a commodity in the oil industry meaning it could have been refined by any company anywhere. He advised I always go with brand name 89 or higher because it was refined to the exact standards of the folks with their name on the big sign at the gas station. 89+ when purchased from the big boys is the as-advertised stuff with detergents and whatever, assuming the retailer doesn't mess it up with sludge in their tanks or shady practices. This executive spent a career in his industry so maybe he drank too much of the cool aid but since I heard this I've stuck with 89 or higher.
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