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Unread 07-25-2013, 11:13 AM   #16
Kev M
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Originally Posted by ColdCase View Post
I think we have a failure to communicate. You seem to keep wrapping your head around a knock sensor an that was not what I was saying. I did not mention knock sensor, but a what is commonly called a predestination sensor or prediction algorithm.. Nothing like a knock sensor. Harley users a what the call an ion sensor system.

Bottom line is that, for the WK2, listening for ping and hearing none gives you false information.
My bad - I came back from a run and did some research. You caught me as I was trying to edit my post.

You might be right - or at least more right than I thought.

My switch from the automotive to the motorcycle and marine industries has left me a little lagging behind in that tech. I'm still seeing pezo knock sensors used, at least in the marine industry. I had forgotten about the Harley ION sensing systems on BTs (and didn't realize they went back as far as they do).

So you're right, it might be far more widespread.

That said, IF IT IS, that makes octane a much less important issue, since those systems are much more sensitive and better at preventing knock before any damage can occur. Yes?

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Unread 07-25-2013, 11:17 AM   #17
Kev M
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Originally Posted by ColdCase View Post
Harley users a what the call an ion sensor system.... That is also used in many new sportsters.
Well, a year or two ago, last time we checked into it on the XLforum that logic was NOT used in the Sportster ECMs. That might have changed by now, but I haven't heard of it.

Certainly I know for certain it was not used at LEAST 07 (introduction of the EFI Sporty - we have one) through 09 or 10.

I'd have to check with someone who has data-logged or gotten into the OEM maps of a later model to tell you for sure if it's still the case.
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Unread 07-25-2013, 02:10 PM   #18
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I believe the reference to 91 octane in the manual is a mis-print...I seem to remember this coming up several times in the last year.

FYI, I run 87 exclusively...and average about 16-17 in normal rural driving and 20-21 on the highway for distances.
2012 Grand Cherokee Overland Summit 5.7L V8 - Winter Chill with New Saddle Interior
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Unread 07-25-2013, 04:31 PM   #19
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Running the 87 at altitude is the same as running the mid at sea level. OP, you'll be fine. I lived in Denver for 7 years and even the dealers would say 85 is fine when regular was specified and 87 was fine when mid grade was specified.
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Unread 08-20-2013, 10:28 AM   #20
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Gasoline is gasoline, and contains the same explosive power, no matter which grade you use. Octane controls the burn (flame-front).

Too little octane for that engine-type means the gasoline may self-ignite from the high temperatures in the combustion chamber, pressure of the rising piston, or a hot-spot deposit on the piston or in the cylinder. This is called pre-detonation (ignition before the spark plug fires). When the spark plug finally ignites, you'll have two combustion wave fronts. When the two wave fronts collide, you'll get knocking. The PCM will retard the timing, lowering your optimal performance. This happens so fast, you won't even know it.

Too much octane for that engine-type means when the spark plug ignites the fuel mixture, the flame front is so slow that your don't get a complete burn, lowering your optimal performance. This unburned fuel goes out through the exhaust, affecting your emissions systems, specifically your catalytic converter. Also, because the exhaust stroke of the 4 stroke engine doesn't completely evacuate the cylinder, unburned fuel molecules remain in the combustion chamber, and will be deposited onto the piston, eventually building up, raising your compression ratio, requiring you to burn premium gasoline to stop pre-ignition.

Higher compression engines create more pressure and heat in the combustion chamber, just before top-dead-center. That's where your power comes from. Remember 13.5:1 compression ratios that required 104 octane? I know - I'm old...

Engineers have designed their engines for a complete burn, and recommend a specific octane to accomplish that. It's that complete burn that will give you the most power and mileage.
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Unread 08-20-2013, 08:22 PM   #21
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Originally Posted by Wasatchwk2 View Post
I know, I did read the sticky. It says the 5.7 is tuned for 89 and can run fine on 87, however, I was reviewing the manual and it says recommended for optimum preformance to use 91 octane.

So I was just wondering what everyone is using for their 5.7? I was planning on filling up with 87, I just wanted to be certian that this will not effect the engine in anyway.

Thank You,
We run either 87 or 91 in our HEMI, mainly 87 through and 91 every other fuel up or so. 89 is not readily available around here just 85,87 and 91 and sometimes you can find 89...
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Unread 08-21-2013, 03:16 PM   #22
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I operate just fine on a balony sandwich but will operate at maximum performance eating steak. I use 89 octane in my 5.7l cause it's recommended. Saying it's OK to use 87 is not good enough for me. It's your Jeep feed it what you like, I feed mine what the Jeep likes.
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Unread 08-21-2013, 06:05 PM   #23
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I know all of the Vette guys use to buy their gas as the additives and everything is controlled more at these stations.
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