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Old 09-22-2014, 11:59 AM   #1
Evergreen6
 
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Seat of the pants octane test - LFX

I'll start by saying I've been a proponent of running the bare minimum octane requirement for cars that get used as daily drivers. Unless the manual stated otherwise, I would use 87 octane.

So I tried my own unscientific test. I had a buddy with me one day, and I handed him my credit card as we pulled up to the gas pump. I told him I'm gonna go grab a soda (would he like something, as well?)- and to please fill the car full (tank just about empty) with any grade of fuel. Pick a grade of fuel, don't tell me, and wad up/ throw away the receipt too.

When I returned to the car with 2 sodas, the car was filled, filler nozzle back on the pump. I had no idea what fuel grade was used, but the car was full. The car, to date, was getting a solid 27.0mpg average on my daily 80 mile commute.

I drove the car back/forth to work for the week (which ended up being 4 days of driving). The one thing I thought was different was the car seemed to pull better in 6th gear. There's a 70mph section of freeway that steadily climbs a mountain pass. With the A/C on, that stretch has always felt sluggish when I'm getting out to pass or trying to accelerate out of a 55-60mph hole as traffic clears for a major exit.

So that was it. A simple, but noticeable change. 5th felt a little bitter too between about 2000-2500 rpm too, same feeling as 6th. Just a little more punch.

Week 2 rolled around. Same buddy with me that second weekend, and I said we're going to do it again. Pick a fuel, any fuel (except diesel). I'm going to go use the facilities and grab a soda--here's my card, don't tell me what you put in.

Rinse, repeat with the work commute. 5th and 6th continued to feel better on the long grade.

The average MPG hasn't been reset since I bought the car, and it had 5500 miles when we started this adventure. By about 6,100 miles the average mpg had gone from 27.0 to 27.5. It's just an observation, and many factors could have played a part. The temperature dropped on the second week, so I wasn't using A/C as much. But the commute was about the same. No extra stops. No extra trips.

So at the end of that little experiment, ready to fill up the car again I sent my buddy a text. "So, what have you been putting in the tank?"

89 octane. Both times. Car has only ever had 87 in it prior to this.



So, there, a blind fuel grade test, 2 fill ups from less than 1/4 tank till full, same gas station, 2 weeks apart with the same exact driving between fillups.

Now-- does anyone have any data to support that the LFX ecm is able to adjust fuel mapping and get better performance out of the engine with increased octane?

It's always run fine on 87 octane, but there was definitely a change with 89. Now I'm trying to find hard data to support it. Help please
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Old 09-22-2014, 12:22 PM   #2
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It's my understanding that our LFX's have a pretty high compression ratio and can benefit from higher octane fuel. They will run just fine on 87 but I think the ecm will pull timing to prevent predetination. This will in turn make things a little more sluggish. I personally have been running 93 in mine from the beginning. I figure, if I can afford it, why not give the engine the best chance to make the power, right? I've read that the LFX's only have a single fuel map table, unlike the V8's, so there is no need to do a fuse pull to get it to run on the higher table if you up the octane, as you would have to do for an SS.
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Old 09-22-2014, 12:22 PM   #3
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There was one member that actually did run some data and it proved to be better even slightly..I'll keep digging


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Old 09-22-2014, 12:26 PM   #4
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Its been stated by a tuner that the LFX has seperate timing tables for octane grades.
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Old 09-22-2014, 12:27 PM   #5
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I switched to 93 awhile back, I feel can feel the difference.
Only hard fact I can give here is I was getting black crap on my exhaust tips with 87 and with 93 that problem almost went away.
So it's 93 for me.
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Old 09-22-2014, 12:28 PM   #6
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Quote:
Originally Posted by snymat68 View Post
It's my understanding that our LFX's have a pretty high compression ratio and can benefit from higher octane fuel.
Yeah, I was surprised when I saw the stats on the LFX. The LFX has a higher compression ratio than the LS3 (11.5:1 for the LFX and only 10.7:1 for the LS3).
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Old 09-22-2014, 12:32 PM   #7
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11:5:1 compression is very impressive.. Not too long ago you needed 103 octane minimum to run that


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Old 09-22-2014, 12:32 PM   #8
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I still get black soot with the 93, lol. I have an intake and some exhaust work though (see signature), so maybe that has something to do with it. Or maybe they just run rich or I hammer on it too much, haha.
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Old 09-22-2014, 12:37 PM   #9
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Quote:
Originally Posted by snymat68 View Post
I still get black soot with the 93, lol. I have an intake and some exhaust work though (see signature), so maybe that has something to do with it. Or maybe they just run rich or I hammer on it too much, haha.
No mods here, and I also have noticed mild soot.

Not as much soot as when I owned a turbo, but still sooty nonetheless.
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Old 09-22-2014, 12:45 PM   #10
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The only way dissect it further would be to log lots of information at the various octanes. The LFX has many fuel/spark tables (not just octane, but ethanol content as well). However, just because they are there doesn't mean it is using them. The Impala LFX is a FlexFuel platform. It has a special sensor in the in-tank fuel to determine the % ethanol and adjust which table it uses.

A starting point would be to log the commanded spark advance, commanded knock retard (KR) and actual spark advance. KR + actual Spark Advance should map closely to commanded Spark Advance for a given fuel table. For example, if the low-octane table has a commanded timing of 26* at 2500 RPMs for a certain load, and the knock sensors are commanding 6* of knock retard, then the actual spark advance should be around 20*. If you put in high-octane gas and you still see a commanded timing of 26* and KR of 4* (actual spark advance of 22*), then it probably didn't change the table it is using.

Now this is if everything else is the same (which it rarely is), and the E39/E39A has several strategies for addressing knock. It can adjust short-term fuel trims and decrease dynamic compression ratio. There are lots of variables to try and monitor to tease this all out. My guess is, there is a parameter in the OS that tells it which table to use on our cars, but I really haven't looked for it.
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Old 09-22-2014, 12:47 PM   #11
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Quote:
Originally Posted by snymat68 View Post
I still get black soot with the 93, lol. I have an intake and some exhaust work though (see signature), so maybe that has something to do with it. Or maybe they just run rich or I hammer on it too much, haha.
Octane should not affect AFR. Higher octane only allows for the use of more advance in the timing, which generally means more power. The definitive way to determine if higher octane fuel will help is the use of a scanner like EFI Live, HP Tuners etc. If your scan shows timing retard due to knock sensors, higher octane fuel will make your car run better, or at least allow you to run it harder.
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