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Curious IGN info using ScanGaugE

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Recently I purchased a scangauge I was curious about the extra information provided by the cruze computers that really drivers dont have access to. Most of the gauges such as intake air temperature, manifold absolute pressure, throttle position sensor and fuel pressure are interesting to keep an eye on.

I found that I was most interested in the ignition timing guage and should note I use 87 octane. The values given for this guage ranged from a high of 49 to a low of about - 8 which if im not mistaken any numbers below 0 indicates knocking or retard. While the cruze was being driven efficently the guage would show 30-49, normal driving conditions between 15 and 25, under load it would show 10 to 0 and under heavy load in my case fast speeds and up steep terrain 0 to - 8. Well I noticed something curious when coasting downhill and as soon as the injectors cut off or whatever the cruze does when coasting downhill I forget the term. Anyways I noticed when coasting downhill gettin 99mpg the ign guage would show negative timing values such as -5 or -2 indicating knocking or retard while coasting downhill ? At the same time if I would slightly be on the throttle downhill it would show efficent timing such as 40-45. So im curious why would the engine show knock or ping while coasting downhill in fuel efficent mode but if the throttle was slightly engaged show high efficiency ign numbers ?

One additonal note , while coasting downhill the dic will obviously display 99mpg max but scangauge was showing upwards of 220 mpg.
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Discussion Starter · #2 ·
Anyone able to comment about why scangauge ignition timing values go negative and show knock or ping while coasting downhill in fuel efficent mode but if the throttle was slightly engaged show high efficiency ign numbers ?
 

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Anyone able to comment about why scangauge ignition timing values go negative and show knock or ping while coasting downhill in fuel efficent mode but if the throttle was slightly engaged show high efficiency ign numbers ?
If the injectors are shut off you really don't have a need for spark. You should never ever never ever have knock or ping, it is very bad. Pre-ignition and detonation can and will destroy an engine. If you are actually seeing knock retard under a load with a scan tool, go with better fuel or don't load up the motor hard. Back to timing, the idea is to end the burn at 24* after TDC, so engine speed, load, and the amount of air/fuel determine when you want to start the burn.
Looking at the original post I'm not sure you are actually looking at knock you are only looking at timing and assuming knock?
 

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Discussion Starter · #4 · (Edited)
But what it seems like to me is the fuel im using is dependant upon the terrain. If im driving on a normal flat surface the fuel is adequate and effiecent but when the terrain changes to an incline or decline the fuel im using becomes obsolete. Is it really that simple ? Is it the fuel or the terrain that causes the pre ignition.

I will be switching octanes from 87 to 91 to test my theory of octane vs elevation as it stands 87 is sufficent.
 

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I don't think you are seeing knock going down hill, it just kills the fuel to such a low level the timing is backed way off, the burn time is short and still needs to end after TDC. Going up hill you add load so the timing backs off from normal cruise because the burn time goes down under higher compression ratio, but it shouldn't be after TDC going up a hill under load because you have more injector pulse width.
Think of it like this with RPM we advance and as pressure increases in the intake we retard.
 

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Discussion Starter · #6 · (Edited)
I don't think you are seeing knock going down hill, it just kills the fuel to such a low level the timing is backed way off, the burn time is short and still needs to end after TDC. Going up hill you add load so the timing backs off from normal cruise because the burn time goes down under higher compression ratio, but it shouldn't be after TDC going up a hill under load because you have more injector pulse width.
I understand the first sentence that makes sense to me. Im not sure im smart enough to understand why the same fuel is unable to provide the energy needed to sustain the power necessary to climb the mountain without causing damage to the engine.

For my own curiosity I need to use 91 octane in an attempt to provide evidence that the values I see using scanguage are in fact affected by the difference in the octane.
 

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Preignition occurs when the spark plugs fire too early while the piston is still on its compression stroke moving upwards, causing the piston to want to move downward, when momentum wants it to go up. Problem is way too much spark advance. Knock sensors in the Cruze are suppose to prevent this and do retard the spark.

Detonation occurs strictly on using a low octane fuel, heat is generated when the piston is on its compression stroke, and low octane fuel will ignite at a much lower temperature than high octane fuel. Has nothing to do with ignition timing and typical cause of detonation is excessive carbon buildup in the cylinders that drastically increases the compression ratio.

With the older high compression engines with a 10.5:1 compression ratio, had no choice but to use a high octane fuel or you will get plenty of detonation. Seen some so bad with added carbon, when you switched off the ignition, engine will still run without ignition but sounded like it was going to fall apart.

Using high octane fuel with knock sensors, lets the engine run with more advance for more efficiency and far better fuel economy.

In my neck of the woods, 87 octane fuel is always ethanol based that uses a very low octane fuel depending on ethanol to effectively increase the octane rating. But done by state run facilities not under the control of the EPA, so you don't know what in the heck you are getting.

And besides alcohol and gasoline does not form a homogeneous blend, is a mixture and can get a bad mix with very low ethanol content, thus detonation. Plus ethanol is highly corrosive for even more problems. Added to this problem problem, farmers around here can make a lot more money selling corn than feeding it to cows, so a shortage of beef and diary products, prices have skyrocketed. A bit of topic, but a major problem. And you don't dare use this stuff in marine and small engine applications.

Using Protorque with my smart phone, not even close to the spark advance numbers you are getting. Appear to be far more normal.
 

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besides alcohol and gasoline does not form a homogeneous blend, is a mixture and can get a bad mix with very low ethanol content, thus detonation. Plus ethanol is highly corrosive for even more problems.
Fresh ethanol fuel bought from a high volume station has none of the issues you describe. Only time E10 fuel separates if enough water is introduced to the fuel. The cruze fuel system as with all modern cars are designed to handle ethanol fuel, ethanol is not corrosive what so ever in this fuel system. Owners manual clearly states all 2012+ GM cars and trucks can run up to E15 (15% ethanol) without any issues.

Since your so into giving us our daily "back in the day" remember adding 'heat' to cars fuel in the winter? that little pint of alcohol was enough to deal with about a cup of water. Current fuels with a 10% alcohol blend no longer need this added alcohol and since 10% is a higher volume than that little pint, modern fuels can handle even more water.

The real issue with ethanol is people leaving the fuel in the tank for months, leaving the tank half full or less allowing condensation to form in the tank. Most of the time this is the reason people have issues with ethanol, improper use and storage of the fuel.
 

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. Im not sure im smart enough to understand why the same fuel is unable to provide the energy needed to sustain the power necessary to climb the mountain without causing damage to the engine.
87 octane is riding the knock sensor heavily even when under light load. I suspect when you switch to 91 octane you will see the motor running much smoother. Part of the problem is not only are you running low octane 87, but no ethanol too. Ethanol fuel has cylinder cooling properties(reduces knock), so regular 87 octane e10 would run better too.

I usually run 91 no ethanol or 93 E10. My car runs better on the E10 fuel but gets slightly better MPG with the no ethanol gas. My local ethanol plant sells 90 octane E15(15% ethanol) for less than the cost of 87octane fuel. My cruze loves this gas, runs so smooth and has so much better low RPM power. I see virtually no knock running this fuel. I do see a reduction in MPG with E15, however no more than E10 fuel.
 

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87 octane is riding the knock sensor heavily even when under light load. I suspect when you switch to 91 octane you will see the motor running much smoother. Part of the problem is not only are you running low octane 87, but no ethanol too. Ethanol fuel has cylinder cooling properties(reduces knock), so regular 87 octane e10 would run better too.

I usually run 91 no ethanol or 93 E10. My car runs better on the E10 fuel but gets slightly better MPG with the no ethanol gas. My local ethanol plant sells 90 octane E15(15% ethanol) for less than the cost of 87octane fuel. My cruze loves this gas, runs so smooth and has so much better low RPM power. I see virtually no knock running this fuel. I do see a reduction in MPG with E15, however no more than E10 fuel.
Exactly. I suspect he might see a little less knock on 87 E10, but under heavy load and low RPM conditions you are flirting with the knock sensors often. Higher in the RPM band for pulling hills knocks less, or running a higher octane will enable you to run at those low RPMs and fully utilize the torque provided by the small turbo to pull you up.
 
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