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Which fuel should I use in my LUV/LUJ? Does it matter?

Posted by Jerry@BNR on 10/26/2016
This question pops up all the time on social media and forums, so we thought it would be helpful to make a blog post about it for people to reference. 

There are a number of articles published by so-called "Experts" that state that there is no benefit to running premium fuel on any car that does not specifically require it. This information is FALSE

On most modern GM vehicles there are multiple timing advance tables. Before we can discuss the engine controller's timing strategy, we must first understand what timing actually is.

Ignition timing, in a spark ignition internal combustion engine (ICE), is the process of setting the angle relative to piston position and crankshaft angular velocity that a spark will occur in the combustion chamber near the end of the compression stroke.

The need for advancing the timing of the spark is because fuel does not completely burn the instant the spark fires, the combustion gasses take a period of time to expand, and the angular or rotational speed of the engine can lengthen or shorten the time frame in which the burning and expansion should occur. In a vast majority of cases, the angle will be described as a certain angle advanced before top dead center (BTDC). Advancing the spark BTDC means that the spark is energized prior to the point where the combustion chamber reaches its minimum size, since the purpose of the power stroke in the engine is to force the combustion chamber to expand. Sparks occurring after top dead center (ATDC) are usually counter-productive (producing wasted spark, back-fire, engine knock etc.) unless there is need for a supplemental or continuing spark prior to the exhaust stroke.

Setting the correct ignition timing is crucial in the performance of an engine. Sparks occurring too soon or too late in the engine cycle are often responsible for excessive vibrations and even engine damage. The ignition timing affects many variables including engine longevity, fuel economy, and engine power. Modern engines that are controlled in real time by an engine control unit use a computer to control the timing throughout the engine's RPM and load range. Older engines that use mechanical spark distributors rely on inertia (by using rotating weights and springs) and manifold vacuum in order to set the ignition timing throughout the engine's RPM and load range.

Early cars required the driver to adjust timing via controls according to driving conditions, but this is now automated.

So now we know what ignition advance/timing is, and why we want more of it. 

GM ECUs employ multiple timing tables. After each refueling, the ECU will attempt to run the "High Octane Timing Table", which like it's name implies, has higher ignition advance values than the "Low Octane Timing Table".

Here you can see the high octane timing table from a LUV/LUJ (GM 1.4T) vehicle.

So the ECU will try to reference this table for ignition timing to get the best possible power, throttle response, smoothness, and economy. 

But if you're using low quality fuel, you'll encounter detonation/knock. When enough of this happens, the ECU will switch to the Low Octane Timing Table, shown below.

As you can see, significantly less advance is commanded on the low octane timing table. This is why you want to run premium fuel. If you use premium fuel, there will not be any detonation/knock, and the engine controller will stay on the high octane timing table, getting you more power and better fuel economy.

There are many, many other tables that the ECU references for final timing, like IAT multipliers, knock fast retard and recovery, and many more. This is just a very basic overview of how the base timing works, so don't panic if you go out to your car and fire up your ultragauge and see low timing values. That doesn't mean you are on the low octane table, but using low octane fuel will get you on it for sure!



Tim Hill
Date: 11/6/2016
Great read! Now I can't wait to get my tune even more! :)

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