DATALOGGING: PART II - Timing
Understand and Diagnosing Timing
Today's focus is on ignition timing. What is timing advancement, what is timing correction, and how can we address it? Keep in mind, this isn't a "how to tune" article, but rather a how-to on reading and diagnosing timing-related issues with data logs.
Timing Advancement, what is it?
Photo Credit: CarThrottle.
Like all modern cars, the EA888gen3 is a 4-stroke engine, intake (down), compression (up), power (down), exhaust (up). Shown above is the compression stroke and the power stroke. To maximize power, we need to maximize the cylinder pressure in the power stroke. There is an actual delay from the time the spark happens to when the air-fuel mixture actually burns. For example, if we waited until top-dead-center (TDC) to ignite the fuel, the piston would have already moved part of the way down before maximum pressure is achieved.
To reiterate, if temperatures are too hot, or pressure in the cylinder is too high on the compression stroke, the gases will explode on their own (without the spark plug). If we advance timing too much and start the burning process too early, a separate detonation can happen. And yet, advancing it earlier helps maximize pressure at TDC. This subject can get very quickly very fast, so here's the TL;DR: we want to advance timing as much as possible without significant correction, particularly at high RPMs (>5500). If you're seeing more than 12 degrees advance on pump gas, give yourself some kudos. About 15 degrees advance at redline is limit for making power.
Reading the Log - Timing
Whether your logging with the JB4, Cobb, or Eurodyne, you should have a series of columns named something to the effect of "Ignition Timing Correction Cylinder <X>". If you're not sure, the values in the cells should look something like this:
Comparison of logger output. Some columns moved or hidden for simpler viewer.
Most loggers show timing correction as a negative number (Eurodyne, VCDS, Cobb), but some loggers do show it as positive (JB4). Either way the absolute value range should be 0-5ish, although some extreme cases could be as high as 7 or 8. If you've ever heard someone say "that's a really clean log" what they probably mean is that timing correction is all 0.
Timing correction means that fuel has detonated at a time other than the spark plug. Specifically, we pull timing when separate denotation happens after the spark. This is known as "knock" because two (or more) flame fronts are created (some from heat and compression, and another from the spark plug), which interfere causing vibrations which sound often sound like a metal pinging. Luckily, our modern cars come with knock-sensors, so we can detect when knock happens, and correct timing.
Example of an "acceptable" amount of timing correction. Lots of cells without timing correction, and timing correction peaks at -2.99, which is still acceptable. We also see the that the timing advancement hit 10 degrees! Great!
The goal here is to see very low numbers in the timing correction fields. What is low? Well, you'll probably get a different answer from everyone you ask, but for me, I don't like to see more than 2-3 degrees correction.
Here's why. Our engines were designed with knock sensors for a reason -- they can correct timing and do it very fast. If we only need minor corrections (1-2 degrees), we incur minimal adverse effects. If we're trying to maximize power but we see no timing pull (straight 0's in the log), this means that we likely could have pushed timing further in advance and made more power. On the other side of things, more than 3 degrees of correction implies that we are having a significant amount of knock. While we can correct for it, it is likely taking multiple iterations to correct it, causing enough knock to loose power, and putting excessive wear on the engine.
This is on the high end of timing correction in my opinion. Consistent correction across every cylinder, exceeding 3* in some cases.
Note that corner cases exist, particularly, at times where we want absolutely no timing correction. One example is a race scenario with extended periods of abuse. The engine is already being stressed enough, so why add extra stress chasing a few hp? Additionally, people running meth and/or race gas may be at point where timing cannot be further advanced in such a way it is beneficial. Because their fuel mixture can support the high degrees of timing advancement, they will also see 0 timing retardation.
Reasons for Timing Correction
If you have 0 timing correction, things are looking good. But if you have is more than you want, it's time to start debugging why. Keep in mind as you read through this, a lot of this issues can be linked together.
[FUEL]: The first and most obvious answer is fuel. The higher the octane, the less likely it is detonate on its own, and thus, prevent engine knock. If you're using cheap gas, try switching to Shell. While there are other great fuels out there, it's one that across the country (and world) is available, and consistently performs. If you're still having issues, you can also try adding an octane booster, or, adding 1-2 gallons of E85. E85 burns cooler, helping keep the cylinder temperatures cooler, thus preventing detonation. While E85 doesn't technically have an octane, you can think of it as an octane booster. Try out different fuels and log again and observe effects.
[HIGH IAT]: As mentioned earlier, the higher the internal temperatures, the more likely fuel is to detonate prematurely. If IAT's are higher, than of course internal cylinder temperatures will be higher. This is a very easy and clear thing to look at in your data log, in the field labeled "IAT" or "Intake Air". Note that in Eurodyne it's in Celcius, while Cobb it's in Farenheit. While there are many factors to take into consideration here, when IAT's start going above 100F (38C) it's likely that this is a contributing factor to your timing correction. (The choice of 100 is somewhat arbitrary, the lower the better in general, but particular problem seem to arise between 100-120F and up). Options are to either add a larger (and better) intercooler and/or add water-meth injection. Water-meth in someways is the best, because a good IC will still only get you to near ambient temperature, while water-meth acts as a cooling agent AND octane booster for more consistent results.
[OVERLY AGGRESSIVE TUNE]: If you've had an EPC light "pop" this is the most likely culprit -- too much timing correction due to an overly aggressive timing map. If you're target timing value is too aggressive for your setup, you'll constantly be correcting for it. Ignition timing on our ECU's is rather aggressive about trying to advance timing as much as possible, so it won't adapt in the same way long-term fuel trims do. Additionally, because our ECU's do try to advance timing, if you previously were running on a higher octane setup, and move to a lower octane setup, you'll likely see an increase timing correction (or even an EPC light) as well. The ECU learned a more aggressive timing map, and will now be correcting accordingly. This particularly happens with JB4 guys running E-blends with inconsistent ethanol content.
One other note here is that Unitronic leaves the EPC fail safe on, so you'll see more EPC lights with Unitronic than others. That doesn't mean what's happening to trigger the EPC light isn't happening with other tuners, it's just means we hear about it more. I have overall seen timing correction issues on 91 octane maps from Canadian tuners (Eurodyne, Unitronic). My assumption is that their Canadian gas which they used for the 91 tune is better than our American 91 octane.
[LEAN AFR's]: While we'll cover AFR's more later, if your AFR's are overly lean, it also can cause premature detonation. You may have heard the term "rich is safe", and there's definitely some truth to that. I'll discuss in more detail later, but if your AFR's are > 14.5 while in boost (> 1 lambda), it's likely contributing to engine knock. Seeing 1.01 lambda isn't something to freak over, but 1.05 is definitely cause for concern. I've seen this issue pop up particularly with people that use cheap grocery store gas. When fuel is imperfect (e.g. water in the gas), the computer is having to make more adjustments to fuel trims. Many factors can cause adjustments to fuel trims, but typically when fuel trims are out of whack these overly learn conditions occur.
[IGNITION COMPONENTS]: If none of the above options seems to explain timing correction, it's time to start looking into hardware. It's also probable that a bad ignition component is causing an inconsistent spark. Start with spark plugs -- I recommend either factory NGK plugs, Denso IKH24, or Brisk ER12S. Keep in mind, If you have RS7 plugs, you can think of them as being "recalled". Lastly, you can try swapping out ignition coil packs. In my personal opinion -- don't waste your time upgrading to RS3 coilpacks. The stock ones work just fine and have made 11.1 second passes in the 1/4 mile.
Today we covered ignition timing. Hopefully you now have an idea of what timing advancement is, and how it helps make more power. We covered how to read the data logs, and when to start worrying about too much correction. We also worked through a number of possibilities for why timing correction is occurring. Next up -- boost!