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Tuning for Power

Tuning for power is primarily about balancing three factors…

  • Ignition Timing
  • Boost
  • Air/Fuel Ratio

…without causing detonation.

Detonation

Detonation is the leading cause of death for high-performance engines. (Over-revving is also bad news, but that’s pretty obvious and adjusting the rev limit is not rocket science.) Before you start chasing power, you should first get a clear picture of what detonation is, what causes it, how to detect it, and how to avoid it. The following links should be helpful:

http://www.streetrodstuff.com/Articles/Engine/Detonation/index.php

http://www.motorcycle.com/how-to/wrenching-with-robchemical-soup-the-mystery-of-detonation-3420.html

Also read this link to learn about how the Subaru ECU uses reduced ignition timing to mitigate knock:

http://www.romraider.com/RomRaider/HowToUnderstandKnockControl

Detecting detonation is fairly straightforward when working with a Subaru ECU that supports the ‘knock sum’ logging parameter. When the ‘knock sum’ counter increments, you’ve probably had detonation. Probably. There’s also a fair amount of false knock (especially around idle and cruise), and even the stock tune will cause some knock sum increments, so it seems that there’s going to be a little bit of random knock even in a good tune. If you pull timing every time you see a knock sum increment, you’ll find yourself running several degrees less timing than you should, and still knocking every once in a long while - I’ve fallen into this trap myself. If you notice the ECU knocking at the same RPM & load point in more than one pull, then you’ve got real knock.

If your ECU does not support the ‘knock sum’ logging parameter, you’ll need to log three parameters and study them carefully:

  • Fine Learning Knock Correction
  • Feedback Knock Correction
  • Ignition Advance Multiplier

Analysis of those parameters is beyond the scope of this document, but the “Subaru’s Knock Control” link above will help you understand them.

Now let’s consider those three power factors again…

Ignition Advance

More ignition advance will give more power, up to a point. The ideal ignition timing would start the combustion process at just the right moment to create peak cylinder pressure at the same moment that the crank angle can most efficiently turn that pressure into torque. For every combination of RPM and load, there is an ignition timing that will produce the maximum torque.

However, with Subaru engines, and straight pump gas, you won’t be able to run that timing unless you’re running race gas, or not making much boost. As you advance the timing, the engine will begin detonating before it reaches maximum torque. So, for WOT pulls, we add ignition advance until it starts knocking, then we back off a little bit to provide safety.

If you’re using race gas, E85, or meth injection, steadily advancing the ignition timing you may not trigger knock before reaching maximum torque. I’ve only used pump gas, so I can’t help you with that.

Boost

More boost will give more power, up to a point. More boost means more heat added by the turbo, and more heat means you have to reduce timing. At some point, adding boost will require retarding more than enough timing to prevent any power from being gained.

If you want to understand the physics behind choosing boost levels, Garrett has a web page for you, and Google will find you several others that tell essentially the same story:

http://web.archive.org/web/20110615150058/http://www.turbobygarrett.com/turbobygarrett/tech_center/turbo_tech103.html

However, chances are very good that someone else has run your turbo on a Subaru motor before. Look at a few boost curves from different tuners, and you’ll probably see a trend. Start with that.

AFR

An AFR of 14.7 gives the best balance of fuel economy and emissions, so this is what we use in the ‘cruise’ region (roughly, everything below 4000 RPM and 1.2 g/rev load). An AFR closer to 12.5 will give more power, because the extra fuel helps to ensure that the combustion process burns every last bit of oxygen. However, even-richer AFRs suppress detonation, thus allowing ignition advance that will give even more power. Richer AFRs also slow down combustion though, which costs power. For turbocharged Subaru motors, under boost, AFRs in the low 11s are pretty typical (11.3:1 - 11.0:1). Most tuners seem to think that’s the sweet spot where you get a good balance of detonation suppression and good combustion. A few tuners prefer AFRs around 12:1 or 10.5:1, but if you need this document, you’re probably better off following the mainstream trends.

Balance

Best power comes from the right balance of all three. Finding that balance is not easy. However, you can leverage the experience of countless Subaru tuners who have probably gone before you with modifications similar to yours. If you pick a typical AFR (11.25, for example), and a typical boost curve (varies by turbo & mods), and increase timing up to the knock threshold, you’ll probably get typical power. And there’s nothing wrong with that.

Opinions differ about the best way to get better-than-typical power. I have some theories, but I haven’t tested them very much yet, so I’ll keep them to myself for now.

Getting it done

You’ll need a way to measure power and torque. Dynos are nice for this, but road dynos can also work.

Choose a reasonable a fuel table and boost curve for your hardware, increase timing until you find the knock threshold, then back it down a couple degrees. Call this your baseline tune. Work on measuring the power output from this tune, until you can do three different measurements that all agree. That’s harder than it sounds, but if you don’t know what your margin of error is from run to run, you won’t know what’s adding or subtracting power, or what’s just random variation from run to run. The difference between a meaningful change and run-to-run variation is very important. It’s easy to be misled if you aren’t familiar with the difference.

Try adding more timing, but enriching AFR and/or reducing boost (to avoid detonation), and see if that gives you more power. Also try adding boost, but enriching AFR and/or reducing timing. Also try leaning out the AFR (say, 0.5 leaner), with less timing advance and/or less boost. If you find that one of these alternatives gives you more power, great! Keep experimenting until you’ve exhausted all of the alternatives (no pun intended).





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Page last modified on February 02, 2013, at 11:09 AM
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