(this page is a work in progress)
The advice on this page pertains to pump gas only. If you’re running race gas or E85, seek advice elsewhere. Some of the general principals are the same, but you can’t rely on the knock threshold to tell you when to stop advancing timing.
Tuning is dangerous. You can blow up your motor if you do it wrong. You motor might even blow up if you do it right, because shit happens. You no longer have a warranty for your motor when you run your own tune - do not expect Subaru to fix your mistakes. They provide a warranty only when you run the stock tune, because they give the stock tunes a very large safety margin. You’re going to take away that safety margin to get more power, which means that if something breaks you are responsible for fixing it.
Logging can be dangerous. It may be tempting to exceed legal speed limits and do things that resemble street racing. Don’t do that. It may be tempting to look at the laptop while you’re logging. Don’t do that, either. The great thing about logging is that you can pay attention to the road while you’re driving, and then focus on the data when you’re not driving.
1.1. What I will try to teach you
If you read this, and all of the other stuff I’ve written about tuning, I hope you’ll be able to create a safe tune for your car, that makes pretty good power. I hope that you will gain a better understanding of what’s happening inside your motor, and how to tune it. And I hope to minimize the probability of you blowing up your motor along the way.
There are dozens of tables in a tune, and this document won’t try to explain all of them. I will instead focus on this set of tables:
- Fueling (including MAF scaling and injector settings)
- Timing (including base and advance)
- Boost control (including boost targets and WGDC)
I will teach you one approach to tuning. It’s an approach that has worked well for me - I’ve been adding parts to my Subaru Legacy for a couple years now, retuning it on a regular basis. So far it hasn’t blown up, and friends who have been for a ride will tell you that it’s got plenty of power.
Many aspects of tuning have been simplified for the sake of getting a working tune that gets most of the potential power from my hardware. Not all of the potential, but I think that I’ve only left a small amount of power unrealized, and I’m OK with that. If you want to get 100% of the potential out of your setup, I still think that this approach is a good start. As I write this, I will make an effort to call out opportunities to gain more power through experimentation.
1.2. What I will NOT teach you
The instructions here are NOT intended to create the most powerful tune possible. They are not intended to create a tune quickly. I prefer safety above all else. This is a time-consuming process. There are quicker ways, and there are ways that will give you more power, and they are outside the scope of this document.
I will not try to teach you more than one approach to tuning a car. Every tuner has their own preferred approach, and many of them are perfectly reasonable - but this document just describes my approach.
I will try to teach you how to do this without blowing up your motor, but I can’t promise anything. If I knew how to guarantee that your motor would survive, I would - but I don’t, so I can’t. If you make the wrong move, you will blow up your motor. Even if you do everything right, something outside your control can still blow up your motor - bad gas, mechanical failure, improper installation of various parts, proper installation of faulty parts, and so on.
If you are not willing to take responsibility for your motor blowing up, tuning is not for you. Take your car to a shop and let them be responsible for it. They probably won’t take responsibility either, but being able to blame them might make you feel better. :-)
2. Before you start
Don’t just dive into this. Reading this document won’t make you a tuner. It probably won’t even make sense the first time you read it. If I were teaching a class, we’d spend the first half of the semester going over the fundamentals of engines, ECUs, and turbochargers…. and then we’d get into tuning. If you are still uncertain of the stuff in the next three sections, you should read up on those before you start.
2.1. Stuff you should know about tuning
You must have a clear picture of how 4-stroke motors work, and how they die. You should understand how turbochargers work. You should understand how your ECU’s various sensors and actuators work.
You should read and understand all of the other documents in my Incomplete Guide To Tuning.
Most importantly, you must be able to create data logs, and read them. If you don’t, then you’re not ready to start tuning. Practice making data logs and reading them, until you understand what those numbers are, what values you should see in them, and what it means if those values are higher or lower than you expect.
You must know what your fuel, timing, and boost tables are. You should be able to predict what will happen when you increase or decrease any of the numbers in those tables. You must know how to verify your prediction using data logs.
2.2. Stuff you should know about your car
Is your car ready to be tuned? Two things can blow up your motor faster than bad tuning: bad installation of good parts, and good installation of bad parts. Before you start tuning, verify that you have no vacuum leaks, fuel leaks, or exhaust leaks. Verify that your throttle plate, wastegate, and diverter valve (BOV or BPV) all work properly. Verify that your AFR and MAP sensors work properly.
If you have a mechanical problem, fix it - and then consider what impact that fix will have on your tune. For example, what will happen to your fueling if you fix a vacuum leak? In some cases, finding and fixing a mechanical problem means that you need to re-examine your tune. Pull timing across the board, cut boost to wasteagate-spring levels, etc, and look for side-effects of the fix.
2.3. Stuff you should know about tuning your car
What do other peoples’ tunes for your car look like? Chances are very good that you’re not the first person to tune your car with your mods. Chances are very good that you can learn from other tunes of similar cars with similar mods. Find some examples and look at them. Compare three tunes for cars with the same turbo. Compare five if you can find five. Chances are good that they will differ quite a bit - but how? Which are more conservative or aggressive with fueling, timing, and boost levels? If you take the most conservative timing, and the most conservative fueling, you’ll probably have a pretty good idea of how to put together a safe - gutless, but safe - tune that you can start from.
Look at data logs from cars with similar turbos as well. How much load (grams of air per crankshaft revolution) are other people seeing? You’ll want your fueling and timing tables to extend at least that high, and preferably 20% higher just to be sure.
3. One Fundamental Assumption
The main reason I don’t attempt to get 100% of the available power is that I tune without using a dyno to actually measure power. This approach won’t work with all types of cars, or all types of fuel.
When using common pump gas (e.g. 91–94 octane), with the Subarus that RomRaider can tune, you will hit the knock threshold before you reach maximum power. Some key aspects of how I tune have been derived from this fact.
4. Some more assumptions
4.1) You’ll start by aiming for reasonably good power, not maximum power. Worry about the latter after you have achieved the former.
4.2) You can find a boost curve (or boost level if you want to simplify further) that will give reasonably good power, by looking at what people with similar setups are running. With that boost level/curve, a WOT pull will take you through a particular set of RPM-and-Load cells in the timing and fueling tables.
4.3) You can make reasonably good power in WOT pulls with an AFR of 11:1.
4.4) You are running pump gas.
4.5) Because you are running pump gas…. Having chosen a boost level and an AFR, you can turn up the timing at each of the RPM-and-Load cells (along that series of cells I mentioned earlier) until you find knock, and then you can back it down a couple degrees. Note that with E85 and race gas, you may not develop knock until you’ve gone well beyond the timing that creates best torque for each cell. So, again, this only works for pump gas.
4.6) After choosing a reasonable boost curve, and a reasonable AFR, and advancing timing until just below the knock threshold, you’ll get reasonable power. Then you can start experimenting with richer AFRs and more timing, or with less timing and leaner AFRs. Or with higher-that-what-other-people-think-is-reasonable boost. Or race gas, or water/meth injection, or whatever.
5. The Plan
In a finished tune, the fueling and timing tables complement each other perfectly and make very good power. Any less ignition advance would leave power on the table; any more ignition advance would require additional fuel; any more fuel would leave power on the table; any less fuel would require less ignition advance.
In a finished tune, the boost control tables will let you run whatever level of boost you want, just by varying the throttle. And at any boost level, you’ll be running a safe balance of fueling and ignition advance.
But how do you get there? In your first tune, you mostly just don’t want the motor to blow up. So we’ll start by creating a safe tune.
Timing is relatively easy, because it’s basically going to be just what you enter into the timing tables. Fueling is more complicated, because it depends on the balance between the MAF scaling curve and the injector settings. Boost control is also complex - it depends in initial WGDC, max WGDC, turbo dynamics, and so on.
I start with a fueling table that’s simple, timing that’s very conservative, and a manual boost controller (MBC). Then I work on MAF and injector settings, so that the tailpipe AFRs actually match what’s in the fueling table. When fueling is good, I turn up the timing, using the MBC to test and tune timing at various boost levels. When fueling and timing are good at all boost levels, I start tuning boost control electronically. When boost is dialed in, I’m done.
(Every time I’m ready to tune boost electronically, I replace something under the hood and start the tuning process over again. I haven’t run electronic boost control for over a year. But, as I write this, I’m one intake swap away from tuning boost again.)
6. Build the first tune
You gotta start somewhere. The tune that you start with should be safe. It should also have fueling and timing tables that have load and RPM axes that extend higher than the limits you expect to see.
There’s more than one way to go about tuning every table. But, as a novice tuner, you need not get too concerned with which way is best - what you should start with is something that’s reasonable. When you get your whole tune dialed in, then you’ll be in a position to start experimenting with other approaches to see what works best for your own car.
If you have multiple sets of fueling and/or timing tables, ask yourself why there are two sets? If you have high-det and low-det versions of some tables, make sure the high-det tables are more conservative (more fuel, or less timing).
If you have cruise and non-cruise versions of some tables, I suggest making them the same. That way you can - at least for a while - forget about the logic that the ECU uses to shift between them. If the tables are identical, then you don’t need to worry about which set is active when you’re logging. You’ll effectively disable the cruise/non-cruise feature this way, but you can tailor each set of tables later, after you’ve come up with a tune that works adequately in all conditions.
6.1) Find a sample tune for your motor and turbo that uses typical AFRs (11ish), and subtract 4 degrees of timing (from the base timing table) at redline and 8 in the midrange. That will leave you low on power, but very safe, and safety is more important when you’re getting started.
6.2) Set up your fuel table. Start by looking at a few other fueling tables. A typical fuel table has a region of 14.7 in the top-left (low load, low RPM), and it calls from around 11:1 at high load. Look at a few tunes to get a feel for how to taper AFRs down from the 14.7 region.
You can almost simplify this to two variables: what AFR do you want to target at high boost, and how steeply do you want to taper from the 14.7 idle/cruise AFR to the chosen high-boost target AFR. I suggest 11:1 for a target AFR, and look at a few examples of fuel tables from tunes based on your motor and turbo to see how steeply the AFR should taper.
6.3) Set up your timing table. You should base this on whatever tune your fuel table most closely resembles. Keep stock timing below 1.0 load, but with a max of 38 degrees. Make of the stock tunes run 45 degrees or more at low load, and this contributes to shift knock.
6.3) Find intake and injector settings suitable for your mods. If you have one, you can figure out the other for yourself via data-logging.
6.4) Fill the target boost, initial WGDC, and max WGDC tables tables with zeros.
7. Add a manual boost controller.
Manual boost controllers are not a requirement by any means, but they do make tuning easier, so that’s what I do, and that’s what I advise. I like using an MBC for two reasons. First, they’re dead simple. You twist the knob, and your boost curve moves up and down. Second, an MBC allows testing various boost levels very easily - and this means it’s very easy to test various MAF ranges and load ranges. I like to periodically re-test the tune at wastegate boost (usually around 10psi) and full boost (usually around 20psi), and a few levels in between.
While you’re running an MBC, the boost control tables don’t really matter, except that you do want to make sure that the fuel-cut boost limit is a couple PSI higher than the boost levels that you’ll be using.
For your first tune, adjust the MBC for minimum boost - you’ll get whatever boost level your wastegate spring provides.
MBCs have two drawbacks. First, they remove the ECU’s ability to cut boost if the ECU sees lots of knock. I don’t think that’s hugely important since the ECU will pull timing and add fuel, but it’s nice to have. Second, they will give you full boost at partial throttle (aka part-throttle-full-boost, or PTFB). Despite what lots of people at NASIOC believe, PTFB isn’t dangerous for your motor. It is however mildly annoying.
To address both of those, I recommend running an electronic boost control solenoid (EBCS, or just BCS) in parallel with the MBC. There’s a great thread about this setup at NASIOC:
8. Get your fueling dialed in.
There are how-to threads about tuning injector settings and MAF scaling, so I won’t repeat the details here. Start with idle. When that’s in the ballpark, work on cruise. When that’s in the ballpark, work on part-throttle pulls, and then WOT pulls at wastegate boost.
9. Get your timing dialed in.
If you’re using pump gas, you can turn up timing in your WOT pulls until you find the knock threshold, the back it down a couple degrees. Don’t worry about timing for the non-boosted regions, just make sure it blends smoothly with the timing curve that you come up with for your wastegate-boost WOT pulls.
Don’t increase timing much in the cells that you’re not hitting yet. Just focus on the cells that you are hitting, and keep the slightly-higher cells 4–5 degrees lower.
10. Add boost, retune.
Turn up the MBC so you get a couple PSI more boost. Double-check your AFRs at the new boost level, then increase timing until you find the knock threshold.
11. Repeat until full boost.
Repeat steps 8, 9, and 10 until you’re running a boost level suitable for your turbo.
12. Tune boost electronically.
It’s tedious work and frankly I’m still not all that good at it. When I feel like I have a good trip on it, I’ll explain.