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Basic AFC Tuning

A "How-to" Guide by Cody Graham

Edited and Adapted by Donald Ashby

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NOTE:  This was originally a post that I saved from "DSMTalk" that tells exactly how to program the AFC on the DSM line of cars.  It applies to the Stealth & 3000GT platform as well, and has been adapted here by Donald Ashby to be more directly applicable to our cars.


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Basic AFC Tuning Guide

0-3 count

ECU advances timing

4-7 count

ECU leaves timing alone

8+ count

ECU retards timing

So anything under 8 counts of knock and you wonít be losing any timing.  Anything higher than that, and the ECU will retard timing.  That is what you want to aim for while tuning-- little to no knock, while running lean enough to get more power.  Leaner plus timing equals more power.

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Tuning the S-AFC for 3000GT & Stealth

Start with a baseline correction based on what size injectors you have:

360cc (Stock):

0% high and low

450cc (DSM):

20% high 25% low


30% high 35% low


40% high 45% low

Try Throttle Points of Low at 30% and Hi at 85%. Then, set up your low throttle trims. 0-4000 rpm's first. Low throttle is under closes loop, meaning your ECU will adjust the IPW based on O2 sensor information. Monitor what the ECU is doing by logging RPM, HZ signal, O2 voltage, throttle, and Low/Med/Hi fuel trims. Here are the fuel trim parameters:

0-125 Hz =

Lo fuel trim band

126-175 Hz =

Mid Fuel trim band

176+ Hz =

Hi fuel trim band

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Low Throttle Settings

Ok, first we will set the Low Throttle setting on the S-AFC, so don't go above 30% throttle during this time.  Set your throttle adjustment points at 1000, 2000, 3000, 4000, 5000, 6000, 7000, and 7500.  (There are 12 Points to set for the SAFC-II, so you might want to choose 800, 1600, 2200, 2800, 3400, 4000, 4600, 5200, 5800, 6400, 7000, 7600.  We'll continue with the discussion for the SAFC-I for these purposes, but the same principles will apply for the SAFC-II.)  To set the 1000 setting, just let the car idle in your driveway.  You will need to be watching your logger and your S-AFC at this point.  Just to get you used to what you will need to monitor, this is what you're going to need to look at--  First check what your airflow is...  If it is under 125 you are in low fuel trim band; 125-175 you are in mid trim band; over 175 and you are in your hi trim.  Ok..., the next thing you need to look at is what that fuel trim is at, i.e., if you're at 75 Hz you need to look at the low fuel trim settings.  Finally, look at what RPM you are at.  All right, now back to the tuning part...

Let the car idle in your driveway.  Airflow should be somewhere under 100hz, and RPM should be about 700 or so.  So you need to take a look at your Low fuel trim.  If your trim is over 100% you need to richen the 1000rpm mark up.  So move it up a bit, then once your O2s start cycling again check the trim again.  If it's still over, move it up more, and repeat until you get to 100%.  Do the opposite if you are under 100%.

Next we will do the 2000rpm mark.  Get on a nice long stretch of road, and get the car in 3rd gear or higher (varies based on speed limit etc).  Now give the car just enough throttle to where your RPMs arenít increasing or decreasing.  At this point, it helps if you have a friend in the car.  Have your friend take a look at your fuel trim (look at whichever one is relevant, based on the chart above).  If the trim is over 100%, add fuel; if under 100%, lean it out.

Finally do 3000rpm.  Once again, just cruise along without allowing your RPMs to vary too much.  Check which fuel trim your ECU is using, then adjust that RPM's set point until that trim level is at 100%.

Now you can go on and do 4000 rpm, 5000, etc.  But I just set the rest of them at what the 3000rpm mark is, and call it even.  Mainly because it isn't very often that I'm at 4000 rpm and not at full throttle.

Now we get to the fun part - high throttle settings!

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High Throttle Settings

This is truly an art.  There are many ways to go about doing it, and this is just my way.  Take it with a grain of salt, don't follow it to the T, and if anything goes wrong make something else up.

All right.  The first thing I do is to set my high settings to the same setting all the way across.  Usually, I use whatever the 3000RPM low setting was across the board as a starting point.

All right now.  I did this solo, but it would probably be easier with two people.  Just have the second person do whatever the first person isn't doing...

OK.  Go find a nice long highway on-ramp. It's best to do this really late at night, since there's less traffic.

1) Start at 2000rpm in 3rd gear, start recording the data log, then wait maybe half a second and floor it.

2) Hold the gear all the way until you bounce off the rev limiter (or if you don't have a rev limiter hold it to whatever you feel like holding it to).

3) Now pull off the highway and into a parking lot. Review the log. What you want to look for is any dip in timing or flattening of timing or anything of that sort.

4) If there is timing getting pulled or flattening out anywhere you need to richen that section of revs up. Look at the corresponding engine speed then add fuel to that section on the S-AFC. Only increase it by 1 or 2% unless you are seeing a massive dive in timing. In that case richen it up a good 5% or so.

5) If there is no timing getting pulled and you have a nice long steady increase in timing you are running too rich. Lean it out across the board using good judgment. I take steps of 3% when leaning things out.

6) Repeat until subtracting fuel gives you knock (i.e. when timing takes a plunge)

A few things to clear up:

First off..., the reason I suggest that you monitor timing instead of knock count while tuning is because of the way that data loggers work.  They poll the ECU for information a given number of times per second.  It's usually around 80 times a second, if you are monitoring 10 variables, since each variable is being updated 8 times per second.  Knock is an instantaneous thing-- the ECU sees it, and then basically forgets about it.  So when your data logger asks for knock, it gets whatever the current knock count is. That means that you will miss a lot of knock counts even if the only variable you are monitoring is knock.  Since the ECU adjusts timing based on knock counts, and the ECU never misses a knock count (unless you have a faulty sensor), you can just watch timing instead:

8 counts of knock (or more) and the ECU retards timing

4-7 counts of knock and the ECU levels timing off

1-3 counts of knock and the ECU slowly increases timing

0 counts of knock and your ECU will greatly increase timing

So while tuning, instead of aiming for "no knock", aim for "no retarded timing"!

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Ok now I'm going to make up a few examples to hopefully clear things up.

(These are fictitious; I made them up while sitting in front of my computer, and they should not be used as a tuning guide, just as examples of what to do in what situation).

Example #1:

Bob has a 3000GT VR-4 at BPU, boost controller (set at 14 psi), air intake, and cat-back.
Bob starts off with low and high settings at 0% across the board.
Bob lets the car idle in his driveway to set up his low throttle 1000rpm point:
Low fuel trim is at 107%
Bob adds 5% more fuel at 1000rpm low setting.
O2s fluctuate and even off and he ends up with a low fuel trim of 98%
He subtracts 1% from the 1000 mark
O2s fluctuate and he ends up at 101%
Good enough, now his 1000rpm low setting is 4%

Example #2:

Jim has a Stealth TT at BPU plus 450cc injectors.
Before turning the car on for the first time he sets the low and high settings to 20 and 25 respectively.
Starts the car up for the first time, and nothing explodes (yea!)
Gets in and starts logging.
Sets his low throttle levels fine, good gas mileage no knock etc.
Starts to tune his high throttle settings:
Floors it from 2000rpm in third gear and hold it to 7000rpm.
From 2000 rpm to 4000 there is a steady increase in timing, at 4000 timing takes a dip then levels off and finally starts increasing again at 6000rpm.
In this situation, richen up the 4000rpm setting but don't touch anything else yet. Work it like you would a book, from left to right, because what happens sooner affects what happens later.
Next run timing is steady all the way to 5000 rpm when it levels off all the way to 7000.
Once again richen the 5000 rpm setting a bit.
Next run there is a steady increase in timing from 2000-7000.
Now comes the power addition stage:
Take your 2000 and 3000 rpm setting and lower them 3%
Make a run, if no "knock" (retarded timing) decreases more, if knock is present then richen it up a little. Then repeat for the rest of your rpm settings.

Happy tuning!

--- Cody Graham ---
--- Donald Ashby ---

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Article ©2003-2004 Cody Graham, All Rights Reserved.
Article ©2003-2004 Donald Ashby, All Rights Reserved.
Other Images ©1995-2004 Bob Forrest, All Rights Reserved.