A post to DSMtuners that people might find helpful. Some day I'll try to do a more complete write up...
The key to getting normal timing with an AFC type device and large injectors is moving a lot of air. Believe it or not, the more air you flow the better off you are. As you raise boost and airflow timing will naturally come down until you reach the highest load map in the ECU. Unfortunately many people interperet this drop in timing as knock and stop short of thier setup's full potential. I'll try to pass on some info on how this works.
With a 2g ECU the highest load map is used when you get over 2.1 g/rev airflow. Timing will reach 10 degrees by 5k rpm, and will peak at 16-17 degrees at 7000 rpm. Intake temps above 84 degrees F or below 34 degrees will cost you one degree of timing. Coolant temps over 206 degrees will cost you 1 degree, and over 226 or so will cost you 2 degrees. Armed with this information, you can see why getting onto this high load map would be advantageous. Anywhere below 2.1 g/rev there is no way to know what target timing is without using DSMlink. But at anything over 2.1 it remains the same and you know what the ECU is shooting for. Anything less is obviously due to knock.
The 2.1 g/rev figure is taken at the ECU, or what you see in the logger. The AFC will cut down this value according to the correction you have entered in the AFC for injector compensation and then target AFR. So more negative the number, the more air you need to flow to get over 2.1 g/rev, and the higher target timing is going to be. The timing that you get in this way is the number one source of knock, and is the problem 90% of the time when people say "I get tons of knock even at such low boost!" ;) It gets worse at lower boost, since airflow is less.
To see where you are at with airflow, use the logger and do a little math. 1Gs guys are screwed without DSMlink, I am not aware of any logger that can display airflow besides DSMlink, but this hardly matters since knock is available. But any 2g logger should be able to display airflow as part of the OBD2 setup, the pocket logger shows it in lbs/minute, which is my favorite unit. But to get to the units that the ECU uses you have to convert that lb value to grams, then divide by rpm (grams per rev). You can see that if airflow remains constant, airflow/rev is still going to drop with rpm. Luckily airflow is usually rising to redline helping to keep airflow/rev relatively level. The problem obviously is that as airflow/rev drops with increased rpm, timing will be going up accordingly. Typically at higher RPM you can get away with a little more advance since piston speed is higher and cylinder pressure is lower (drop in VE). But it's not always desireable.
Some examples. If you are logging 25 lbs/min at 6k rpm, you are getting 1.89 g/rev. Timing will be over the 16-17 degree figure by some amount since you are below 2.1 g/rev. More timing equals higher tendancy to knock. But, if airflow was 30 lbs/min at 6k rpm, airflow/rev would be 2.27 g/rev, and timing would be predictable, allowing you to spot knock easily.
The problem with large injectors and an AFC is that airflow at the ECU will be lower than what it actually is at the motor. Lets assume you are running 650s and are going another 10% leaner to get closer to ~11:1 target AFR. Thats a -41% correction. You would actually need 50 lbs/min through the sensor in order to read 30 lbs/min at the ECU. :eek: (30 divided by 1- .41) You can see where this is going. With 550s, you would only need ~42 lbs/min to get 30 at the ECU. The larger the injector, the more timing you are going to get, because of the lower airflow.
This helps to explain why people with large setups can still run very fast on an afc and large injectors, while the regular guy with a 16g struggles with knock.
The key to making a setup like this work is to know what target timing is, which then allows you to see how much timing has been pulled due to knock. And that is hard to do at less than 2.1 g/rev since we don't have access to what timing is on each load level below 2.1. I have the maps, but not the corresponding airflow values.
This all applies to 2g ECUs of course. The figures given here will not be correct for a 1g ECU. Target timing is a little higher, and I don't know what airflow value corresponds to the highest load map.
If nothing else, this post should provide some insight to the true value of having a system like DSMlink instead. It really is worth every penny, I won't run a car without it. But with some knowledge of how the ECU works and what effect your tuning devices have on that ECU, you can do some more "educated" tuning that simply watching for timing to drop. This really is a vast topic, and this post barely scratches the surface, but I hope people find it helpful.
