Performed on a 1G DSM, but may apply to other cars.
Karmen Type Mass Airflow Sensor:
To get things started, I’d like to describe the way a Karmen type MAS (this is the MAS that Mitsubishis use) functions. If you’ve ever seen a MAS you’ve seen the metal "honeycomb" that is in the sensor housing. The purpose of this honeycomb is to create a swirl of air known as a Karmen Vortex. As airflow passes through the honeycomb, the vortex is created and the sensor’s eye counts it, this is how the sensor measures the air and it is sent to the ECU for interpreting in Hz.
The fact that all airflow must pass by the eye and be counted presents an issue. What happens when there are too many vortices for the sensor to count? This is called an overrun, and the sensor will begin to miss counts. Also, when the MAS sees too much airflow, it hits fuel cut. Fuel cut generally occurs when the ECU thinks the stock injectors should be at their limits, this is to protect the car from getting knock/pre-ignition. How did Mitsubishi counter this problem? It’s simple. They added a second air passage to the MAS. Now, the second passage has a honeycomb just like its upper counterpart, but it’s different in the fact that it does not have an eye, nor does it meter airflow at all! The car’s ECU can get a good idea of airflow by knowing how much air flows through the second passage in relation to the first (metered) passage. For example, if the flow proportion were 2:1, then the ECU would calculate airflow like this:
Total Airflow Reading = First Passage Airflow + Second Passage Airflow = 75.
See how simple that is? It is possible to still overrun the MAS even though it has two passages (2nd gen. Cars have three un-metered passages and one metered passage). The general solution to this problem is to allow more airflow through the lower chamber, and thereby letting more airflow through without the eye seeing too much and getting overrun. The easiest way of doing this initially is by removing the lower (second) honeycomb. This will not have any adverse effects because there is no eye in the lower passage that will require a honeycomb. (Note: Never remove the upper honeycomb, the air will not be swirled properly in order for the car to count the airflow passing through the chamber.) By removing the lower honeycomb, the car will run a bit leaner because the ECU never "sees" this additional airflow and therefore does not add extra fuel to the mixture to compensate. For example, instead of running at the original 2:1 top to bottom ratio, the MAS lets in a ratio of 2:1.25.
In more extreme situations, such as higher boost levels, one might take a Dremel tool and cut out ALL restrictions in the lower portion of the MAS (AKA hacking the MAS) and thus creating an even further lean condition. This can be potentially dangerous since an overly lean condition can lead to pre-ignition or knock, which will eventually destroy an engine if it is left unchecked. One method used to combat this lean condition would be to buy some sort of fuel tuning device such as an Apex’i S-AFC. With the right tools, one could tune the cars fuel delivery to compensate for the extra un-metered airflow, and benefit from a more free-flowing MAS with an even further delayed MAS overrun and fuel-cut. Caution should be exercised when hacking the lower chamber.
That’s all I’m going to cover concerning the MAS, on to AFC tuning tips.
Apex’i S-AFC Tuning:
The S-AFC works by intercepting the signal sent by the MAS, modifying it, then sending it to the ECU. For example: The MAS sees 500 units of airflow at 4000 RPMs and sends it to the ECU. But wait, the user wants the car to run richer because his MAS is hacked. So now the user wants to add 5% more fuel to the car at 4000 RPMs using the S-AFC. The S-AFC intercepts the MAS signal to the ECU, adds 5% to the original airflow value (500 units remember?) and sends the new signal to the ECU, which is 525 units of airflow. In order to make up for the additional 25 (5%) units of airflow that don’t really exist, the ECU will add the appropriate amount of fuel to the mixture to compensate, which in turn makes the car run richer.
The S-AFC has eight adjustable RPM points where the user can modify the airflow signal to the ECU + or – up to 50%. It also has high and low-throttle settings. Generally, most people will set the RPM points at 1K-2K-3K-4K-5K-6K-7K-8K. Another setting that can be adjusted is the throttle points. This tells the S-AFC what you want it to consider High Throttle and what to consider Low Throttle. Anything in between the two settings will result in fuel correction being averaged between the two accordingly. Mine is set at 30% and 70% throttle. The best tool for tuning a S-AFC on a DSM is by utilizing a data logger setup, such as a Pocket logger, or TMO Data logger.
To tune for low throttle settings, you will want to monitor the cars low, medium, and high fuel trims, additionally, you’ll want to keep an eye on O2 trims. Fuel trims are the ECU’s ability to keep the car running with an ideal fuel mixture during part throttle operation under 4000 RPMs. In 91-94 DSMs, fuel trims can range from 60% (really rich) to 139% (really lean). Your goal here is to get the fuel trims as close to 100% as possible. At idle, you’ll use your low fuel trims to get an idea of where your car is. If it is over 100%, add fuel at your 1000-RPM (idle) setting. If it is under 100%, take away fuel at your 1000-RPM setting. Now for the rest of your fuel trims, you can use this as a general rule: low fuel is up to about 50 HZ, medium fuel is 30-40 mph cruise, 50-200hz high is 70 mph, 200-350hz. Hz can be monitored with the S-AFC. You will try and get your trims as close to 100% as possible, or a little leaner (110%). Note that above 4000 RPM the ECU goes into open loop mode and runs very rich, so just set your 5K-8K the same as your 4K. O2 trims may be monitored during tuning also. They will sweep up and down constantly and ideally you want them to sweep between 80%-120%. That’s all for low settings.
Here’s the fun part, tuning for high settings. Start out by monitoring RPM, Timing Advance, Throttle Position, and Knock Sum. At about 3000-RPMs floor it until redline. Now check your data logger readings. Was there any knock at any RPMs? If there was, then you are possibly running lean at that RPM, and you will want to add fuel accordingly until the knock goes away, if not, then start again in third gear and go until redline once more. Was there knock this time? Repeat the process until you are knock free***. Now go back into third gear and lean out the RPM points until you encounter somewhere between 1-5 knock counts across the RPMs. At this point, your car is running just lean enough to make good power without the ECU pulling ignition timing (timing = power). Ideally, you want your timing to drop when you got WOT, then rise steadily all through the gear. If you are knocking, the timing will flatten out, and if you have really bad knock, timing will drop, even as far as 0* of timing advance.
Some people (myself included) leave their cars right here. Others will tune for zero knock count and you will want to richen up your settings just a tad to get zero knock count. At zero knock count, the car will not make as much power as before.That’s all for high throttle tuning.If you did everything properly, your car should be tuned very well and you’re good to go. If you have a 2nd Gen. DSM or other similar OBD-II car (3KGT) that does not read a knock sensor, then you can use O2 Voltage (with a grain of salt) and Ignition Timing Advance to tune at High Throttle. If you see timing drop off dramatically, then the car is registering a lot of knock and is pulling timing to protect the engine, you are running lean at the point where timing is pulled. I hoped that this article helps anyone who was looking for some tips.
***If adding fuel does nothing to help your knock, it is also possible to get knock from running extremely rich. One such case where this may occur is due to a major boost leak. For example, if you have a big boost leak in your UICP (for some reason or another), your MAS will meter all of the incoming air (pre-turbo) and tell the ECU to add the appropriate amount of fuel. Then , a lot of that metered air will escape via the boost leak, but the ECU does not know this. Now you have way too much fuel for the amount of air you are getting. This can cause detonation.
