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Tuning Concepts with DSMLink: Fuel Tables

The following article will help describe the use of the fuel adjustment tool with DSM Link Engine Management System, and give you a better understand of the fuel delivery system in your DSM.

Note: This guide is to help you understand how the Fuel Dialog Box for DSM Link works and is in no way a complete explanation on how to run or tune your car. I am not held responsible for you causing damage to your car. All examples given are just that, examples! Don't assume any setting I have plugged in here will work. Every car is different and every tune requires attention to detail, and concepts of sound tuning knowledge. Also, before we start, I will assume you have read the ENTIRE instruction manual, right?

Okay, let's talk about what the settings in your DSM Link are, and what they do. Unlike an AFC, the fuel table is much more of a straight forward application, and works in direct correlation with the injector pulsewidth signal in the ECU. Instead of intercepting an ECU signal and creating a compensation table, you work directly with the fuel system. What changes you make, are made, simple as that.

DSMLink: Integration and Application

DSM Link is sometimes confused with a stand-alone management system such as AEM, when indeed it is still a piggy-back system. Although the program includes many stand-alone simulated features, it is simply a computer chip integrated into your Electronically Programmed (EPROM) ECU. The advantage to this is you are able to retain many vital stock features (such as knock sensor) while be able to simulate a quality stand-alone management system, through a simple, easy to use interface. Also, DSM Link is exceptionally priced well below AEM.

RPM-based adjustments:

The RPM sliders are only used during certain modes of open loop operation in the ECU, primarily the WOT open loop mode. Basically it's a straight forward compensation table in relation to fuel injection that you can use to fine tune you're A/F Ratios at WOT. They will only apply during those conditions, simple as that.

Global Settings:

The global setting on DSM Link is fuel setting that will have a direct effect on injector pulsewidth at all times. When you purchase your DSMLink chip, ECM Tuning will automatically program your starting global point for your size injector at the time of purchase. The DSM Link program can handle any size injector from stock 450cc, to a whopping 1600cc. 0% compensation being 450cc (stock). The equation for figuring out your injector compensation start point is as follows:

(450cc/ Inj. Size) -1 = (% of compensation)

(450cc/ 720cc) – 1 = (37.5% compensation)

Dead Time Settings:

Dead Time settings are fuel based settings that are applied all the time, same as Global. This value is simply a measurement of injector pulsewidth. With larger injectors, you don't just get more fuel flow; you also get slower response. Basically dead time is the amount of nanoseconds of "dead-time" between injector pulses. (How long they spray/don't spray) This is important for many reasons; mainly ensure the proper timing of fuel flow in relation to airflow. If you're dead time setting is too high, then the pulsewidth is too high, and fuel isn't spraying when it's suppose to.

Here is the procedure outlined in the DSMLink Manual:

Use the long-term fuel trim (SelectedLTFT) value calculated by the ECU as your guide. The following outlines a good procedure for getting the dead time right for your setup.

1. Car must be fully warmed up then must be operated at cruise for about 10 minutes followed by idle for 5 minutes to get stable LTFT values.
2. Compare the LTFT at idle and cruise. You want both to be right about zero (+/- 5% or so).
3. If the two are about the same, dead time setting is good.
4. If they are equal but non-zero, adjust the global setting.
5. If idle LTFT is more positive than cruise LTFT, increase the dead time, less, decrease.

A minor quirk in the implementation of the dead time value adjustment requires that any value entered into this dialog must be evenly divisible by 15. The software will automatically make appropriate adjustments for values you enter that are not evenly divisible by 15.

Also note that other long term fuel trim variables do exist. These can be used instead of SelectedLTFT if you wish. Depending on your ECU type, these will include LTFT Lo, LTFT Mid, and LTFT Hi. These are the individual long term fuel trim variables inside the ECU. Which one is used at a given point in time is determined by logic inside the ECU. It's mostly based on airflow. Low airflow typically seen at idle will cause LTFT Lo to be used while airflow typically seen under cruise conditions will usually result in the use of LTFT Mid (LTFT Hi on 1Gs). Whichever value is selected for use by the ECU is what you will see in the SelectedLTFT variable.

Fuel Pressure and Pump Selection:

As we all know, you can't just slap a program in there, throw some big injectors on, and expect to go super fast! We gotta move dat gas! Fuel Pressure and Pump size have to be directly correlated to your pulsewidth. If you have adjusted your fuel pressure beyond the factory default, you will need to take this into account when calculating the global fuel scale and, to some extent, the dead time as well. Increasing the base fuel pressure increases the flow rate of the injector and slightly increases the expected dead time values as well. The increase in flow rate can be calculated like so:

Here is another example from the DSM User's Manual:

FlowFactor = sqrt (NewPress / OldPress)

For example, let's say you have increased base pressure from the factory value of 43.5psi (2G) to 55psi:

FlowFactor2G = sqrt (55 / 43.5) = 1.1244

Note that a manual 1G will use 37psi instead of 43.5psi:

FlowFactor1G = sqrt (55 / 37) = 1.2192

Applying this adjustment to the example above running 720s gives the following global fuel adjustment:

Adj2G = 450 / 720 / 1.1244 – 1.0 = 0.5559 – 1.0 = -0.444 or –44.4%

Adj1G = 450 / 720 / 1.2192 – 1.0 = 0.5126 – 1.0 = -0.487 or –48.7%

All this is saying that when you do your equation to figure your global settings, you need to account for adjusted fuel pressure. Take that calculated value and divide it by the injector size compensation and complete the equation.

Now that you have an understanding of how the fuel adjustments are made, and what the different setting you can comprise, were are ready to move on to the next section. Airflow map adjustment. Happy Tuning!
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