Please explain timing map in simple terms

[4gfun]

Could someone explain timing maps and load axis in simple stupid terms?

Is it an accurate statement that one end of the timing maps will be when my foot is to the floor and the other end is when my foot is barely touching the gas.

Everything in between would therefore be different positions of the pedal (e.g. TPS)?

Thanks for explaining in advance!

 

[Melbowski]

In its simplest form load = airflow. The more air flowing through the MAF, the larger the load number. It is mainly pulled from the MAF reading, and then tweaked slightly by some other variables. Here's a quick snipet of a post I made on 3Si concerning the fuel/timing tables:

The ECU mainly uses two maps to control the motor: Fuel and Timing. Both maps run via calculating load vs RPM. The ECU determines load using throttle position, MAF reading, Barometric pressure, temperature and a few other variables. The ECU determines the RPM via the Camshaft and Crankshaft position sensors (1ga's use only the Camshaft position sensor). This table is the fuel map from a 2gb DSM (my '98 Talon TSi/AWD to be exact):

You must be logged in to view this image or video.

The ECU uses this table to determine what fuel ratio to use. It's pretty simple, it calculates load and RPM then uses the cell where they meet. If the value falls between two cells, it uses a linear value between them. I.E. if 100 load is 12:1 and 110 load is 13:1, it'll use 12.5:1 at 105 load. Fuel ratios for gasoline have 3 main areas: less than 14.7:1 (rich) equal to 14.7:1 (stoich) and greater than 14.7:1 (lean). I.E. 12:1 is rich, 15:1 is lean. As a rule of thumb: too rich kills power, too lean kills motors. It can also be noted that excessively rich conditions can also damage the motor.

You must be logged in to view this image or video.

This is the stock timing map of a 2gb DSM. The ECU uses the same load/rpm calculations to pick what cell to use as it does for the fuel map. Timing can make BIG power, but it can also be very dangerous to your motor. The number in the cell that is positive refers to the degrees of timing to fire the spark plug before top-dead-center (when the piston is at the absolute top of its travel). Numbers that are negative refer to the degrees of timing to fire the spark plug after top-dead-center.

You'll notice that as RPM increases, timing also increases. This occurs because it takes time for the flame-front of combustion to move across the combustion chamber, creating the pressure needed to push the piston back down and therefore make power. As the RPM increases the plug must be fired earlier to keep combustion at the proper point so that the moving piston doesn't out-run the pressure-front, as that would simply waste the power. In contrast however, adding too much timing and firing the plug too early can cause knock which will damage the engine.

The only thing that tells the ECU there's a problem with timing is the knock sensor, which pulls timing when it hears knock (random spontaneous combustion of the fuel in the cylinder not emanating from the spark plug). Knock damages your motor, especially if it's continuously repeated, and should be the one thing a tuner avoids above all else. The knock sensor system is a reactive system: I.E. it will not pull timing to save you from knock until after you have knock. It will then quickly forget (within seconds) that you had knock to begin with. So the next time you hit the same load/RPM again you will knock again, and again, and again. Rinse and repeat until your motor self-destructs.

It should also be noted that there are High and Low octane fuel/timing maps that the ECU can interpolate (go between) if knock is excessive and constant. However, the ECU will continuously attempt to return to the the High Octane map by default. The Low Octane maps are designed solely as a safe-guard in case you get a bad batch of gas or accidentally fill up on lower octane fuel; do not count on it to save your engine long-term.

With this information in mind, you can directly combat knock in two ways: by running a richer fuel mixture, or by reducing timing. However, this is also a double-edged sword as running more rich and reducing timing also reduce power output. As a tuner it is your job to find what combination of fuel and timing make the most power for your specific setup. Most tuners find that running a bit more rich so they can add a little more timing makes more power than running less timing to use a leaner mixture.

You must be logged in to view this image or video.

This is a timing map that Ceddy posted on DSM tuners to help show what each part of the table controls; the red/white dots show an example of the path of cells the ECU would follow during a 3rd gear pull in a lightly modified 2gb DSM. Do note that the timing and fuel maps use the same load/rpm axis calculations, so the plot would look very similar on a fuel map. The map/labeling is very general but it's a good visual representation:

As you can see by the map Ceddy mocked up, it generally follows the idea that the more you push the gas, the higher the load. However, remember that load roughly equals airflow, and on a turbocharged vehicle you can make plenty of boost (airflow) without hammering it to the floor. Therefore pedal position is only a relative measure of where you'll end up on the table.

Hope that helps you out, and always feel free to ask more questions if you're still confused.

 

[delta448]

Excellent post, except for one thing...

The ECU determines load using throttle position, MAF reading, Barometric pressure (usually a MAP sensor but our cars use a MDP sensor), temperature and a few other variables.

The stock MAS actually contains 3 separate sensors. It includes measurements from karmen frequency, BARO and IAT sensors. Since the mass flow out of the BOV is recirculated behind the MAF sensor on a stock setup, there is no need for the ECU to know manifold pressure, since the mass flow measured through the sensor will not change before it passes the intake valves. Mass entering the sensor will be the mass that enters the engine, at least until you VTA your BOV or develop a boost leak.

The Manifold Differential Pressure sensor is only there to check for proper EGR function.

 

[Melbowski]

Excellent post, except for one thing...


The stock MAS actually contains 3 separate sensors. It includes measurements from karmen frequency, BARO and IAT sensors. Since the mass flow out of the BOV is recirculated behind the MAF sensor on a stock setup, there is no need for the ECU to know manifold pressure, since the mass flow measured through the sensor will not change before it passes the intake valves. Mass entering the sensor will be the mass that enters the engine, at least until you VTA your BOV or develop a boost leak.

The Manifold Differential Pressure sensor is only there to check for proper EGR function.

You are 100% correct good Sir! As noted at the start of the post it was a quick copy/paste job of an old post of mine on 3Si from almost a year and a half ago. I wasn't as keen on all the details then and I'm still no guru now, so imperfections were sure to arise. Luckily for me I have the forums and you Wisemen to cover my back.

Thanks for taking the time to clear it up, I appreciate it.

 

[TexasTurbo]

Too rich will also kill motors.