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Hello i found this relating to a Link Ecu of which i am more familiar with but is relevant to most ecus

There are a couple of separate things to discuss here. Â The fuel equation setting, what type of load parameter you span your fuel table with what happens to volumetric efficiency with altitude.

Volumetric Efficiency and Altitude

On non-turbo engines, volumetric efficiency does not change with altitude. Â Volumetric efficiency is the ratio of the volume of gas the cylinder receives per charge to the volume of the cylinder (and chamber). Â The key point is volume. Â At higher altitudes, the air is less dense, and there is fewer oxygen molecules. Â The volume of air consumed remains the same, but the mass (or more importantly the amount of oxygen) is less. Â Seeing as there is less oxygen, you need less fuel.

The story is slightly different for turbocharged engines where the efficiency of the turbocharger changes with air density and exhaust backpressure. Â Most people do not have this information and therefore roughly assume VE is remaining constant with altitude.

It may seem that the intake manifold pressure is the only thing influencing how much air the engine breaths as it is what pushes the air into the cylinder, but various combinations of running conditions can give the same manifold pressure but require a different amount of fuel due to different volumetric efficiency. Â Imagine going up through RPM at WOT on a nonturbo engine, manifold pressure is always 100 kPa, but at every RPM there is a slightly different fuel requirement due to the engines volumetric efficiency changing.

The fuel equation setting. Â

I am not going to cover all options, just the MAP setting. Â When the fuel equation mode is set to MAP, the intake manifold absolute pressure directly influences the amount of fuel delivered. Â Ignoring all other corrections, in MAP mode, if you double the intake manifold pressure, you will get double the fuel. Â Likewise if you reduce the intake pressure (by high altitude) you will get proportionally less fuel.

The final amount of fuel delivered is a function of MAP and the number in the fuel table. Â Half the number in the fuel table, half the fuel!

The Fuel Table Load Axis

The fuel table load axis decides what parameter is used to select the fuel table number used for a given load. Â We will restrict ourselves to just the MAP and MGP options for this conversation. Â A reminder that MGP is the difference between atmospheric (barometric) and manifold absolute pressure. Â It is 0 when the manifold pressure is the same as atmospheric, negative when the manifold is in a vacuum, and positive when the manifold is at a higher pressure than atmospheric (boost).

It is important to note that the parameter selected to span the load axis does not directly influence the fuel equation, rather the fuel table number it helps to select influences the fuel equation.

Putting it together

I will base the examples around a NA engine as it is simpler to explain. Â Remembering that VE is constant with altitude, so if we want our fuel table to represent a VE curve, then the ECU must take the same number from the fuel table for a given RPM and load (eg WOT) at sea level (eg 100 kPa) and the top of a mountain (eg 80 kPa).

If we had our fuel table spanned with MAP (rows at 80, 90 and 100). Â At sea level, WOT we would be in the 100 kPa row. Â Lets say that row has 30 as the fuel number. Â Lets say we are delivering 3.000 ms pulse width. Â We close the throttle until we get 80 kPa MAP. Â The fuel delivered is reduced by the use of MAP in the fuel equation to 80% of what we had at WOT (now 2.400 ms). Â But at the same time we have throttled the motor and altered its volumetric efficiency (maybe by altering the intake dynamics). Â This means to achieve the correct AFR we need a smaller fuel table number in the 80 kPa row (eg 25). Â So the actual fuel delivered is 2.000ms.

Now we drive to the top of a big hill where the atmospheric pressure is 80 kPa. Â At WOT the MAP reading will be 80 kPa. Â So, we would want 80% of the fuel required at sea level (100 kPa). Â So that would be 2.400 ms fuel. Â But as WOT puts us in the 80 kPa row and the fuel table number is 25 there we only get 2.000 ms fuel. Â This is 66% of what we had at sea level, not 80%. Â Uh oh, we are a bit lean!!!

If instead we had used MGP on the fuel table load axis, WOT would be in the 0 row at sea level and at the top of the hill. Â This is great because at WOT we have the same VE at the bottom and top of the hill, so we are in the same row in the VE table at the bottom and top of the hill. Â Our number 30 would be in the fuel table at our WOT row (0 kPa MGP) and the delivered fuel at sea level would be MAP = 100 kPa, table = 30 = 3.000 ms and our delivered fuel at high altitude would be MAP = 80 kPa, table = 30 = 2.400 ms fuel. Â Exactly 80% the fuel at sea level