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Leo Chabot

MAP or MGP for fuel table

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Hi everyone,

I'm running a G4 plug-in il my Turbo V6 WRX Subaru.

What are advantages to use MGP or MAP for main fuel table for turbo engine? (and atmo engine ?)

My point of view for turbo :

I tune my fuel point for 120 MAP and 100 BAP so 20 MGP in table.

Another day there is 95 BAP so for the same MAP (120 because boost is regulate in absolute pressure) the MGP point is 25 ? So the ECU is going to inject more fuel whereas there is the same air mass in cylinders (because 120MAP !)

Am I Right  or not?

 

Thanks !

Leo.

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HI Leo

In MAP mode you loose barometric compensation. This applies to turbo or NA applications.

The engines VE gets worse as altitude increases requiring a reduction in fuel.

We tested this on a a very large hill in central Otago (the race to the sky)

The 135 turn course climbs from 450 meters (1500 feet)  to 1500 meters (5000 feet)

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The ECU has two sensors one is for MAP and one is the barro the MAP cal matches the MAP to the barro sensor at the same pressure as both sensors are measuring absolute pressure.

So a cal on a 105 day and a cal on a 95 day will give the same result.

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OK.  Always confusing to explain, but I will try put this one to rest...

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

Volumetric Efficiency and Altitude

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

The story is slightly different for turbo charged engines where the efficiency of the turbo charger changes with air density and exhaust back pressure.  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.  Imagine going up through RPM at WOT on a non turbo 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.  

I am not going to cover all options, just the MAP setting.  When the fuel equation mode is set to MAP, the intake manifold absolute pressure directly influences the amount of fuel delivered.  Ignoring all other corrections, in MAP mode, if you double the intake manifold pressure, you will get double the fuel.  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.  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.  We will restrict ourselves to just the MAP and MGP options for this conversation.  A reminder that MGP is the difference between atmospheric (barometric) and manifold absolute pressure.  It is 0 when the manifold pressure is the same as atmospheric, negative when the manifold is in 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.  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).  At sea level, WOT we would be in the 100 kPa row.  Lets say that row has 30 as the fuel number.  Lets say we are delivering 3.000 ms pulse width.  We close the throttle until we get 80 kPa MAP.  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).  But at the same time we have throttled the motor and altered its volumetric efficiency (maybe by altering the intake dynamics).  This means to achieve the correct AFR we need a smaller fuel table number in the 80 kPa row (eg 25).  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.  At WOT the MAP reading will be 80 kPa.  So, we would want 80% of the fuel required at sea level (100 kPa).  So that would be 2.400 ms fuel.  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.  This is 66% of what we had at sea level, not 80%.  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.  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.  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.  Exactly 80% the fuel at sea level!

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Great explanation.

So even with MGP on the load axis the ECU is using absolute pressure (MAP) for the basis of fuel corrections. The advantage is that MGP better identifies the correct cell (VE) on the fuel table if the altitude (air pressure) changes. The assumption is that VE does not change with altitude (air pressure), and hence that VE variations with load are caused by changes in throttle position rather than changes in manifold pressure.

I hope I got that right.

Thanks

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Thanks for your explanation Ashley !

I'll try to be clear.



I understand, but not really this part, the most important :

"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. Exactly 80% the fuel at sea level!"


Can you detail it ?

I understand that in MGP setting there is a correction based on MAP value to determine pulse width.

So without other correction : Pulse whidth (ms) = Master * (table value/100) AND maybe * (MAP value in kPA/100) just when MAP is less than 100kPA ?




I find this in the help, but NOT only for MGP setting, is it the explanation ? "The value of Master is the injector pulse width (in milliseconds) that results if the active cell of the of the Fuel Table is set to a value of 100 and the manifold absolute pressure is 100kPa" 

What happens when MAP is below 100kPA ? In MGP and MAP setting.



I'm very interested, thanks to answer.

(I don't understand why there is no more equations/specifications/flowcharts/code..etc in the helps to understand how the ecu works, It's a pity !)

Léo.

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Hi Léo,

I will ask Ashley to have a look at your reply. In the mean time, here is something that will help you and others determine the correct settings to use:

photo loadaxis.png 

Cheers,

Scott

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Hi 

How is that on ignition side? In my eyes we can't run the same ignition timing in this two cases because of different VE as stated above. As a example for a NA engine 

1. We tune the engine on the dyno at 100kpa baro, 80kpa MAP with part throttle wich the engine has a VE of say 80%. We tune ignition timing to the knock limit. 

2. We drive the car at 80kpa baro, rich same cell as we had on the dyno with part throttle, but now with WOT, which means we have a higher VE (say 100%) , which means we have effectively more oxygen in the cylinder. This means we can't run the same timing without struggle with knock!

Thats how i understand that, maybe i'm missing something? 

How is that on the new firmware 5.2.2 Should we use same axis setup as before? 

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"The volume of air consumed remains the same, but the mass (or more importantly the amount of oxygen) is less.  Seeing as there is less oxygen, you need less fuel."


Sorry but I'm not that sure. If there is less ambient pressure which forces the air into the combustion chamber, there can't flow the same volume of air into the combustion chamber, isn't it? 

Or is that effect part compensatet through less exhaust pressure?

Thanks for answer! 

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