EFI Systems

[Bobby Fazio]

Excited about my new FAST system and still reading instructions while my car is still in Virginia! How does load index differ from traditional speed density? Every track I race puts me at 97.5-99.5 kpa going downtrack so I figured I would change my y-axis to include a row for 97.5, 98.5, 99.5 kpa so I would have a row for any track I race at. When I first started I had a 95kpa row and the next row was 101kpa, so I could never get it exactly right going from a 97.5 track to 99.5 track. It would work for one track then have to be fixed for the next track. The differences between my three new rows are very minimal, just a little bit in the peak torque area just to keep the AF ratio consistent at all the northeast tracks.

Sounds like load index speed density calculates a load percentage based on the difference between Baro and manifold pressure, so does that mean I need an accurate row for (for example) 96%, 98%, 100%, etc.? I'm not yet understanding how the two types are different or how load indexed is the "easier" one to use. I'm guessing at Atco it would be close to 100% while in Norwalk it would be down to 90% or so?

[pbp1]

Bobby, you are exactly right. Load Indexed Speed Density calculates load as MAP divided by Baro. The flaw with standard speed density is that as you go to conditions where you have lower barometric pressure, you will run in a lower row in your fuel, spark, etc. tables because your vertical reference in these tables is straight MAP. What we have always done to compensate for this is just tune those rows the same so that your tune stays the same, but what if you are running back in a good barometer situation with a throttle stop? Now, you will be in a lower row in the table, but you have entirely different airflow in your engine. You have to remember that the fuel table in any speed density application is actually an airflow table. The numbers in this table represent VE or Volumetric Efficiency. Simply put, how well are you filling the cylinders at this load and RPM point. If you think about it, you can see the rub, In the low barometer example where you are at high altitude, the Volume of air you are putting in the engine does not change, only the density of that air changes. In the second example where you are in good air (high barometer) but you are using a throttle stop, you are actually affecting the volumetric efficiency of the engine because the throttle blades are hindering the cylinder filling. Because of this, to properly tune, you need decreasing VE numbers in your table as you come down from the top row. Now if you do this, you will be in trouble when you go up in elevation because you will be lying to the ECU about the volumetric efficiency. Load indexed speed density fixes this because when you turn the key on, the system samples the MAP sensor to get a "Baro" reading and basically calibrates that MAP reading whatever it is as 100% load (or the 100% row in the tables). Now, you can build your tables properly to reflect the decreasing VE as the MAP comes down, but when you go to high elevations or "Bad Air", the system calibrates whatever the MAP reading is at key on as 100% load. So no matter what air you are in, you will always run in the 100% load row in your tables as long as you are wide open.
Now, for those who will say that as you get into bad air , low baro, high elevations, etc., you need less fuel, I agree, that is correct. But the MAP reading is already taking care of this in the air density part of the math that is going on behind the scenes. No need to put in a false (lower) VE number in bad air because the ECU already knows the air is bad from the MAP and Air Temp readings and it calculates the correct amount of fuel accordingly. In this density part of the equation, the adjusted or indexed load is not used, it uses the true uncorrected MAP reading just as it should to calculate how dense or "good" the air is.