EFI, Alpha-N vs Speed Density fueling Strategies

[pbp1]

I’m sure this is going to bring out some strong opinions but I have been involved in some conversations lately about the Pros and Cons of Alpha-N vs Speed Density fuel tuning strategies. This prompted me to start a discussion about why I am a fan of Speed Density over Alpha-N.

For those of you that don’t know what I’m talking about, with modern EFI systems, there are two main strategies for fueling an engine, Alpha-N and Speed Density.

In Alpha-N mode, the system uses Throttle Position to measure the load on the engine. It then uses the throttle position and RPM to identify a cell in a 3D table that fuels the engine at that specific Load and RPM. In Alpha-N, the tuner puts a value in this cell that represents the amount of time they want to turn the injector on (pulsewidth) or Fuel in pounds per hour. The FAST XFI can use either Pulsewidth or Pounds of fuel per hour. In this mode, the tuner is basically using the system like electronically controlled mechanical injection. This is under utilizing the potential of modern EFI systems because it cannot make accurate corrections to engine fueling to compensate for changes in atmospheric conditions. Alpha-N is a direct way to tune an engine for a given set of conditions but if the conditions change, guess what, you have to re-calibrate or re-tune your fuel tables. The XFI offers Barometric pressure and air temperature correction tables that are capable of trimming the fueling when these conditions change but this requires the tuner to manually setup these tables, and throws another unknown variable into the tune vs Speed Density which automatically and accurately changes fueling to compensate for atmospheric changes.

Speed Density fueling strategy uses atmospheric pressure (measured by the MAP or Manifold Absolute Pressure sensor) and air temperature to calculate the density of a given volume of air in your engine at a specific set of atmospheric conditions. The ECU uses the same standard constant laws of physics about the relationship of the density of air to the Pressure and Temperature as your weather stations and all Dynos use. Once the ECU has calculated the density of the air, it just needs to know the volume and your desired Air/Fuel ratio to calculate how much fuel to put into the engine. The displacement of the engine is a constant so the VE or Volumetric Efficiency is the variable that you use to properly calibrate or “tune” your system. The VE is a measure of how well the cylinders are being filled compared to their potential volume. Once you have found the VE value that causes the engine to run at your desired or target Air/Fuel ratio with no O2 or other corrections, then you have discovered the accurate VE for your engine at that particular load and speed (in that particular cell in the VE table). The VE of the engine does not change unless mechanical parts are changed. Your engine will flow the same VOLUME of air in Denver as it does in Gainesville, the air is just more dense in Gainesville therefore it needs more fuel to get to the same Air/Fuel ratio. Once your VE table is properly calibrated, you have an accurate Air Flow table for your engine. The benefit to the racer is that once you have properly calibrated your VE table, you will not have to re-tune or re-calibrate your fuel table from track to track anymore. The ECU will do this for you. Just so you know this is not some un-proven theory, with his permission, I want to provide a real world example of a successful race program that relies on True Speed Density. Jeff Dona runs his SS LT1 Firebird in Speed Density, Open loop and he has run his car in 1.015 to 1.095 correction without changing anything in his tuneup and the actual Air/Fuel ratio never strayed more than 2% from the target Air/Fuel ratio. This allows a racer to focus on other aspects of his race program without worrying about his or her tuneup every time the engine goes from the dyno to the track or from track to track.

Some EFI manufacturers offer a hybrid Speed Density mode that allows the tuner to enter fuel in pounds per hour instead of Volumetric efficiency. The problem with that is that you lose the benefit of TRUE Speed Density as you have overridden the ECUs potential to accurately compensate for changes in atmospheric conditions. The tuner can set up some compensation tables but why guess at the needed changes in fueling when TRUE Speed Density uses known laws of physics to compensate accurately.

The engine does not know or care how the ECU decides how much fuel to inject into it. It is possible to have two tunes for an engine, one in Alpha-N and one in Speed Density and they both inject the exact same amount of fuel at all loads and RPMs, the only difference is, when the air gets better or worse, the Alpha-N tune will still provide the same amount of fuel while the Speed Density tune will compensate properly. I am not talking about the engine running richer or leaner, I am talking about the fact that as the air gets worse, it takes less fuel to get to the same Air/Fuel ratio!

I am not trying to put down anyone who uses Alpha-N fueling strategy as I know people who do so successfully. If you do use Alpha-N and are happy with your program, stick with it. I just wanted to provide some facts and information on how Speed Density uses modern EFI to its full potential.

I welcome comments and questions.
David Page
Fuel Air Spark Technology

[Tony Curcio]

I can see how using a Speed Density map would get the last bit of performance on a heads up run, or in a no-breakout index class such as Comp.

But, if the afr is changing while going down track, doesn't that make it harder to predict an accurate dial in? Whether using a weather station formula, or one's own ratio of performance loss or gain per 100 feet altitude change, don't these calculations assume the afr is not being adjusted from run to run, let alone during the run?

[Ed Wright]

I run mine in Speed Desity. I consider AlphaN the easy button. They can tune it on the dyno, and ship it, leaving adjusting for conditions to the ownner.
Set the break points in the V.E. table right and once you see the Baro that causes it to reference a different row in the V.E. table and get the fueling right you won't have to change anything when you get in that air next time. First time I went to Great Bend KS mine was running 4th row down. Never had before (higher elevation) so dialed it in. Next time I went there it was fine. Once you get it dialed in, you seldom have to change anything for weather changes. I have never run mine in AlphaN. Been Speed Density for ever.

Tony, why do you think the air/fuel would be changing going down the track? I don't get your reasoning.

[Tony Curcio]

The above description says it's using the MAP sensor to determine conditions. Doesn't that mean it's adjusting any time the engine is running?

[Ed Wright]

Originally Posted by Tony Curcio

The above description says it's using the MAP sensor to determine conditions. Doesn't that mean it's adjusting any time the engine is running?

Why would that change? It changes a little in the top of the gears, but seldom to a different cel at a given RPM. My A/F stays dead nuts one end of the track to the other, unless I want it different. Mine does like it a little different at higher RPM, so I put it there.

[pbp1]

Tony, if the atmospheric conditions are changing while you are going down track enough to affect performance, those conditions are to blame for you missing your dial in, not your EFI system. There is nothing an ECU can do to offset those changes, it can only provide the correct amount of fuel to maintain a consistent Air/Fuel ratio so that the change in performance is proportional to the change in air. In reality, it is doubtful that the air can change enough in the 9-10 seconds to cause you to miss your dial in.

[pbp1]

Tony, the A/F ratio is not being adjusted, the fuel is being adjusted to keep the A/F consistent in spite of air changes. The MAP sensor is constantly registering changes because it is reading the "atmosphere" inside the manifold and that is what the engine sees. It's all about maintaining a consistent, or at least a desired Air/Fuel ratio.

[Bobby Fazio]

Originally Posted by pbp1

Jeff Dona runs his SS LT1 Firebird in Speed Density, Open loop and he has run his car in 1.015 to 1.095 correction without changing anything in his tuneup and the actual Air/Fuel ratio never strayed more than 2% from the target Air/Fuel ratio. This allows a racer to focus on other aspects of his race program without worrying about his or her tuneup every time the engine goes from the dyno to the track or from track to track. [/FONT]

I thought Open Loop meant there was no such thing as a target A/F ratio since no correction is present? Basically it's a carburetor, no? What if it is 65 degrees and 500' DA during Stock Q1 and then 80 degrees and 2000' going into E1, wouldn't you have to make some VE changes? My stocker runs Open Loop Speed Density, Sequential fire, and was deadly to the .001 second during Q1 and Q2 but lost 1 tenth in Q3 and another tenth in E1 at the Summernationals because it got hot on Saturday.

I guess what I want to know is what method is the most consistent for bracket racing? Predictable dial-ins? And not having to hook the laptop up before or after every pass to add/subtract fuel or guess on a fuel setting before first round?

[Signman]

Originally Posted by Bobby Fazio

I thought Open Loop meant there was no such thing as a target A/F ratio since no correction is present? Basically it's a carburetor, no? What if it is 65 degrees and 500' DA during Stock Q1 and then 80 degrees and 2000' going into E1, wouldn't you have to make some VE changes? My stocker runs Open Loop Speed Density, Sequential fire, and was deadly to the .001 second during Q1 and Q2 but lost 1 tenth in Q3 and another tenth in E1 at the Summernationals because it got hot on Saturday.

I guess what I want to know is what method is the most consistent for bracket racing? Predictable dial-ins? And not having to hook the laptop up before or after every pass to add/subtract fuel or guess on a fuel setting before first round?

Bobby,
Cells in fueling table of FAST 2.0 are VE values for the given load and RPM. Those values are developed by achieving the optimal HP for that cell (generally done a a dyno) recording the AF ratio. That optimal AF ratio is input the closed loop AF table in the related cell and becomes the target in closed loop or your tuning target open loop when adjusting at the track. In speed density open loop the system does not adjust to the live AF input from the O2 sensor but does use air temp and manifold pressure to calculate the optimal fueling to achieve that VE value in each cell of the fueling table. In closed loop AF valued from O2 sensor is used in addition to air temp and manifold pressure.
Manifold pressure is effected by outside barometric pressure and engine vacuum with any restriction in the intake system.
Once the correct VE values are input the car should be very consistent and write time slips. In my case it does when I leave it alone.
With the FAST system you can run Alpha-N with MAP sensor open to atmosphere, I have not done it but others have been successful as XFI 2.0 has a change making it more to pressure changes seen from MAP sensor input. So you can use ALPHA-N and not need to tune over different or changing weather conditions.

See you soon!

[GUMP]

Originally Posted by pbp1

.....Once the ECU has calculated the density of the air, it just needs to know the volume and your desired Air/Fuel ratio to calculate how much fuel to put into the engine.....

.....Once you have found the VE value that causes the engine to run at your desired or target Air/Fuel ratio with no O2 or other corrections, then you have discovered the accurate VE for your engine at that particular load and speed (in that particular cell in the VE table).....

.....Once your VE table is properly calibrated, you have an accurate Air Flow table for your engine.....

These three sentences speak volumes.

I can have a new combination running well in Alpha-N within 3-4 dyno pulls. I have also found that it is pretty easy to dial a car from my log book once I have a couple of runs on it. I also don't have to worry about sudden changes in performance due to too much fuel correction.