EFI, Alpha-N vs Speed Density fueling Strategies

[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!