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Alpha N downsides?

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I have an aquaintance running an EightStack ITB system on a small block Ford. Using Speed Density and having some issues.

I am running a similar system on a BigBlock Ford using Alpha N and running fine. I understand AlphaN is the preferred system for the ITB with short runners.

He received some comments "I always run in Speed Density as long as the system is plumbed to give a good vacuum signal to the 1-Bar map sensor. Alpha-N is very problematic for well-documented reasons."

In earlier posts, Simon commented Alpha-N was the preferred system however I don't have a comeback for this. Does anyone know what the problematic well-documented reasons would be?

No, Alpha N is by far more appropriate for ITB's. The two main downsides I can think of are really only relevant to very basic outdated ECU's (and maybe some current USA offerings LOL), any modern ECU should be able to do it well.

The first would be some older ECU's have no means for baro correction in when Alpha N mode and it is quite necessary if the car is driven over a change in altitude or if the local baro pressure changes with weather significantly.

The second is for decent low load control you need relatively fine resolution on the fuel table axis at small throttle openings. Typically say 0%, 2%, 5%, 10% etc. Many older ECU's didnt have flexible breakpoints (they used say fixed 10% or 33% steps) and some didnt even interpolate the axes.

^ I'm with (not so) stupid

Possible downsides:

1. Need to make sure the mechanic never adjusts the TPS sensor, changing the measured voltage at low throttle angles can cause drivability problems.

2. Some of the older/cheaper TPS designs seem less reliable than MAP sensors, especially on a setup with solid engine mounts and lots of vibration.

3. If you're running a true old-school Alpha-N config without any MAP sensor at all, you might need to make some compromises to avoid occasionally running too lean when load changes at idle or very low throttle angles. For instance, a MAP sensor might detect the change in load when a radiator fan kicks on but the TPS sensor doesn't show it.

That said, I really like how Alpha-N engines drive, especially if there is also a MAP sensor installed to handle minor variations in load. I try to use both TPS and MAP for fuel calcs on all engines that have reliable TPS and MAP signals, including turbocharged engines.