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Alpha-N calculation?

EFI Tuning Fundamentals

Discussion and questions related to the course EFI Tuning Fundamentals


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How about Alpha-N calculation? There are times that this is the only way to go (N/A running ITBS, aggressive cams etc.)

Alpha-N can essentially operate the same way as a MAP based load axis. You can still operate a VE table with TPS as the load input. The pressure input into the ideal gas law can be derived from a MAP balance tube that samples all 4 throttles. Another option that's commonly used is to use the MAP sensor to read barometric air pressure. This works, although it only gives a true manifold pressure reading at high throttle openings so the VE numbers aren't accurate at lower throttle openings where true manifold pressure will be much lower than barometric pressure. The other option that is quite often used is where alpha-n is incorporated with an ECU that uses an injection time based fuel model where you are simply requesting a specific injection pulsewidth in each cell.

But when running ITBs does the balance tube read actual vacuum or due to sort runner length and high throttle plate area the reading is much higher than what it should be?

What you'll find is that at idle and low throttle openings you tend to see a reasonable vacuum level. The vacuum also increases at higher engine speed and low throttle openings. Depending on the size of the throttle bodies though you generally end up with no real vacuum above perhaps 30-40% throttle which is one of the reasons you can't use MAP as the load axis on an itb engine.

What I have noticed with my tune so far is that at slight throttle openings with less then 20% throttle (above 20% using TPS below MAP) the engine is running rich and above 20% running at my target AFR with slight lean on throttle openings! If I increase the Tip-in enrichment the engine runs a lit bit reacher! I should note that I haven't installed an idle air valve on my ITBs and I'm trying to have the idle without this! (Cold start is a pain). Should I change the MAP-TPS relation? Should I run TPS at low RPM and then switch to MAP? or decrease the level that the TPS is engaged lets say 6-7%?

I would like to congratulate you for your work as it has been very helpfull and very educational and also to thank you for any advice in advance!!!