Alpha N vs Speed Density

[Orange65]

I have read that Alpha N is used for naturally aspirated engines with low vacuum signals (less than 10 in). I have also read that throttle body choice is more critical with Alpha N. I would like to hear from those that have tuned high perf NA motors- do you prefer Alpha N or Speed Density for this type of application? What about sensitivity to throttle body size in Alpha N mode- have you seen this in your experience?

My engine runs less than 10 in of vacuum and while running the fuel table on my MS1, it idles in the upper left hand quadrant of the table. I can see that that is really going to restrict tuning points as I have eliminated about half of them. Is there any way to change the Y axis such that I am working in a smaller range of pressure on the Y axis or is Alpha N my best choice? This is a street driven BBC with a long duration cam.

[BigAL]

From what I understand, running Alpha N is basically like running a carb, it's the lease efficient way to run EFI. Speed density would be the better way to go since you'd be able to use a MAP sensor, I was lookng at running Alpha N with my IR intake that I'm putting on my 406, but after reading about it, I changed my mind and am going to fab up a small plenum to reference a MAP signal. I'm Scott can give you a better understand of it than me. Hopefully he chimes in...

[dieselgeek]

I've always had better luck with Speed Density than Alpha N but it's easy to try both. McConnel's N/A SBC idles with maybe 4" of vacuum and works fine with the common plenum manifold, but I've used vacuum manifolds with stack EFI too. Upgrading your $12 MS1 processor with the MS2 would be a simple way to run the MS2-Extra features like "MAP sample window" which is REALLY handy for unsteady MAP signals, you pick the actual crank angle that the MAP samples are taken, say from 30-120* ATDC where the vacuum signal is highest, and you get a really nice load reference. I'd try it with the MS1 first and see how it goes. It's simple to upgrade processors later.

[BKBridges]

That's a pretty cool feature re: MS2 MAP sampling.

[Orange65]

Thanks for the replies. I have the truck running and have done some driving with it. I think I have some bugs in it still. The AFR varies up and down constantly according to the AEM wideband- ranging from 13.7-15.2 while driving for instance over a 20 second window. It shows in the logs as well. It is constantly varying. I have a fuel pressure gage on order so I can watch fuel pressure to ensure no bad fuel pump. I am also going to change the VE table to make the Y axis have even steps on it. Right now, it steps 5 kpa mostly with a few 10 kpa steps. I am not using anything less than 50 kpa when cruising, so I am going reset the table to start at 45 kpa and go to 100 kpa in even 5 kpa steps. I don't know if that will do anything, but it will make the table prettier.

Does the AFR normally vary while driving under a constant load?

DG- the vacuum signal varies a good bit. I may try your idea with the MS2.