question on SD vs Alpha-N

[Yves]

Engine is SBC with ITB's and megasquirt 2 V 3.57 run on SD right now. The ITB's have a vacuum chamber underneath the runners and it has given a fairly good map signal.

During some testing that had to do with injector dead time, I noticed the following : I have started here and she seems to go really rich immediatly causing the vacuum to be too low (around 80 kPa) instead of the usual 69 kPa. This makes the engine run in a cell with a higher VE value which makes the engine even more rich.

I also noticed that when not loaded and running at higher engine speeds that the vacuum stays almost the same as at idle (around 69 kPa and a VE of close to 30)

When stumping the throttle when standing still, she seems to go lean (misfire ? ) and I cannot solve it with a AE.

It would almost seem that the map value are not a good representation of the actual airflow into the engine.

Would it be better to go to 100 Alpha-N in this situation ?

[DaveEFI]

ISTR there being a ITB mode which effectively uses alpha-N at high engine loads and speed density at low ones?

[grom_e30]

i use the itb mode on my bike its a 600cc inline 4 engine at idle the best i could pull is 85kpa so sd was out of the question, i switched over from alpha-n to itb and now have the bike at about 38-40% load at idle, it drives pretty good better than it did using alpha-n at lower throttle % i found alpha-n to be a bit inconsistent but at wide throttle it was good. where as itb gives me the best of both worlds with only 1 fuel table. you just have to get used to thinking of the load as neither tps or kpa.

[jsmcortina]

I setup a Vauxhall red-top on MS3 and ITBs using ITB mode. Worked out well.

James

[Yves]

Yes, I know, at low KPa the variation of VE from one cell to another is pretty close, but up high the number fluctuate with 10% or more, so might be that ITB is the best logarithm to use.

I did some tests with it today and seemingly the engine (even cold) doesn't seem to like to run below 13 AFR. I ran a WUE curve that was set by the autotune feature but it was way too rich. Apparently being way too rich causes it to drop in rpm and vacuum, which makes it end up in the higher kPa area with an even higher VE value at low rpm. I cut down on the WUE enrichtment values and seemingly around 105-108% seems to work well. Even IAC seemed to pull up my rpm's at idle which I couldn't get done before.

Now I have a couple of questions on setting up my table to deal with this. I currently have a lowest rpm column of 700, 1000, 1300...the engine idles around 1000-1100 when hot. Cold it can drop some to 800-900.

Do I move a column in between 700 and 1000 say a 900 and use the VE values at the higher kPa as before and set the 700 column to 27 VE which is my normal VE at idle in order to prevent it from going too rich ?

Also, since the engine didn't like to be rich, could this mean that at WOT I do not need to go to 12.5/1. I currently have it setup like that.

[Yves]

Since I'm running 2 seperate airboxes to feed each cylinder bank. Would it be possible to put 2 maf sensors in parallel and input the voltage into megasquirt as a measurement for airflow ?

[suberimakuri]

If ITB mode seems confusing, I would setup an Alpha-N map to get you going.

Alpha-N is straight forward, the VE table is fairly linear with ITB's, easy (easier).
Make sure your accel enrich is also TPS based, low % change figure.

You can revisit ITB mode once you're up and driving round.

[Yves]

Actually I have it running on SD right now. So getting it running is not the bad part (aside from some wiring issues I have right now)