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Old 01-25-2008, 03:21 PM   #1
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Default Starting Logic

At the risk of double posting, I want to put something here I'm looking at on the MS-II extra development forums. Probably those interested should look there. But I wanted to tap into the brain power here.
Original Thread:

Well, I have a feeling I should read the source code before posting this, but I'm curious for the insights of those with a lot more experience than I.

A few nights back, playing with my JimStim, I noticed three full sets of cam signals before the MS "started". Three full engine rotations. That seems a long time to me. I'm sure it wants to see a "double pulse" on the cam signal (i.e. to uniquely identify cam position) before committing to start.

Playing the other side of the fence for a moment (I'll use the 99+ Miata as an example, I feel it's representative and I'm familiar with it) which works like so:
The crank has four teeth, two marks some distance before TDC, and two are just somewhere in between.
The cam had three teeth, a single at TDC cyl1 firing, and a doublet at TDC cyl 2 firing. Not exactly, but this is close enough.

To my simplistic view, two things will tell you where you are. Either
1) You see a signal on the cam sensor. You know you are near TDC on cyl 1 or 4
2) You see three pulses on the crank. You know you are a few degrees before TDC.

On a bank fired motor you can fire the ignition on coil pack A shortly after condition 2. You will have passed TDC, and even if you are a little retarded, the extra kick will get the motor going faster. For condition one, you likely can fire coil A by the same logic.

You could make the argument that you need to let that opportunity go by (though I don't think I would agree), but at most, two crank teeth later, you could fire the coil.

This should work for any bank fired motor. A missing tooth could be fired after only a few teeth to establish a time base and then a miss.

My only guess is that this is a noise rejection scheme, or to maintain compatibility for non-bank fire cars. Of course, nothing would keep you from firing pairs of coils until you achieve sync on a dedicated coil motor.

So, now that I've laid out the simple version, someone explain to me why it's a bad idea. The theory, while simplistic, seems workable to me.
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