I've been driving my car for the last 6 months with an obvious electrical problem...I know about patience.

 

Lol

 

I know that the US-spec '90-'93 cars had a baro sensor on the ECU board. I'd assumed that any Miata (or any car) with a VAF sensor would need one, as it's the only way to be able to correlate volumetric airflow to actual air mass. Without knowing barometric pressure, the ECU would not be able to distinguish between 100 CFM in San Diego (which is at sea level) and 100 CFM in Alma, Colorado (3.156 meters / 10,578 feet above sea level) despite the fact that there is 30% less air per cubic foot in Alma than in San Diego.

So I can pretty much guarantee that as barometric pressure increases, you are going to see an increase in fuel PW and a decrease in spark timing at all operating conditions, because an increase in barometric pressure will cause an increase in computed load.



Do you have a two-channel scope? I ask because I would think that RPM would be easy to measure without using the MS- just attach CMP to one of the scope channels, set a counter for frequency, and divide.

Or you could always do a parallel install on the bench.

 

Yes, it is a 2Ch 60Mhz scope. I am using both channels to correlate the injection timing (one probe on NE, one on the injector) and the ignition advance.

 

Thanks reverant for finding out this information.
Hopefully this info leads to better understanding of what the stock ecu does for startsup and other conditions.

 

Well, there you go. If you've already got Ne (which we often call CKP, I mis-spoke earlier when I said CMP) connected to the scope, then all you need to do is dial out the horizontal far enough to do a frequency count on that signal (I'm assuming you have a digital scope with built-in measurement functions) and you can then derive RPM from that without needing the MS. Just divide the frequency of Ne by 120 and you have RPM.

If your multimeter does frequency counting, you can use that instead, and not even have to bother adjusting the scope's timescale.

 

Any new nuggets of wisdom extracted from the stock ECU?

 

Yeah, the stock ECU fires the injectors half the times the Megasquirt does, and that fucked up everything that I've measured so far, concerning the injectors. I need to figure out how to get the MS to fire the same number of injections as the stock ECU.

 

this still the early 89-93 ECU i.e. batch?

How very odd, almost like simultaneous injection on the MS, with 1 squirt per cycle?

 

When you said the ECU adjusted idle by altering advance, what ranges are we talking here? Thanks

 

Exactly. It does half the injection events, without about twice the pulsewidth.

Still on the 89-93 ECU. I also have a 94-95 ECU to play with, but no stim for that year yet.

 

That was with the stock 01-05 ECU. I did not measure any numbers as that was with the ECU in the car, when I was still running a parallel setup. When I switched the ignition control to the MS, the idle was all over the place (the idle valve was still under the command of the stock ECU). Switching back to fuel only gave an instant rock-solid idle. My spark map was flat at 10* during idle, so the only reason that would explain the oscillations (+/- 200rpm) would be that the stock ECU uses the ignition as well to control the idle.

Btw that was the main reason why I decided to go for a full standalone setup.

 

altered the baro sensor pressure and seen any results?

 

I can't WAIT for the 94-95 ECU info. Which ECU do you have; there were 2 in 95 in the U.S. IIRC.

 

You know, I never noticed this before.

Rooting around on my hard drive, I found the following scope trace, which I must have taken on a running car, given how ugly the INJ pulse is:



My notes say that this is the INJ 1/3 wire, but that's about it. What bugs me is the filedate- this picture claims to have been taken in 2008, and I'd have already been running a Megasquirt by that time. Maybe the date got corrupted in a file copy operation...

But yeah, there's clearly only one injector shot per engine cycle here. At moderate to high load, I would think that the MS's two-shots-per-cycle would, in theory at least, produce slightly more even fuel distribution, as each amount of fuel will spend the same length of time "lingering" in the port regardless of which cylinder it is on. At idle, however, it may well be that the stock ECU's method is superior, as one shot of fuel can be more precisely metered than two half-shots.

Alas, this capture was taken at idle, so perhaps the stock ECU switches to two-shots-per-cycle at some higher load condition? Can you test for this?

 

I can't with the onboard baro sensor. At least not someway I can think of, I couldn't see a port on it somewhere.

 

BPL9 ecu. Got it from ChicksDigMiatas.

 

Up to 2000rpm and full load on the AFM (pot), it was still 1 injection event per cycle. I haven't tested further.

 

there is a hole in the small window of it, I was figuring a small tube glued onto that connected to syrunqe (spelling) and pressure gauge

 

2002 ECU. Look at the funny injection sequence - semi batch or something?!?!? It is fully batch below 500rpm - but only during the first cranking event. Above 500rpm this is what you get:



It goes like this:

1-2
1-3
4-3
4-2

Oh and y8s...alternator no chargy below 300rpm.