Megasquirt Cold Starting Pulse Widths

Dr. Hess wrote: Right now it isn't firing at all. In the past when cold, I'd crank it a while, then stop, then try again and it'd fire, if that's what you mean, Eric. Otherwise, I'm going to assume it is in batches and not all 4 injectors at once. My injectors are wired up in 2 banks of 2. The priming pulse probably hits all 4 at once. The motor has to go around once for the MS to figure where it is to determine which coilpack to fire. The timing pickup is off the distributor hanging off the back of the exhaust cam with 2 teeth removed out of 24, creating a 12-1 pulse pattern. Here's a pic of the Megasquirt derived computer I built for my wife's Sportster. http://www.drhess.net/images/bbms3.jpg The HD collector's cigarette package is shown for size reference only and not to be construed as a smoking endorsement.

The latter is what I was asking. Ok, so in theory, it should need two total injections to fire.

Oh, and ONE prime pulse? Or is the prime pulse used to crank it?

I suppose I could go and learn the code, but I think it would be much more useful to you if you learned it. If you know what I mean.

Eric

There's one prime pulse when you first turn the ignition on. You can set how much (mS) to prime. Then it uses the lookup table posted by dan_efi above to get the cranking pulsewidth.

I can work my way through assembler. I just don't like to. I am a very good C programer, ANSI C in particular, but I can muck through C++. I wrote my suspension design software for the Locost in Borland C/C++.

From the Megatune help file:

Help said: Note: Cranking pulse occur at every ignition event, while running pulses only occur at the interval specified on the constants page; if you have a 4 cylinder engine and are taking the tachometer signal from the coil, then you will get four (4) cranking pulses per cycle and depending on settings one injection per cycle while running.

Each cylinder is getting 4x of cranking pulsewidth for each intake stroke. If you are getting sufficient voltage to keep MS online while cranking, I would look at decreasing the 20F pulsewidth to maybe 10ms. Those 295cc injectors could very well be dumping too much fuel right now.

Also I think opening the throttle while cranking would effectively lean the mixture, as you are no longer allowing the intake to hold any vacuum.

Thanks for finding that, Dan. That makes sense and jives with what pkrstr8 and alphadriver have been saying. So 16mS x 4 is 84 mS. That may be a bit fat with my big OEM injectors.

I tried to start it when I got home this evening and managed to get it fired. It has warmed up to 29F. So, I took it to O'Reilley's and got a new 770 CCA battery. That spins that 4 cylinder pretty good, 10.5:1 compression and all. Tomorrow morning I'll fiddle again and see what happens. As I said, you only get one shot at it a day.

Dr. Hess wrote: Thanks for finding that, Dan. That makes sense and jives with what pkrstr8 and alphadriver have been saying. So 16mS x 4 is 84 mS. That may be a bit fat with my big OEM injectors.

stick to saving lives, doc. leave the math for the rest of us.

16 x 4 = 64 !

Yeah, 2^6.

Well, it started this morning. Ambient temp was 29F, block temp was in the low 30's. I dunno how fast it cranks now with a fresh 770 CCA batt, but it is a lot faster. I think that last battery has been going bad for a year. I leaned out the 20 and 40F buckets to 8mS and it fired on about the 2nd revolution. I had to "catch" it with the throttle to keep it going for the first 3-4 seconds, then it started idleing fine after that. Now, there is a big difference between 29F and 19F, so I'll keep fiddling with it each cold morning.

Dr. Hess wrote: Thanks for finding that, Dan. That makes sense and jives with what pkrstr8 and alphadriver have been saying. So 16mS x 4 is 84 mS. That may be a bit fat with my big OEM injectors. I tried to start it when I got home this evening and managed to get it fired. It has warmed up to 29F. So, I took it to O'Reilley's and got a new 770 CCA battery. That spins that 4 cylinder pretty good, 10.5:1 compression and all. Tomorrow morning I'll fiddle again and see what happens. As I said, you only get one shot at it a day.

It's not 4x a normal pulse, since with banked injectors, you already get two pulses per cycle. It's twice the amount.

BTW, DON'T rely on the prime pulse- that's going to be very unreliable. If you do a 24 hr soak, the remaining pressure in the fuel system will be 0, but after 8 hours, there will be some. Yes, the fuel pump turns on when you key on, but there's a delay from when it turns on to when you actually see flow in the injectors. Basically, it will be a very un-robust solution. We deal with similar problems.

That 64ms crank pulse is very reasonable at 70F. Last time I checked on a PFI motor, that's about how much we fuel to fire on the first try. Closer to 100 will be better for 20F.

Seriously. More fuel. Don't be afraid.

Leaning it out on future rev's isn't a bad idea, but you will risk not firing.

To do this really well, and accurately, you need a real WB sensor, and good data. As an example, we've currently got like 8 cars up in Thompson, Manitoba doing cold start testing, with full instrumentation. Thankfully, 80% of the work I personally do is at a specified 70-80 F.

Eric