little random post

[robertisaar]

got to thinking about the igniton again....

with 5mS dwell periods.... and each coil getting fired once per revolution(waste-spark)....

the EST circuit has to transition from high to low every time for a coil to charge, then discharge. if the EST line remains high for 5mS for each coil, that's 15mS per revolution... 15mS revolutions = 4,000RPM. so beyond 4000RPM, 5mS dwell periods aren't possible.

so let's look at 3mS dwell periods.

3mSec X 3 coils = 9mSec. 9mSec dwell periods = 6,666 RPM.

2.5mSec = 8,000 RPM.



so.... depending on the real-world coil saturation time, you could realisticaly lose spark energy above ~6700 RPM...

with a 4 cylinder application using the same coils, they hit 10,000 RPM at 3mS before it becomes a problem.

with a 8 cylinder application(93-99 northstar and LT5), 5,000 RPM....

then again, this is assuming that the ICM isn't doing anything differently. i read it somewhere from someone who stuck a scope on a 60V6 ICM, but supposedly, the ICM limits current to the coil to ~5 amps or so. if that weren't limited, then they could certainly charge up faster considering their primary resistance is actually really low. i think somewhere around .5 ohms when i last checked....

more assumptions would be that the ICM doesn't change the current limit based on RPM.... a lot of "what ifs".

[Superdave]

I'd snapshot a waveform of it but i don't think i like the idea of hooking my $5k Scope up to a plug wire.. lol

[robertisaar]

no need to deal with the high voltage side of things.... either a signal of the EST circuit at high RPM or of one of the coil packs primary side terminals. both would be at roughly 5 volts.

EDIT:

or i could hook up my bench and take a look at the dwell period the ECM is generating.... with the 555 circuit i'm using for pulses, i have damn near no resolution up there though. tap of the knob goes from 4000 to 8000.

[robertisaar]

instead of double-edit, a new post:

you know...... i don't think it would be that difficult to create your own CDI coils/controller and just use the falling voltage signal from the ICM for a specific coil pack to know when to start triggering the plug.... essentially combining the existing parts for the logic of selecting the correct cylinder and the advantages of CDI ignition. using an Arduino(i love those little things), just have to watch 3 interrupts, when the circuit transitions low, have the coil(s) responsible for that cylinder pair fire off in rapid succession, get at least a couple of sparks to fire at lower RPMs, set up transition points so that the multi-sparking doesn't interfere with the possibility of the waste-spark system accidentally setting off the companion cylinder WAY too early......

now i just need to look into what all is required on the coil/cap side of it. could literally plug all of it into the ICM.

[Superdave]

I thought i remember reading that the output of the ICM to the coils was some sort of higher voltage out of an ignitor or something like that.. That's what stopped me from using a DIS coil to fire the dual plug setup i was going to use on the go kart.

[robertisaar]

i might have to find the article i read up on.... i THINK it was from a goldwing fanatic who was adapting the GM DIS stuff for his bike, but i'll need to dig around some more. IIRC, it had scope patterns for a few interesting things.

[robertisaar]

http://home.comcast.net/~loudgpz/GPZweb/Ignition/CBheiModCavCoil.html

took a while to find, and the info is a bit different than i remember.... does have good info though.

http://home.comcast.net/~loudgpz/GPZweb/Ignition/IgnitionCoils/IgnitionCoils.html

as does that in the "cavalier coil" section.... mostly:

Resistance: 0.402 ohm
Inductance: 2.8mH
This coil reaches 5.5 amps in 1.2 msec with 14 volts applied.

not that i play with inductors much..... but from what i get out of it, is that lower voltages take a longer time to fully saturate the coil compared to a higher voltage. considering they're hitting dwell periods of just over 1mSec with 14 volts applied, and the ECM commands roughly a 5mS dwell period in steady-state conditions, i would ASSUME it's getting fed significantly less voltage.

[robertisaar]

http://www.searchautoparts.com/motorage/maintenance-repair/gm-dis-part-i

http://www.msextra.com/forums/viewtopic.php?f=94&t=7287

that's interesting as well. apparently, UP TO 10 amps being fed to the coil. 10 amps at .4 ohms is 4 volts.

why GM chose such a low voltage, i'm not sure.

[Superdave]

This i'll have to play with then...

My plan was to mount a 3500 CPS on an adjustable mount above the flywheel on a 5 HP Briggs (stock coil location), then feed the timing pulse to a DIS coil and fire 2 plugs. I need to be able to adjust the timing because of the wild cam and this will work out better than offset flywheel keys.

If i get a chance i'll test it this weekend.

[robertisaar]

when in doubt, voltage divider?

and from the sounds of it, you're basically bypassing the need for an ICM for the B&S engine?

[Superdave]

yah, i just need about 25 degrees. Would be cool to be able to control it with a '7730 so i could take it down to about 10 degrees at idle. I'm using a jr dragster cam with a stupid amount of duration.


It's going to be a long winter, maybe that's a good basement project.

[robertisaar]

designing my own PROM emulator using two seperate banks of RAM.... whether or not i actually build it, IDK, but it at least is giving me some interesting problems to solve. if i could figure out what the solid-state equivalent of a single-pole double-throw switch is(rather than the single-pole single-throw nature of what i am using), it would be so much simpler, but i'm just not that up to date. from the looks of it, switching the address and data busses of the two RAM banks between two processors will require 8 74244 ICs(each of which control 8 circuits, so up to 64 circuits can be controlled), which has been the most complicated part of the process to figure out.

i get the feeling that what i'm really looking for is a some type of multiplexer IC? anyways, this certainly seems like a doable scheme thus far. basically, program one of the RAMs(which will be converted to NVRAM quite easily) before giving power to the ECM, turn-key and go. once you see a change you want to make, do it, upload the new BIN to the emulator, it writes it to the other bank of RAM, when it finishes, it hands it off to the ECM and then is ready for more changes to make to the first RAM.

of course, once i get this far, the other ideas start rolling in..... the processor doing the RAM writing has a BUNCH of 10-bit A/D ports to play with..... can run ALDL through here instead of through a seperate cable..... has a lot of leftover inputs/outputs.....

essentially, i can make my own AutoPROM..... though to program PROMs, i would need to add yet another address/data bus...... but at least it wouldn't have to be the complex dual arrangement explained earlier.....



i think i have too much time to think about these kinds of things.

[robertisaar]

looking around at S10 stuff because i want my own mini truck to putt around in(the 92 2.3/5 speed ranger annoys me at times, especially with no exhaust), found this interesting little chunk of info from the GM sales brochures back in the day.... 1991 to be specific.

the 2.8 actually seems to not drop off as badly as the 2.5 or 4.3 do after passing their torque peaks.



then just a larger version of the 2.8 graph:

[robertisaar]

MOAR iron-head..... possibly the lamest version ever built, the U-body 3.1 TBI.



now to see if i can find the LB8, iron LH0.... already looked for a GM L32 chart, but couldn't find it....

[Superdave]

I always thought the U body 3.1 would be an awesome start for a nasty budget hybrid, I assumed it had the 7x notched crank.. and they probably haven't been abused much.


Add that to the list of things i'll probably never do.. lol