Spark mod changes on DIY's Site
 

"Option 1: Spark Output to factory Miata ignitors: (Recommended)
Parts Needed for PCBv3 Output Mod using factory ignitors: (2) 270 to 330 ohm 1/4w Resistors"

I have been going through my megasquirt stuff, specifically miata mods and it seems that the required resistors on the spark outputs has change from 1K Ohm, to 270/330 ohm.

Anyone know why, and what difference this produces?

I think it produces a smoother signal. IIRC Joe Perez mentioned the change in one of his threads.

Yup. The igniter turns out to be a surprisingly capacitive load. When you first start driving it high, it consumes a lot of current and drags the signal down. So with too big of a resistor in the pullup, you get an ugly rising edge on the trigger signal. IOW, the voltage on the line starts very low, and works its way up as the igniter's current requirement tapers off. It doesn't seem to cause any actual harm, but it isn't "right", it probably affects dwell just a tiny bit, and it's likely on the hairy edge of working vs. not working.

So the change to the resistor value was to address this concern. By using a smaller value resistor, more current is available to power the igniter, thus there is less voltage drop, and so the rising edge of the trigger signal is much sharper.

The only real concern with resistor size here is the amount of current being sunk through the drive transistors (Q6 and Q8) when the coils are not on (the CPU turns on these transistors to pull the circuit low when it wants the igniter off.) The devices in question are the 2N3904 (on the 3.0) and the MMBT3904 (3.57) both of which are rated for 200ma continuous collector current. The LED itself is going to pass about 10ma, and so even if you went with a 100 ohm resistor in this circuit (which is what I'm using) you'd still only be passing 60ma total, which is well within the transistor's comfort zone. If you do go with 100 ohms, make sure to use 1/2 watt resistors as the power dissipation in the resistor will be exactly 0.25 watt and you don't want to cut it too close.

Effects dwell huh. This should be quite an interesting change.

I don't have anything as low as 330 on my desk, I have 470 and 100 ohm 1/4 watt. Ill start with the 470 and see what happens.


Thanks Joe, ill report back.

It's a tiny effect. A while back I scoped the trigger voltage vs. coil current on the stock ignition system, and found that even though the trigger rising edge was a bit grungy, it didn't seem to have a huge negative effect on the timing of the coil current. Tenths of a millisecond maybe. Like I said, it works as-is, this is mostly to satisfy purists like me who don't enjoy seeing ugly signals.

Here's a later scope capture of the same thing. This was with the EMU driving the stock igniter and coils. Yellow line is trigger voltage, blue line is coil primary current:

http://img34.picoodle.com/img/img34/...lm_3f63c21.gif

As you can see, the trigger is being significantly drawn down, and never gets anywhere near 5v. The stock ECU was slightly better in this regard, but still showed the same effect. Despite this, the current profile of the coil is close to what I'd expect to see in a healthy system. Note that in this image, it's only dwelling for about 3.7ms and thus the coil is not reaching full saturation, although it is tapering slightly. Never could figure out why the EMU did that...

Interesting stuff.

I wish I had one of those USB interfaced oscilloscopes.

I picked up 330ohm resistors because im just ---- like that, and will put it all back in the car this afternoon.

It would be interesting if this "tiny" effect, produces a big difference, at least for me.

Just an update

"Tiny effect" = 3.5 ms of running dwell.

After putting the computer back in the car I was FINALLY after about 8 months of shit was able to start and run my car with 3.5/2.5 dwell, vs the 6ms I was using since winter started and 5/4 I was using in the summer.

Also, now my fuel readings are totally off, very rich during low map.
I even had to reduce my warm up enrichments by almost 30%.

Thanks for the explanation Joe, it really helps explain why the new resistors have helped to reduce the required dwell on the megasquirt.

P.S. I sent the same question to Joe Cramer, and he said that they made the change because on a select few cars they were getting weak spark with 1k ohm resistors. It just so happens that with my Jewish luck, I am one of those.

I'll be doing this when I get back to cookeville. Maybe the '99 coils don't suck so bad after all.

Wow! I guess the 1.8s are a lot more finicky about their trigger voltage than the 1.6 cars. I'm really surprised that it had such a huge effect.

Still, glad it's working for you. :bigtu:

But now lets figure out why exactly.

Other then the external ignitor on the 1.6 what else is different with the 1.8 cars? Unless that is the only difference and now that I think about it, probably explains why it works on the 1.6 cars and not on the 1.8.

Just as Chad says, I am now also curious about the worthiness of the stock coils. If the megasquirt was producing a weak spark, and now it isnt, the stock coils should be just as good as the COP's if the prior effect of the larger resistors was effecting them as well.

Chad I am assuming you built my MS with the higher 1K Ohm resistors since it is identical to yours. If so, while my car is offline, I want to pull my MS out and do this as well. I've got access to a good soldering station so it should be cake.

So, to be sure I have this right, the overall potential effect is stronger spark with less/same stress on the coils?

Yes

To be more correct, its a stronger trigger voltage. Which I assume means it gives a longer fuller signal to the coils, if the signal is weaker the coil isnt sparking or maintaining the spark for the correct length of time due to voltage drop.

I have an OG (early) MSpnp and Savington is finishing my COPs...does this affect me?

Call DIY and ask them. I honestly don't know if the MSPNPs have always shipped with the lower value resistors or if that change was made some time into the production run. Only they can tell you.

I called DIY for clarification. Matt pointed out that this hasn't been tested on any NB coils. But he also didn't indicate any major worry, i.e. our cars won't blow up. I hope. :)

He said that "for dummies" people like me just need to put one 330 ohm resistor from pad 5 to pad 3, and another from pad 5 to pad 1.

If doing so would really make my car's coils work better and produce a stronger spark, negating the need for COPS, then I'd be very happy.

John, your MS was built with the 1k ohm resistors. I didn't know about this mod until recently.

Thanks. I'll be doing this as well then, though I suspect given my current engine state that you will be testing it before I. :)

One other note: evank has a V3.57 that was using the built in 1K pull-ups, so removing the normal pull up resistors would be something of a pain.

So do 1.6 users need to modify this as well? Or just 1.8?

Huh- I never even noticed that the 3.57 had built-in pullups, but looking at the schematic, there they are! What a thoughtful addition. And as it happens, I have pdexta's MSPNP sitting on my desk right now, and sure enough, R58 and R60 are 1k on his as well. I'll be changing those. :D

Looks like they used 1206-size parts, which are pretty easy to work without special tools. I'm used to dealing with 0402s, but these things are massive by comparison. If somebody with a 3.57 board wanted to do the mod, there's no need to remove them however. Installing a 330ohm resistor in parallel with the existing surface-mount device will yield an effective 250 ohms, which is right in the ballpark of where we want to be. With a small 1/8 watt resistor, it should be pretty easy to tack it onto the sides of the existing device.



This mod will be of theoretical benefit to everyone. It's quite possible that the variance in igniter performance is not just from year to year, but from car to car. When I scoped mine, the actual dwell wasn't being severely affected, but there was some measurable effect.