dwell setting

[big mike]

This is a carry over from another forum.. I hope someone can answer this here.

I'm using ms-1 v.3 internal vb921 on a dodge neon

I wouldn't suspect dwell if it's breaking up that low in the rpm range.

I'd be careful with dwell that high. One of my ignition modules failed after about 3000 miles set at 4.2.

There is the possibility that the thermal interface material I was using between the VB921 module and the case was inadequate though. But still I'd be careful.

Steve

OK now I'm confused. When I had dwell set at 4.0, I was informed that it was too low.... Breaking up at high rpms + 8k.

Now I see that 4.2 is considered high, 4.1 gave the same high rpm break up. Can someone explain this

Yeah, this is new information. But like I said, my heatsink solution may have been inadequate.

This has just brought up a good point. Boost Junkie mentioned that dwell with the Mitsubishi ignitor was only 3.5ms and worked fine. So why are we having to run such high dwell with these VB921 modules. Dwell is a function of the coil, not the ignition module.

I have a scope but not time. As soon as I free up some time I'll try to figure out what the dwell is on a factory setup. Although I'd imagine it's variable depending on RPM.

Steve

I'm also using a MSD coilpack

[Bernard Fife]

big mike,

Did you see this: http://www.megamanual.com/ms2/configure.htm#dwell

Lance.

[BlackHat]

Can you control dwell with a MSD 6a ignition module? Or does the module control this directly?

In this case I wouldn't be using vb921 so I'm thinking the MSD 6a would control that, but thought maybe you'd know off the top of your head?

[big mike]

lance no I did not see that.


this is the coil that I am currently using http://www.msdignition.com/coil_blaster ... 9_8239.htm

I wish I knew how or what to measure...another reason I purchased a preassembled unit

[Bernard Fife]

big mike,

Since the link you give lists both the primary resistance and the inductance, you can caculate the dwell using the forumla in the link I gave earlier.

I am a little suspicious of the list 0.051 Ohm primary resistance, which seems quite low, though. I suspect it's supposed to be 0.510 Ohms. You might email MSD to confirm this.

Lance.

[big mike]

* Do not use misfiring and high engine speeds and loads to set the dwell - misfires under those conditions will likely be caused by other factors, and will lead to you setting very long and damaging dwell periods. Recall that the dwell is shortened with higher rpm in any cases, so increasing the maximum dwell setting doesn't help at higher rpms. For example, with a 3.5 millisecond dwell time, and a 2.0 maximum spark duration, the rpm at which the dwell begins to be reduced is just 2727 rpm on a V8.

* Do not increase the dwell to create a 'hotter' spark. Increasing the dwell above the 'saturation point' does not make a hotter spark, it only makes a hotter coil. For example, for a 2.5 millisecond saturation time, the current is increasing to it's maximum for 2.5 milliseconds. If this is 6 Amps, then the average current during this time is about 3 Amps. During this time, much of the energy being supplied is building the magnetic field. However, after 2.5 milliseconds (once saturation of the magnetic field is achieved), the current is a constant 6 Amps, all of which is heating the coil, so it presents a much larger load on the coil. So leaving the dwell too long is a problem. As well, the VB921 goes into current limiting mode if the current gets high (6.5 to 7.5 Amps). If the current is kept below the current limit, the VB921 should not heat up much at all. However, if run with a significant amount of current limiting, it heats up pretty fast (which can ruin it). That's why you should lower it as much as possible without misfires under normal operating conditions. If you need a hotter spark, you need another coil.

How do I fix the high rpm misfiring? It's not a hotter spark i'm trying to get, it's the unit not acting like in spark cut a rpms above 8k

[Bernard Fife]

How do I fix the high rpm misfiring?

big mike,

If your dwell is correct, and you don't have other problems (bad wires, rotor out of phase, cross firing, etc., etc., etc.) then you need a better coil to cure high rpm misses.

As it says above, 8K rpm is generally well above where the coil will begin limiting dwell (for a single coil) just because of the shortened time avaiable, so increasing the dwell won't help at all - it just make a hotter coil (from the increased dwell at low rpms). So you can increase dwell to solve low rpm misfires, but you should not increase dwell to solve high rpm misfires (this is implied in the linked material as part of how to set the dwell - you do it at low rpms).

A better coil may have a shorter dwell and/or a lower resistance & current capacity, making both a hotter spark and a higher percentage of full dwell at high rpms.

However, I'd check some of the other possibilities mentioned above first, as these are generally more likely.

Lance.

[boost junkie]

Lance, the issue is if we let the stock computer run the ignition then there's no high rpm miss problems even with the stock coil. There is definately something funky with either the hardware or software. We just don't know what it is yet.

This is a dodge neon wasted spark coil pack on MS1 v2.2 and v3.0 boards with VB921 ignitors. Firmware is 029q2. From what I've experienced, it's not nearly as much of an issue using an external ignitor from a 4G63 turbo mitsubishi.

I searched for some coil specs and came up with:

Primary: .951ohms
Secondary: 12kohms
Inductance: 7mH

[Bernard Fife]

Shawn,

Yeah, I totally agree that something funny is with the hardware/software rather than the dwell.

My point was that cranking up the dwell isn't very likely to help, and will likely burn up the coil. I think it's much more likely to be something in the hardware (whether that's the high tension side of the wiring, or in the harness or MegaSquirt itself) or software.

You know the inductance and DC resistance, so you can calculate the dwell you need (you still need the current, but the VB921 is good for 6.5 Amps, and you can try calculating for a few Amperages between 3.5 and 6.0 Amps, to give you a range of values to experiment with in the vehicle). This isn't going to solve the high rpm miss, though.

Lance.

[big mike]

Shawn/Lance,

Ok

so I turn dwell down in car to a normal 3.5. Car seems to run the same as it did before. No noticable misfires or high rpm missing.

it works so who knows