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| Chosing dwell for an after market coil. https://www.slantsix.org/forum/viewtopic.php?t=42585 |
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| Author: | Sam Powell [ Sun Nov 14, 2010 7:59 pm ] |
| Post subject: | Chosing dwell for an after market coil. |
If I chose to fire the coil directly with the EFI ECU, I must set the dwell for my coil in the tuning program. Since this is an after market coil, I am unsure of how to do this? The FSM is not of much use. Can anyone point me in the right direction to figure this out please? As long as I was firing the coil with the MSD box, (which I may still do), I did not have to concern myself with this. Since I have had an MSD box for over ten years, I am kind of out of touch with this bit of automotive technology. I posted this in the engine section since I think the question itself is independent of the EFI considerations and it will be more widely seen here. I feel like I am kind of in no mans land now. I don't fully understand the EFI thing yet, and yet I am losing touch with all the old basic carb tech stuff. Thanks for your help here. Sam |
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| Author: | emsvitil [ Sun Nov 14, 2010 8:31 pm ] |
| Post subject: | |
How many ohms is the coil? With a 3 ohm coil, I'd use stock dwell. The lower the coil resistance, the lower the dwell. You need to avoid coil saturation (will just overheat the coil, without and spark improvement) There's a coil output simulator somewhere on the web that may help (need to find it again, then I'll post a link). An HEI will calculate the dwell. Don't the HEI's with more than 4 pins send a signal to an ECU (square wave???) and then trigger a spark when they get a signal back from the ECU? If this is the case, it might just be what you want........... |
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| Author: | emsvitil [ Sun Nov 14, 2010 8:42 pm ] |
| Post subject: | |
Do you have all the specs for the coil? (inductance.....) And how many amps can the ECU take for the coil circuit? |
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| Author: | emsvitil [ Sun Nov 14, 2010 9:00 pm ] |
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Try this: http://cyberdave.org/HEICoilInfo.html (only glanced at it so far.......) |
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| Author: | Joshie225 [ Sun Nov 14, 2010 9:43 pm ] |
| Post subject: | |
Sam, Are you looking for an answer in unit of time or in degrees of crankshaft rotation? |
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| Author: | emsvitil [ Sun Nov 14, 2010 9:56 pm ] |
| Post subject: | |
Here's the simulator http://www.bgsoflex.com/igncoil.html |
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| Author: | Sam Powell [ Mon Nov 15, 2010 5:38 am ] |
| Post subject: | |
Wow, I had no idea there was so much to this. I thought a coil was a coil. I read the entire linked page on HEI Coil info. And found it very educational but somewhat over my head in some areas. Josua, I will do a little research and find out the answer to your question. I think it wants units of time. I will check and get back to you. The coil I am using currently is an MSD SS. It is listed on that web page as having the following specs. 1. Primary R ohms.---- 0.35; 2. Primary L;--- 7.0mH; 3. Turns ratio:;---- 70 4. saturation time@13v 7.5 amps;----3.87 5. Max primary energy mJ.----168 So here are the questions that come immediately to mind. Will the MS ECU handle the primary of this coil? If not I either do not want to fire this coil directly from the ECU or will need another coil. It seems that since the slant is a low RPM engine, the coil requirements are not as high as with a high rpm 4 or 8. What is the meaning of Primary R and Primary L? I missed that in my reading if it is there. Here is the bottom line. I am not married to any system at this point. I will change any or all parts of the ignition system if I can know for sure they will work. I clearly do not understand this well enough at this point to trust my judgment. Any good system that will work is on the table for discussion. I have this old MSD box I have been using for ten years. It seems to work fine, but maybe it does not. Maybe it has been marginal all along, and I don't know it. The main advantage right now is that the MSD box provides a trigger signal the MS II ECU can see. I cannot get this ECU to read the sine wave output of my distributor, even though the MSD box sees and reads it fine. Everything is set up correctly as near as I can tell. Since the ECU DOES see the square wave output of the MSD box, I simply need a circuit to supply square wave conversion from the distributor to get this all to work. An HEI module was used for this with the early MS II ECU boxes that did not include a VR circuit. This one is supposed to, but it does not seem to see the out put of the Mopar pickup. I would be happy buying an HEI module and scrapping the MSD box if I knew which one to buy and how to wire it. The wiring diagrams in the MEga Manual show only the newer 7 pin versions. I suppose that is fine, and I would be happy to buy that and wire it in if I was sure everything I have is compatible, and is configured right.. And compatible with the coil I select. I have posted on the MS forum of the problem getting the ECU to read the Mopar VR output. If they can lead me to a fix, then this will be a non issue. But I have been struggling with it for weeks now. Thanks as usual. Sam |
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| Author: | Sam Powell [ Mon Nov 15, 2010 6:12 am ] |
| Post subject: | |
I manipulated the dwell calculator and assuming the RPM never drops under 850, which is about where it idles best now, a dwell setting of 20 keeps the primary power draw at just under 7 amps, with the MSD SS, which is B&G's recommendation. Does this sound reasonable? I tried to do a print screen and paste here, but that did no work. It seems as if the dwell you select in the software is going to determine to some extent if the circuit in the ECU can handle the load. A dwell of 30 degrees which was the starting default put the idle draw up around 15 amps, it you idled at 500 RPM. To be honest, if I could get this car to idle reliably at 500-600 RPM I would do it. The large injectors for the turbo requirement are pr0blematic for idle. Although the better resolution with the new MS processor does provide more stable idle. Thanks for that link. That link is also listed in the other thread you gave us. Sam |
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| Author: | Sam Powell [ Mon Nov 15, 2010 4:58 pm ] |
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The tuning program asks for dwell in time. The default is set at 3 ms. Looking at the chart for the MSD SS, what dwell time is appropriate? How do I translate degrees of rotation to time? According to the calculator 15 degrees is good. Sam |
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| Author: | emsvitil [ Mon Nov 15, 2010 5:20 pm ] |
| Post subject: | |
Not answering any particular question, but if you look at the coil simulator, dwell is given as CRANK degrees. Normal dwell figures are DISTRIBUTOR degrees.... Not sure if you double or half to get required number for ECU......... |
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| Author: | Sam Powell [ Mon Nov 15, 2010 7:22 pm ] |
| Post subject: | |
If you were setting dwell at the distributor you would half the crank degrees would you not? Since it turns half as fast. However, the ECu is configured in terms of timing mark degrees, I think. The question is how do these translate to time in ms's? It is set up with a default of 3 ms. I guess you could calculate the MS it took the crank to rotate 20 degrees at 850 RPM. Let's see if I can do this right. 850/60=rpm per sec. which is 14.16 RPS 14.16*360=degrees turned per second =5099.9 degrees per second 5099/1000=degrees per MS=5.09 3*5.09= degrees moved in 3 ms. =15.2 at the crank If my math is right, then 3 ms is about right to keep the coil from overheating. Did I think that through correctly? If so, I could likely raise the time a bit. The math would show by how much. I could turn the math around backward and solve for time instead of degrees. Sam |
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| Author: | emsvitil [ Mon Nov 15, 2010 10:33 pm ] |
| Post subject: | |
I'll do the math differently 850 rpm = 14.1666666 rps or 1 revolution takes 1 rev / 14.166666 rps = .070588 sec * 1000 = 70.588 ms 3 ms / 70.588 ms = .0425 * 360 degrees = 15.3 degrees.......... Sounds good to me..... |
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