drifting timing with rpm - Page 3 - Miata Turbo Forum - Boost cars, acquire cats.

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Old 11-23-2015, 11:36 PM   #41
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codrus recently scoped the GM coils. He has data.
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Old 11-24-2015, 12:10 AM   #42
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I believe the LS coils will fire if they think they're being over-dwelled. This would lead to advance at high rpm.
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Old 11-24-2015, 12:21 AM   #43
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<Stupid double post>
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Old 11-24-2015, 12:21 AM   #44
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I scoped the LS3 coils, not the truck coils. They're probably different, but:



(There's an obvious typo on the second line in the chart).

The first three columns are voltage, because saturation dwell varies depending upon the input voltage to the coils. This is why the MS3 has a curve. I measured it at various points because I was curious how much it varied, but it's the ECU-observed one that really matters because that's what's used as an input to the MS3 dwell curve. The delta between ECU-observed voltage and coil-observed voltage may vary by car, though.

Note that you don't necessarily want to saturate the coils all the time. For maximum coil life you want something around the minimum dwell that still produces a strong enough spark not to misfire, and the saturation level merely indicates the limit beyond which you're not going to get any more spark strength.

--Ian
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Old 11-24-2015, 12:25 AM   #45
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Quote:
Originally Posted by stefanst View Post
I believe the LS coils will fire if they think they're being over-dwelled. This would lead to advance at high rpm.
I don't think so, I ran them at something like 2-3x the saturation time during my testing and they didn't fire prematurely.

--Ian
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Old 11-24-2015, 12:27 AM   #46
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How did you alter the voltage?
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Old 11-24-2015, 12:28 AM   #47
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Quote:
Originally Posted by Joe Perez View Post
In the absence of scope data, I'd tend to trust the manual. Also, ideal dwell time varies with voltage. There's no one single number which is ideal for all cars.
A number of people have pulled the GM dwell curve out of the factory computer, it's a 3d table with RPM and voltage as the inputs. It goes from 0 to 8000 RPM and 0 to 26 (!) volts. I dunno how GM expects the Corvette to run at 1 volt of system voltage, but....

--Ian
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Old 11-24-2015, 12:29 AM   #48
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Quote:
Originally Posted by aidandj View Post
How did you alter the voltage?
I borrowed a variable power supply from Jason.



--Ian
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Old 11-24-2015, 12:30 AM   #49
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And the MS3 only has a 2d table correct?
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Old 11-24-2015, 12:31 AM   #50
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Ah, didn't even think about running it off something besides the battery. Duh.

I had a 30amp variable power supply at school. I miss that.
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Old 11-24-2015, 12:33 AM   #51
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Quote:
Originally Posted by aidandj View Post
And the MS3 only has a 2d table correct?
The MS3 has a bunch of different ways of controlling it. IIRC, there's one method that's RPM-based (an old one I think) and another one that's got two 2d tables, one off voltage and the other off MAP. I think I have mine with a voltage curve and a flat MAP curve.

--Ian
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Old 11-24-2015, 07:43 AM   #52
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Here is the current dwell - voltage correction table.
Timing change must be hapenoing outside of the MS control or it would apear in the logs? attached is the log from the video on an earlier post, it shows a steady 10 deg when the actual timing was 10btdc deg at idle to over 20btdc at 6k rpm
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Old 11-24-2015, 12:23 PM   #53
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OK, this is starting to make sense.

Your coils were automatically discharging due to over dwell. This is something that some variants of (not all) LS coils do. The coil manufacturer felt it best to discharge the coil, and save its life, and fire the charge in advance of the ECU control. This saves your coil and destroys your engine. Good stuff.

I would replace all 4 coils and set dwell with a scope. But based on your observation with a timing light, 4ms is a better number than 5ms.
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Old 11-24-2015, 12:27 PM   #54
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Quote:
Originally Posted by Ben View Post
OK, this is starting to make sense.

Your coils were automatically discharging due to over dwell. This is something that some variants of (not all) LS coils do. The coil manufacturer felt it best to discharge the coil, and save its life, and fire the charge in advance of the ECU control. This saves your coil and destroys your engine. Good stuff.

I would replace all 4 coils and set dwell with a scope. But based on your observation with a timing light, 4ms is a better number than 5ms.
Exactly what Ben said. All of it.

FWIW I run the IGN1A Race coils, and they will not auto-discharge on you, and are actually a bit stronger than the GM coils. I run mine at 3ms dwell, .030" gap, and no misfires at 28 PSI. With 4.5ms I could run .040" gap with no misfire at 28 PSI. Currently at 26 PSI, 3ms, .030, works great.
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Old 11-24-2015, 12:48 PM   #55
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I really appreciate everyone's help on this!!
I have ordered a scope and will try to confirm Ben's theory by using the coil test setting with > 5ms dwell to see if it is firing before the 5ms, i will post the coils p/n if i can find it to hopefully help others from having the problems i had.
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Old 11-24-2015, 01:25 PM   #56
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Quote:
Originally Posted by HHammerly View Post
I have ordered a scope and will try to confirm Ben's theory by using the coil test
Wow, that was quick.

If you don't already have one, you will also need a current probe capable of measuring steady-state DC. (Most cheap current probes are AC only).

If you don't have one, you can construct a cheap substitute by just wiring a very small-value resistor in series with the +12 to the coil primary, and using the scope to read the voltage which develops across the resistor.

I'd suggest something like 0.1 ohm. It doesn't have to be precise, as we're interested not in knowing the exact current but merely the shape of the curve. Just be sure it's rated for at least 1w, as it's going to develop some heat.

Example; 0.1 Ohm 5W Resistor Wire Wound 5% Tolerance

Also, be aware that the act of inserting the resistor will change the dwell curve slightly, by reducing the effective voltage at the coil. This will skew your observations by a few tenths of a ms.

Last edited by Joe Perez; 11-24-2015 at 01:38 PM.
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Old 11-24-2015, 01:51 PM   #57
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I need a current probe. Time to go to surplus.
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Old 11-24-2015, 02:26 PM   #58
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You can get reasonably decent ones off of eBay for under $100. This is the one that I have: CP 06 AC DC Current Probe 10kHz 40A 6 5mm Jaw Size | eBay

The build quality is very poor, but the actual performance is pretty decent. As compared to the $5,000 Tek probe we had at Harris, it was accurate to within a few %. More than good enough for automotive use.


The ones made by Hantek also seem reasonably well-reviewed, no first-hand experience: http://www.ebay.com/itm/NEW-Hantek-C...AAAOSweW5VPgRX
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Old 11-24-2015, 02:30 PM   #59
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I had one at school. I'm just going to check the salvage carts until I find one. I'm not in dire need.
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Old 11-24-2015, 08:30 PM   #60
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We use this one:
https://www.picoauto.com/products/cu...-current-clamp

No comparisons against a $5k clamp, but it's held up over 8 years of shop use, so I think it's pretty good.
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