MSD Race Ignition and PICO HT

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MSD Race Ignition and PICO HT

Postby hegge » Thu Oct 16, 2003 1:50 am

Guys,

I am a new user of the PICO scope and was wondering if there is any limitation to the HT pickup? I am planning on using it on an MSD ignition and racing applications. The voltage could exceed 100K I am told. Will the pickup work okay if I create a custom range and scale it accordingly?

Thanks,
Doug
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Postby ricardo » Thu Oct 16, 2003 8:44 am

Hi Doug,

Thanks for the enquiry. You should be ok with the HT pickup lead, essentially what it does is attenuate the signal by 10k:1 so your 100kV will show up as 10V on the scope. You can use your own custom range to display the 10V as 100kV.

Best regards,

Ric
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Postby dhegge » Fri Oct 17, 2003 2:35 am

Okay Guys,

I had a chance to hook the PICO up to the race engine today... I created a custom range expecting to have ignition HT secondary peaks of 100K. I connected the HT pickup around the Taylor 8mm race wire. Fired the engine and a suprising thing happened.

The signal was only indicating 10K volts... which is way low for an MSD ignition. I am wondering if the race wires maybe effecting the capacitance pickup of the MT probe? Perhaps the wire exhibits a different electrical characteristic or something?

Also I was connected to plug #1 and the spark was jumping all around the screen perhaps this has something to do with an .880 lift CAM :) Makes it kinda hard to trigger off of. My old standby Tek 465B Analog scope actually had a much easier time triggering.

Any Ideas?
Thanks,
Doug
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Postby ricardo » Fri Oct 17, 2003 10:07 am

Hi,

If you email me a .psd file I can have a look at your settings, increasing the number of samples per trace and selecting a lower timebase so that you just get one spark on the screen should improve things.

As far as I understand, just because the coil is capable of producing 100kV, it doesnt necessarily mean that you will get a 100kV spark. I imagine it would depend on the size of the gap and amount of compression?

Tom - Have you got any thoughts on this one?

Cheers,

Ric
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Postby Autonerdz » Fri Oct 17, 2003 8:41 pm

You are correct, Ric. The KV demand will be influenced by the largest gap in the circuit, as well as the timing of the spark and how much pressure is in the cylinder when it fires. The fuel mix also has a large influence. (lean drives up demand)

Your maximum KV requirements will be experienced under a snap acceleration. At this time, the spark will occur closer to TDC (higher compression) and the mixture will be quite lean for that instant as the engine takes a big gulp.

Never would I expect the KV demand to go anywhere near the max coil output. A stock normally aspirated angine KV demand should be under 25K. For full blown race applications, the demand would be higher under maximum demands.

High energy coils are better able to keep the spark lit as long as the burn is consuming fuel. Higher KV demands shoeten spark time, as there is only so much energy available.

To reliably capture the firing line, which is only a few microseconds, you will need to have the samples maxed out at 32,000 and have less than 100ms on the screen. More time will require the use of the min/max selection which will accumulate passes on the screen to display the max KV.

:shock:
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Postby Carl Grotti » Sat Oct 18, 2003 7:26 pm

Hi Doug,

The highest output ignition coil made by MSD would be the Pro Power Coil with a maximum output of 55,000 volts. They can be used on the 7, 8, and 10 systems. For most racing applications, 40,000 volts is plenty and widely used. If it's good enough for "Top Fuel", you should be ok. The primary voltage may reach 500 volts on some of these. This won't hurt the scope if you use a 20:1 attenuator. Under no circumstances would you need to use any scaling higher than 50 KV for secondary. You'll probably find yourself wanting to scale at 27 KV for the top end.

Not sure what you mean by having trouble triggering. I think your referring to the varying firing KV spikes. Like Tom said, the firing line is very short in duration. Somewhere in the 10-15 uS range. If your scope didn't sample at the exact time when firing KV reached it's peak, you simply won't see it. Also mentioned and very important for the 212/3, make sure your samples per scope trace is turned up to 32,000. Even with this setting, you'll still see varying KV spikes. The amount of variation depends on the timebase your using. Try to keep this at 10 mS or less. On a race engine, you should come close to getting all events on the screen at idle. If not, come off idle just a little and you will. At 20 mS/div, the sample rate roll off becomes apparent. Min/Max will fill in shadows of the previous peak passes (filtered) if your interested in focusing in on the firing KV for a consistant resistance issue.

The KV demand for ionizing the plug gap is important, however, the spark line tells us much more about the combustion process. This is why I don't get too hung up on firing KV.

Carl
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