Sender: <marv@lancaironline.net>
To: lml
Date: Mon, 14 Aug 2006 05:19:07 -0400
Message-ID: <redirect-1330341@logan.com>
From: "Charles R. Patton" <charles.r.patton@ieee.org>
Subject: Re: [LML] EI sparks
In-reply-to: <list-1320923@logan.com>
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I'm confused.  I thought all spark plug wires were shielded in 
aeromotive applications (radio EMI reduction being the prime reason.)  
So the capacitance is fixed by the wire/shielding/wire length 
combination.  Mounting close or mounting on insulators would make no 
difference.  Even the P-leads (low-tension side) are typically shielded 
for the same reason. 
Charles Patton
LNC2 360JM


Paul Lipps wrote:
>   On a waste-spark system, two plugs, 180 apart in the firing order, 
> fire in series; one just before the top of the exhaust stroke, and one 
> near the top of the compression stroke. So for all practical purposes, 
> they fire simultaneously! On the Lycomings, it's 1&2, 3&4, and 5&6 on 
> the 6 cyl. So if cylinder 1 is normal and 2 is not, for instance, it 
> would indicate possible breakdown in the #2 wiring, a fouled plug, or 
> bad connections, as Scotty pointed out. The coil transformer secondary 
> is totally isolated from ground. The voltage leaves one terminal, 
> jumps a plug gap, travels across the engine, jumps the other gap, then 
> returns to the other coil terminal. It's a good idea to keep the 
> high-voltage wiring mounted away from any metal, as there is 
> capacitive coupling from the coil to ground. This coupling, along with 
> capacitive coupling from the HV wiring to ground, can form a 
> capacitive voltage divider which reduces the output. Excess wiring 
> capacitance, acting in shunt with the secondary resistance, can create 
> a voltage drop which reduces peak voltage and slows rise time. On my 
> Lancair, I use non-metallic standoffs where possible to keep the HV 
> wires away from the engine. It doesn't have to be a lot; 1/2" 
> works. It's also prudent to keep HV wiring from closely parallelling 
> other wires so as not to induce voltage transients in them; they may 
> be part of sensitive circuitry. These considerations apply to all 
> waste-spark systems, not just LSE!