I was talking with a friend at the field the other day and it
got me thinking about how are gas engines ignitions are wired....

Can someone explain why the negative wire for the spark plug
is run directly to the plug instead of a simple ground wire screwed
to the block of the engine?

As it stands now we only have a pressure fit around the spark plug
for the negative side of the ignition, I would think a separate wire
bolted to the frame would be a more reliable setup. I also would think
that a single wire (the positive wire) running to the spark plug we could
have a nut or screw to secure that wire wire to the spark plug. That
make room for a very nice shield boot to slip over the plug, and room
for a much heavier shielding on the single positive wire.

Thanks for your help.

Hi,
You need a real good ground. The makers of electronic ignitions who ground the plug just want to eliminate possible problems.

Dave

That setup was and may still be available from some suppliers- on aftermarket ignitions
FWIW-- I asked this same question of an ignition designer -
It has to do with the grounding path and the rf radiation.
Appoarantly -the best setup is a proper, fully shielded HT cable -Which is pretty well std now .
The return path is well contained
Also -on the circuit you describe - should there be a loose connection - the radiated rf is fierce and will swamp any of the radios we use .

Keep the ground lead short, use a ring terminal, tap the head...NO problem
The ground lead to the crankcase is long and may cause problems

A proper ignition with the circuit INSIDE the cup mount is internally grounded..Guess who has been making one like that for 18 years ?

Lycoming?

Funny

"As it stands now we only have a pressure fit around the spark plug
for the negative side of the ignition"

NO thats the +ve side
the engine is the earth side so you cant use it as the +ve feed

But isn't the pressure fit of the cup more prone to a bad or loose connection
then a bolt?

No, the pressure fit metal cup on the DA an many others are
the negative side of the ignition. The feed wire is in the center
of the cup/wire and it is shielded.

My question still stands... Besides the possibility of a loose connection
with a bolt, why would the manufactures rely on a pressure fit for the
ground/negative side of the plug?

For that matter why rely on a spring tension on the positive side when
the plug could have a bolt or nut to attach the positive wire?

Do you think a pressure fitted cup on the hex head of the spark plug
is less or more reliable then a wire bolted to the block of the engine?

Who?

When you use a CH ignition with the metal caps, 10mm or 14mm, there is a clamp to tighten so there is a postive lock on the plug. Seeing the number of rubber cap leads damaged by people in a hurry to get them off, I thing that they are tight enough.

Advent.. it is..

Ralph, He is the gentleman who made the post.

C&H also had a mount with ignition inside, called Mod U El mount..
D&B engines have the Reichmuth ignition circuit inside their twin mounts also...

Sorry Advent I missunderstood what you were saying, mine are grounded at a connection somewhere else, the shield doesnt make the ground contact anyway so I still dont understand what you mean...oops I get it now, If your particular ignition doesnt have the cap grounded elsewhere and just relies on grounding to the -ve side of plug, then it is indeed a bad design imo

That is why I asked the question. This is my first gas engine
and it seems like a bad or cheap design. I never really gave it
much thought intil we where sitting and talking about it one day
at the field during a bench flying sesion.

I looked at the CH ignition and it does use a clamp around the
plug, that is better then a pressure fit, but i would still rather
see bolt for the ground and a wire screwed to the plug for positive.

Thanks for all the info.

You're trying to re invent the wheel....C&H has an ignition with a rubber spark plug cap that uses the wire shielding screwed to the cylinder for a ground..NO ONE makes a screw on spark plug wire, they went out with the Model A Ford and 7/8 pipe thread spark plugs.. Only NGK makes a resistor spark plug with a removeable top. With the resistor wire used for the spark plug there's no way to screw it on...
What's wrong with the present system ?