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[aaronjb]

I stuck NGK single electrodes into the Omega (after LPGing it) once as a trial and there was no improvement over GM twins that were cheaper

True, if GM twins are cheap you might as well use them The Omega is hardly a highly tuned powerhouse even with LPG I doubt the ignition system is at all 'marginal'

I switched to NGK copper plugs in the other cars that specified (expensive!) Iridium or Platinum plugs, though (those are also specified for service interval reasons only; they give a weaker spark than copper AFAIK)

[Andy B]

... The spark is vertical between the centre electrode and a random ground prong, the other three ground prong ends are essentially shielding it from the cylinder.

(Well that's the theory, anyway)

Have never looked at a twin or quad plug then? You theory holds for the old style single earth plug, but the twin & quad plugs have the earth arranged around the outside of the central electrode so the spark is horizontal between the 2 or 4.
I'm with Chris (as long as he uses a bit of copper slip when he refits them  ) as the combustion chamber will see as much spark from a quad as it will see from a twin.
However when I removed my quads last year after a couple of years  (20/25k miles??? :-/) they looked quite 'worn' so I replaced them with twins.

but WTF do I do ?  :-/

[aaronjb]

I guess 'new' quads are different from the ones I looked at 10+ years ago then (which were exactly like I described).. does answer how they reduce wear on the centre electrode then (since you'd be eroding four sides of it rather than the top of it, IYSWIM)

[feeutfo]

why would GM put quad plugs in cars if twins were better?

To extend the plug life to a supposed 80k. Shame they are all loose by 60k...

So twins give 80k life? In theory?

I see the theory on doubling the number of electrodes to spread the wear over 4 instead of two. Got that, that's why i fit quads.

I have trouble understanding how a cylinder full of compressed fuel by a ratio of 11.1 or whatever it is will ignite worse with two extra electrodes.... ? the spark radiates around the four, facing the piston. I don't get it. The flame front starts with spark. Theres nothing between it and the piston crown.

Surely? What am I not seeing ?  :-/

Twins give 40k, quads give 80k... In theory

Don't completely understand why it shields it, but that's the theory. In reality I don't really see much difference but as the twins are slightly cheaper and I change them annually I'll go with them

Yes, TB has it fuddled again. That was my point. Silly. 

[feeutfo]

I guess 'new' quads are different from the ones I looked at 10+ years ago then (which were exactly like I described).. does answer how they reduce wear on the centre electrode then (since you'd be eroding four sides of it rather than the top of it, IYSWIM)

No mate, the spark is horizontal or parallel to the piston crown radiating out from the centre.

Theres nout there to shield the spark. Quads for me thanks. 

[Andy B]

....
Theres nout there to shield the spark. Quads for me thanks. 

You'll be struck down for disagreeing with OOF hierarchy ............... 

[Lazydocker]

....
Theres nout there to shield the spark. Quads for me thanks. 

You'll be struck down for disagreeing with OOF hierarchy ............... 

Having said I'll go for twins, I might change my order to quads and we can do a side by side comparison with TB's... At the end of the day, it's only pennies in it price wise :-/

[feeutfo]

....
Theres nout there to shield the spark. Quads for me thanks. 

You'll be struck down for disagreeing with OOF hierarchy ............... 

Having said I'll go for twins, I might change my order to quads and we can do a side by side comparison with TB's... At the end of the day, it's only pennies in it price wise :-/

Not the money as you say. Just want to understand what we're talking about here. Logic, or my logic anyway, says quads all day.... For twice the miles...? In theory.

Reality might be more 40k for quads, 20 for twins? ESP if lpg'd...? <- note. Question. Not a statement. I don't trust vx's desire to lengthen established service schedules to suit fleet managers budgets.

[hotel21]

....
Theres nout there to shield the spark. Quads for me thanks. 

You'll be struck down for disagreeing with OOF hierarchy ............... 

Hardly...

Personally, was giving an individual understanding, nowt else.

And I can do those smileys too.... 

 

    

[Andy B]

....
Hardly...

Personally, was giving an individual understanding, nowt else.

And I can do those smileys too.... 

 

    

I wasn't really thinking of you ..... 

[TheBoy]

why would GM put quad plugs in cars if twins were better?

To extend the plug life to a supposed 80k. Shame they are all loose by 60k...

So twins give 80k life? In theory?

I see the theory on doubling the number of electrodes to spread the wear over 4 instead of two. Got that, that's why i fit quads.

I have trouble understanding how a cylinder full of compressed fuel by a ratio of 11.1 or whatever it is will ignite worse with two extra electrodes.... ? the spark radiates around the four, facing the piston. I don't get it. The flame front starts with spark. Theres nothing between it and the piston crown.

Surely? What am I not seeing ?  :-/

Twins give 40k, quads give 80k... In theory

Don't completely understand why it shields it, but that's the theory. In reality I don't really see much difference but as the twins are slightly cheaper and I change them annually I'll go with them

I think quads are marginally cheaper, but the cost is not relevent here, as its only pence.

[TheBoy]

why would GM put quad plugs in cars if twins were better?

To extend the plug life to a supposed 80k. Shame they are all loose by 60k...

So twins give 80k life? In theory?

I see the theory on doubling the number of electrodes to spread the wear over 4 instead of two. Got that, that's why i fit quads.

I have trouble understanding how a cylinder full of compressed fuel by a ratio of 11.1 or whatever it is will ignite worse with two extra electrodes.... ? the spark radiates around the four, facing the piston. I don't get it. The flame front starts with spark. Theres nothing between it and the piston crown.

Surely? What am I not seeing ?  :-/

Twins give 40k, quads give 80k... In theory

Don't completely understand why it shields it, but that's the theory. In reality I don't really see much difference but as the twins are slightly cheaper and I change them annually I'll go with them

Yes, TB has it fuddled again. That was my point. Silly. 

Eh? How can so many people misread that?

GM put quads in to extend the inteval from the (over optimistic) 40k to the even more over optimistic 80k....


....simples....

[TheBoy]

I believe the theory is, the more bits sheilding the spark from the fuel in all directions, rather than just the downward side, the worse the plug is for its function.


Note, LPG specific plugs, such as NGK Laserline, are single electrode...

[Marks DTM Calib]

Nobody has said it shields the spark, it does shield the flame front....

Given the denisty of fuel air mix once compressed there is always going to be something to ignite, its the speed that the flame can spread to the rest of teh mix that is impeded.

Just remember, these plugs were ONLY fitted to get the service interval upto 80K miles....i.e. above the 60K mile benchmark the fleet operators look at (similarly the twins were fitted to get the interval upto 40K)

[feeutfo]

And we're discarding them over the benefits because of that...?