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Spark plug gaps


jagnut66
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Hi, 

I have had cause to replace the spark plugs in my Herald, which, of course, meant I had to check the gaps on the new plugs I installed, plus the spare set I like to carry.

I repeated this for the spare sets for my Minors, which I ordered at the same time, incidently the plugs and gaps are the same for both cars, all of which set me thinking.......

I have done this for years now, following what the manuals advise for different cars, without ever questioning why and simply accepting that it was the correct procedure.

But what determines the gaps for different cars, does 0.001" really make any difference and who determined what the gapping should be and how to calculate it in the first place?

This is a question brought about by curiousity, by the way, before anyone starts reaching for the Uzi 9mm's ammunition mags.........😄

Best wishes,

Mike.

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34 minutes ago, Badwolf said:

Mike - Interesting thought considering that it now appears to be recommended to run modern tyres at a higher pressure than our manuals suggest. 9mm's at the ready for the pedantics??

Modern tyres are a lot different to those fitted as original equipment, construction and materials have changed. When it comes to spark plugs and gaps, I too will be interested in what the collective knowledge has to say.

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The plug gap is a matter of compromise. If too narrow, the spark is physically too short to reliably ignite the charge, and you get misfires. If too wide, the HT may be insufficient to ionise the charge and make a spark in the first place. So the optimal gap depends on how much energy is available, which depends on the coil and the distributor, but also on the compression pressure you expect (so the engine's VE, CR and timing, as well as typical driving style).

There's also a consideration of service interval. Plug gaps tend to expand with use, so the gap you set at a service has to allow for the amount of expansion before the next service. The MGF, due to its awkward engine location, has unusually long service intervals for plugs, so the ignition system was specifically designed to tolerate very wide plug gaps.

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yes ours had a service interval of 10,000  before change  a modern on FI and all the emission and very different electrodes can be 20.000 

the basic gap is as Rob says and the time it takes to jump the gap and timing advance are all things to consider

yes the pressure in the chamber makes a lot of resistance as to where and how the spark jumps

who used the old champion plug tester and cleaners  the plug looks to work ok till you applied 100psi air pressure and then the spark leaps all over the place 

put that into the combustion pressure at way more than the air line  and a plug and gap that looks fine  will be useless under pressure 

from the experience of misfiring cars having plugs with an R in do not work well with our limited HT of around 22KV 

where as a modern produces well over 32KV 

unless you upgrade the HT voltage and power i would stick with a factory spec that   works , its not as if many classics do many miles between services 

unless youre a travelling salesman in a classic belting up and down the motorways  ( does that even happen now??}

Pete

  

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3 minutes ago, Pete Lewis said:

unless youre a travelling salesman in a classic belting up and down the motorways  ( does that even happen now??}

A few years back there was an article, which ran to at least three pages with pictures, in Practical Classic about a chap in Scotland who used a Rover P4 to travel all over (I beleive he was some sort of rep), he did a deal with his company, so he could run a classic P4 instead of a boring modern. Mind you, first he had to prove it would be reliable enough for work use (I suspect the plugs were one of the items he renewed). Once he'd done that he spent the allowance for a company car on his Rover each year.

Fair play to him. I've always liked the idea of owning a Rover P4, one day maybe.

Best wishes,

Mike.

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The above refers to plugs of the 'classical', single-ground-electrode type.  There, the ground electrode overlies the central electrode and may be bent to adjust the gap.   The spark jumps axially.

An old design, but one that has been adopted for modern needs, has multiple ground electrodes, two, three or even four, around the central electrode, so that the spark jumps radially.    These are designed not to be adjusted, because the spark will jump to only one electrode, ignoring all the others, until that becomes worn.   The worn gap will inhibit sparking and another electrode will bear the current, until that too wears away.   Thus there can be much longer service intervals for the plugs, a policy that endears them to fleet managers.  Certainly, a life of 100,000 miles is now considerd usual.

Of course there are many designs for spark plugs, even one like this.  Try setting the gap on that!

333705123_FlamespinningElectrodeSparkplug.jpg.8790416727d36f4993241b3b572ea914.jpg

 

 

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  • 3 weeks later...
23 minutes ago, Andrew said:

Just going off tangent a bit can anyone explain the lost spark ignition on moderns please 

Usually referred to as "wasted spark", which is more accurate.

If you have a single coil firing four (or more) cylinders then you need some way to direct the high voltage to the right cylinder. This is the top part (and the original part) of the distributor, where a rotor arm points somewhere close to the right HT lead. The energy has to jump this gap as well as the one in the spark plug. Also, the distributor adds cost, especially when you're no longer using the low tension part of it because the spark is scheduled by a computer.

For maximum efficiency, the computer could schedule sparks on an individual coil per plug. This means there's only one spark gap, so the coils can be smaller. However, you need lots of them, and lots of output drivers from the computer, and you also need the computer to know not only the crankshaft angle but also which half of the two-rev (four stroke) cycle it's on. That all costs.

Wasted spark is a middle ground. The computer only has one coil driver per pair of cylinders, and you only have one coil per pair, too. That coil drives both spark plugs, usually with one of them at the "wrong" polarity (it's cheaper and easier to put the coil between the plugs rather than having twin secondaries, plus the electromagnetic coupling makes twin secondaries potentially troublesome). What this means is that the computer knows when cylinders 1 and 4 are approaching TDC and it's time to fire the spark. It doesn't know which one needs the spark (because it's on the compression stroke) and which one doesn't (it's on the exhaust stroke) but that doesn't matter because it's going to give them both a spark. One of those sparks ignites the mixture and provides power. The other spark does nothing - it's completely wasted, except that wasting it makes the whole system cheaper.

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