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(Post Link) post #1 of 10 Old 1 Week Ago Thread Starter
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Fun with spark plugs

Lately I've been noticing a significant (but not huge) loss of power at lower rpm (from idle up to about 3000 or so), accompanied by a slight increase in engine vibration, and some apparent slight misfiring at idle (though this could be a perception created by the increased vibration, the cylinder might still be firing?). On inclines which the car was once happy to climb in a higher gear the engine was now feeling a bit strained and wanting a lower gear. A bit more than usual clutch slippage had become required to move off from rest. Above 3000rpm the engine still performed well. Engine is a 2.0L TS.

The plugs were only 50,000km old, and AR reccomends plug change at 100,000km, so they ought to have been OK. Pulled them out anyway and found all eight to be somewhat encrusted with dry deposits (seen worse though, on plugs that still fired). Cleaned them on a wire wheel, taking care not to touch the insulator with the wheel wires (which could conceivably leave steel track marks on the ceramic, potentially creating a misfire if the high tension voltage earthed through the tracks rather than jumping the gap). I poked the ceramic insulators with a sharp scriber point, cracking off most of the deposits. I then scraped the electrodes with the sciber point to remove remaining deposits from the electrodes.

With the plugs now more or less free of deposits, on the secondary plugs I could see significant 'scalloped' spark erosion on the underside of the the earth electrodes. This means that the effective gap had increased significantly compared to new. On the primary plugs I found significant scallopped erosion on the sides of the centre electrodes, again meaning that the gaps had significantly increased. The centre electrodes of the primary plugs should look cylindrical in side view, most of them were decidedly no longer so (with one exception).

So it was a reasonable bet that the engines' ailing performance could well be due to bad plugs, despite them being only about half way through their projected life span. Not wanting to spend significant money on new plugs (having already strained the automotive budget on recent parts and fluid purchases for our other two cars), I decided to have a close look at the used plugs that I had on the shelf. I had sixteen other used plugs, eight that were in the engine when I bought it (unknown KM), and another eight from my recently acquired parts donor car (also unknown KM).

After cleaning these as above, from the total of 24 plugs I selected eight with the least apparent electrode spark erosion (very little). I ended up with only one plug that had been in the engine recently, and seven from either the original plugs (in the engine when I bought the car) or from the donor car (don't know how many from the donor car or from the original plugs).

All symptoms have gone. The engine now runs more smoothly and with significantly more power at lower rpm, and I have evaded the significant cost of eight new plugs...

It occurs to me that with two plugs per cylinder, if one fails then the cylinder will still fire, but performance will reduce. It's my understanding that using twin plugs will allow the use of a later ignition event while still allowing peak cylinder pressure to be reached at X ATDC (I think this is around 8 ATDC, the crank angle at which peak pressure most effectively translates into force acting on the crankshaft, but don't quote me...). The point is that being able to 'retard' the ignition timing (i.e. not so far BTDC) reduces pressure rise before TDC, so reduces 'pumping loss' as the piston is rising against an expanding flame front for fewer degrees of crank rotation.

This is the advantage of using twin spark plugs, as I understand it (possibly simplistically...). If we have twinned plugs, and one fails, then the effect is (I think) somewhat similar to a single spark plug engine (per cylinder) with significantly retarded ignition timing.

This could include;
Loss of power (directly from the affected cylinder).
Possible increase in vibration (remember setting the timing on cars with distributors, and the vibration increasing if either too advanced or too retarded?).
Hotter exhaust valves (potentially burning a valve).
Hotter cat converter as more hydrocarbon gets burnt in the exhaust system rather than in the cylinder.
More unburnt hydrocarbon being detected by the ECU (which might then inapproriately lean off AFR for more than one cylinder, so indirectly reducing power from more than just the cylinder with one failed plug?).
Etc...

Regards,
John.
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(Post Link) post #2 of 10 Old 1 Week Ago
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I was under the impression that the 100,000km service interval is with platinum/iridium tipped spark plugs which generally are not able to be cleaned or serviced. Copper end spark plugs usually can be cleaned up safely with no negative effects on the tips, but these usually have shorter 30,000 km or less lifespans. I'm not too sure on the plugs in the TS motors however.

Still, it seems that you fixed up your issue for now without a significant outlay in time and money on new plugs. The 100,000 service interval is probably also based on everything else being correct with regards to fuel/air ratio and short vs long trips, oil consumption etc which would all affect the lifespan of a spark plug.
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The TS plugs are platinum tipped. Platinum and iridium are harder and more heat resistant than the copper or nickel alloys used in conventional plugs, so more resistant to becoming 'rounded' off' over time (sparks don't propogate as well from 'rounded' electrodes as they do from electrodes with well defined sharp edges).

The secondary plug (PMR7A) seems to have a copper post (probably, due to the superior electrical and heat conductivity of copper) with a small piece of platinum attached the top of it (I think this is the litte 'pointy' bit on the centre electrode). NGK describes this plug as having a "Fine Platinum tipped centre electrode and Platinum tipped ground electrode". It's hard to see (at least on a used plug), but a magnifying glass reveals a small 'bump' on the end of the ground electrode, which I assume must be it. Judging by the erosion pattern, it doesn't look like the old plugs (the discarded ones) were sparking to the platinum tip on the ground electrode, but to the metal near the platinum tip...

The magnifying glass does not reveal similar 'bumps' on the ground electrodes of the primary plugs (BKR6EKPA). NGK describes this plug as having a "Platinum wrapped center electrode", and the ground electrodes as "multi-ground" (no mention of platinum on the ground electrodes).

I suspect "multi-ground" plugs work (extend plug life over conventional plugs) by sparking between the centre electrode and whichever ground electrode happens to present the easier path to ground for any given ignition event. As this electrode (or the side of the centre electrode adjacent to it) erodes away and its' gap increases, it must eventually become the harder path to ground, and then the spark must jump between the centre electrode and the other ground electrode (which has become the easier path due to still having a smaller gap). I can't see the spark ever jumping between the centre electrode and both ground electrodes simultaneously, but switching between each ground electrode, which it may do multiple times over the life of the plug as erosion takes place, thus extending service life.

I can't see why these plugs couldn't be cleaned, so long as the platimun tips are not damaged by doing so, or the platinum wrapping (which implies a coating somewhat thicker than mere 'plating') hasn't been eroded away.

So far so good. The engine is running much better than it was, but I'm confident that my 'refurbished' plugs are very unlikely to last as long as a new set would...

Regards,
John.
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(Post Link) post #4 of 10 Old 1 Week Ago
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That's a good official description of the OE NGK spark plugs. With the reasoning you added, hopefully it will keep everyone right about the plugs;
PMR7A plugs have wear resistant platinum in a needle tip.
BKR6EKPA simply have twin earth electrodes which give a larger spark area. Only one of these electrodes fires at a time- the one of least resistance.

If a plug gains heavy deposits, have a look at the fuel or engine oil which is being used. That will likely be the cause but it can be hard to find the root cause accurately due to time and distance involved.

I've noticed spark plug wear with TS engines before and letting the BKR6EKPA plugs go the distance in a JTS is a reliable way of wrecking ignition coils. It is far cheaper to fit new spark plugs earlier than scheduled.

Obviously erosion and deposits are influenced by driving conditions as well. I'm not surprised spark plug efficiency reduction can be detected at even half the scheduled distance. I've wrecked spark plugs by 48000 miles with hard driving before. Smoothness of the engine was reduced as was its eagerness. Just remember spark plugs erode faster if an engine is turning at high rpm a lot.

Last edited by Fruity; 1 Week Ago at 09:35.
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I had a couple of part of NGK Platinums in my TS last around 70k and it was still running fine.

When I compared new for old it was obvious they were worn but not as bad a I'd expected still firing ok. Compared to the rate our MiTo eats plugs when I change them every 18k the Platinums are the way to go for life expectancy.

That car was run for most of it's life on super unleaded so don't know it that helped. When I did have the misfortune to use supermarket unleaded it ran badly .. the lack of punch was so obvious.

And when I had to change the head gasket there was little in the way of deposits to clean up on the piston crowns or valves ..

cheers, Gary

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When I compared new for old it was obvious they were worn but not as bad a I'd expected still firing ok.
One of my points was; if one plug in a twin plug cylinder wasn't firing, how would you really know? The cylinder may well still fire on the other plug, you wouldn't 'lose' a cylinder, just a less than entirely happy engine with some relatively minor power loss and perhaps some slight increase in vibration...

Regards,
John.
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I have found that the platinum tip plugs do not last the distance of 100,000km (or 60,000 miles). I generally change them at 36,000 miles, and have had no problems.

The smaller 10mm plug is also fragile. The thread casing is prone to breaking if kept in the engine for a long period (that is up to 5 years). One broke on me (number four cylinder so eighth in the sequence) and it was really fun removing the spark plug casing from the head.

The trick with TS engine longevity is to double up on all service requirements.
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Quote:
Originally Posted by Spider95 View Post
I have found that the platinum tip plugs do not last the distance of 100,000km (or 60,000 miles). I generally change them at 36,000 miles, and have had no problems.

The smaller 10mm plug is also fragile. The thread casing is prone to breaking if kept in the engine for a long period (that is up to 5 years). One broke on me (number four cylinder so eighth in the sequence) and it was really fun removing the spark plug casing from the head. [IMG class=inlineimg]https://www.alfaowner.com/Forum/images/smilies/titanic.gif[/IMG]

The trick with TS engine longevity is to double up on all service requirements.
I had a brand new small plug fail after 100 miles in my old twinny. The ceramic detached from the metal part and flames shot out burning coil lead and cam cover. Damn thing I was stuck in patchway waiting the AA on Xmas day!!
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Originally Posted by johnlear View Post
One of my points was; if one plug in a twin plug cylinder wasn't firing, how would you really know? The cylinder may well still fire on the other plug, you wouldn't 'lose' a cylinder, just a less than entirely happy engine with some relatively minor power loss and perhaps some slight increase in vibration...

Regards,
John.
The main plug is between the valves, right in the centre. If that stopped working it would be obvious.

If the small spark plug at the side stopped working it would make no discernible difference. That is the theory.

CF1, CF2 and early CF3 engines had the HT wires pair cylinders 1&4 and 2&3. On these engines, a coil would either power both main plugs or both emission plugs. It would be obvious or not at all discernible. If it was obvious, swap the coil from paired cylinder and drive home normally.

Later CF3 engines had the coil power both plugs in its cylinder. Again, a coil malfunction would be obvious.

If the fault was in a spark plug, for instance, the spark may track down the side of the plug. If the coil is too weak though, because the circuit is broken, the other plug powered by the coil also stops firing. That is a benefit of later stick coils for a single spark plug; a misfire only affects a single spark plug and not possibly 2.

In all instances of 16v Twinspark, wasted spark strategy is used. I suspect this made more difference to cleaning emissions, or at least giving catalytic converters an easier time than extra spark plugs ever did.

Perhaps having 2 effective pressure wave fronts asymmetric in each cylinder was useful for obtaining efficiency and torque without the need for really hot camshafts. That is debatable as is benefits and deletion of twin spark technology when the JTS replaced it.

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The main plug is between the valves, right in the centre. If that stopped working it would be obvious.

If the small spark plug at the side stopped working it would make no discernible difference. That is the theory.
I do agree that if you had the choice of which plug failed, then you would choose the secondary plug, because a centrally located spark is better than an offset spark. However, lots of engines have substantially off centre spark plugs. The single offset plug still fires the cylinder quite effectively, if not quite as efficiently (as a centrally located plug).

My understanding is that having twin plugs creates a wider flame front, and tends to mean a faster and more complete combustion, though I suspect the effect is quite minor (or twin plugs would be a lot more common). With twin plugs, if the primary plug fails I can't see why the secondary plug wouldn't still fire the cylinder effectively enough, though the possibly less complete combustion may show up in the exhaust emissions (emissions possibly being the real reason why twinned plugs are fitted). Power would be affected, but not to a major degree (I suspect...).


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Originally Posted by Fruity View Post
CF1, CF2 and early CF3 engines had the HT wires pair cylinders 1&4 and 2&3. On these engines, a coil would either power both main plugs or both emission plugs. It would be obvious or not at all discernible. If it was obvious, swap the coil from paired cylinder and drive home normally.
"Emission plugs"? As I said, I suspect that attempting to meet emissions requirements is the main reason for the use of the twinned plugs, because of the ostensible wider flame front / faster and more complete combustion. But plenty of other engines make good power with a single plug, and still manage to meet emissions requirements...


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In all instances of 16v Twinspark, wasted spark strategy is used. I suspect this made more difference to cleaning emissions, or at least giving catalytic converters an easier time than extra spark plugs ever did.
IMO, 'wasted spark' ignition (the 'wasted' spark occuring BTDC on the exhaust stroke) is at least partially a cost saving exercise. With wasted spark it is possible to trigger the ignition very accurately once each crank rotation from the crank position sensor, meaning that there is no need to fit an additional sensor to detect camshaft position (keeping in mind that the crank and cam shafts don't rotate at the same speed, so with single spark the ECU must 'know' which stroke the crank is on in order to fire the plug on the correct piston upstroke, which can only be done with a camshaft position sensor if the spark only fires on every second crank rotation).

And / or, not relying on a camshaft position sensor means that the ignition timing can be more stable and accurate. It eliminates timing 'scatter' caused by the timing chain or belt allowing the camshaft to momentarily slightly over and / or under rotate relative to the crankshaft (which does happen, due to inertias in the camshaft and shaft drive, and belt drives suffer less bcecause there is less mass and inertia in a belt than in a chain). This causes small more or less erratic fluctuations in the valve timing, and if the ignition timing is reliant on a camshaft position sensor, in the ignition timing as well ('wasted spark' eliminates this ignition timing 'scatter'). However, these affects are minor, lots of very good engines trigger the spark timing from camshaft position sensor(s) with adequate accuracy.

I've heard it postulated that the 'wasted' spark helps to ignite unburnt hydrocarbons in the exhaust gasses as they leave the cylinder, thus improving emissions. I have serious doubts as to the veracity of this theory. Near the end of the exhaust stroke any gasses will be very depleted of either oxygen or fuel, or both. If the exhaust gasses still contained significant unburnt fuel, then the gasses would be extremely oxygen depleted, the oxygen having been consumed a lot earlier in the combustion process. If the gasses were oxygen rich, then the gasses would contain very little unburnt fuel, the fuel having been consumed a lot earlier. Either way the gasses would be very difficult to ignite, the 'wasted spark' is highly unlikely to achieve it (IMO...).

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Originally Posted by Fruity View Post
Perhaps having 2 effective pressure wave fronts asymmetric in each cylinder was useful for obtaining efficiency and torque without the need for really hot camshafts. That is debatable as is benefits and deletion of twin spark technology when the JTS replaced it.
IMO, twinned plugs are a dead end. There are theoretical benefits, which may be partially realised in practice, but even AR has abandoned it. I suspect that cost vs benefit isn't good enough, and at least as good results can be achieved in other ways with single plugs. This isn't to say that it is not an issue if one plug fails in a twin plug design, the engine has been designed around having the two plugs operating...

Regards,
John.

Last edited by johnlear; 1 Week Ago at 00:16.
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