Twin plug heads - Alfa Romeo Forum
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(Post Link) post #1 of 11 Old 20-10-09 Thread Starter
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Twin plug heads

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Hello.
Please forgive this newbie to your site if I I'm in the wrong place with this - perhaps a moderator can relocate it - but I have a question and I'm literally itching to have it answered.
My car is a European two seater, mid engined, turbo charged with 6 sp sequential gearbox and a reputation for frail engines, the existing explanation for which I do not find plausible.
That'll be a smart then! Why I am on an Alfa owners site is this. I suspect the problem with the smart is related to failure of a plug or plug lead, leaving one or more cylinders firing on only one plug rather than the two per cylinder it should be firing on. So what I am asking here is if anyone has had this happen on a TS Alfa, and what the outcome was? I would expect the 8V models to suffer more than the engines with 4 valves per cylinder (if at all). So if anyone has experienced failure of one plug - with the affected cylinder still firing on the remaining plug, will you please please please reply to this post with as much information as you can?
Possible / probable symptoms are burned exhaust valve and / or overheated piston, possibly only affecting one cylinder. It is possible that the coil layout may make this impossible, but my suspicion is it could have happened.
If you can help me here, I promise your input will be appreciated. Many thanks for reading so far.
Thanks, F.
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Twin spark alfas use a big plug for combustion and a small plug for emmissions control, they fire after combustion, as apposed to the smarts where they use both for combustion.
As for the plugs breaking down in them they are very expensive platinum plugs that cost about £80 for a full set with a service interval of about 70,000miles.
Coil on plugs fire main plugs with leads going from the coil to the secondary plug.
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the big and small plug are both fired simultaneously, from different coils. In fact, every ignition event fires 4 plugs.


anyway, burned valves are very uncommon, if at all, on TS engines. Neither the 8v or 16v suffer from this. high rpm power suffers on the 8v if only one plug fires per cilinder.
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Quote:
Originally Posted by UKSPF View Post
Twin spark alfas use a big plug for combustion and a small plug for emmissions control, they fire after combustion, as apposed to the smarts where they use both for combustion.
As for the plugs breaking down in them they are very expensive platinum plugs that cost about £80 for a full set with a service interval of about 70,000miles.
Coil on plugs fire main plugs with leads going from the coil to the secondary plug.
The official change interval for the platinum plugs in the TS 16v engines is actually 60,000 miles. However they have been known to deteriorate before this, notably the platinum tips can fall off and cause problems in the cylinder, also the outer thread casing on the small plugs have been known fracture due to thermal stress (accentuated if any oil is seeping through the rocker gaskets and going down the plug holes), or over-tightening, or a combination of all of these!

So 48,000 miles is a much better (and safer) change interval than the recommended 60,000 miles for these plugs.
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(Post Link) post #5 of 11 Old 21-10-09 Thread Starter
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[QUOTE=Cuore_Sportivo_155;2976090

anyway, burned valves are very uncommon, if at all, on TS engines. Neither the 8v or 16v suffer from this. high rpm power suffers on the 8v if only one plug fires per cilinder.[/QUOTE]

Thanks. From my initial post you guys have got a handle on the info I'm after. With the Alfas it seems that if a plug fails then it's noticed as soon as you use the revs. My original suspicion survives for two reasons. High revs aren't always necessary in the course of smart driving. They are all about low down turbo grunt and therefore it is possible that it could go unnoticed. Secondly the effect with such a small bore size could go undetected. If it happens though, it would have the effect of retarding the ignition timing - a recipe for burned exhaust valves and other heat related issues if ever there was. These little engines are knocking out around 100hp/litre and are thus pretty stressed thermally speaking.
The 964 and 993 911s have twin plugs and have issues if one plug fails but are robust enough to usually survive. The Merc V6, 3 valves/cyl also ran twin plugs so I guess I had better go and ask about them!
Incidentally, the reason given for the smart problem is that bore wear occurs (on one cyl only) which allows oil past the rings which then burns hotter than petrol and burns the exhaust valve. All of this on one cylinder only. I don't buy any of it!
Anyway thanks for your info - and if anyone else has anything they think might be of relevance - please post!
Cheers meantime, F.
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This is just guessing and may be stupid because I have no idea how are the plugs on Smart located, but in case they are in opposite sides of the cylinder and only one fires, the burning process is starting only on one side of the combustion chamber and spreading to the other side. This can lead to uneven pressure build up as the gases don’t press on the piston vertically as with one plug in the center but more on the side that is first ignited and this can force the piston to cross in the bore and wear the cylinder unevenly. Might not be very realistic considering the small bore of the Smart, but the pressure of the turbo multiplies the forces on the piston and if they are not even….

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(Post Link) post #7 of 11 Old 21-10-09 Thread Starter
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Originally Posted by gotcha View Post
This is just guessing and may be stupid because I have no idea how are the plugs on Smart located, but in case they are in opposite sides of the cylinder and only one fires, the burning process is starting only on one side of the combustion chamber and spreading to the other side. This can lead to uneven pressure build up as the gases don’t press on the piston vertically as with one plug in the center but more on the side that is first ignited and this can force the piston to cross in the bore and wear the cylinder unevenly. Might not be very realistic considering the small bore of the Smart, but the pressure of the turbo multiplies the forces on the piston and if they are not even….
Far from stupid! You have highlighted something that hadn't occurred to me. The increase in side thrust may well be an issue here.
The common theory is that the problem begins with bore wear which then leads to valve burning, usually on one cylinder. I am convinced that the problem originates with the loss of effective ignition from one plug, (possibly by oil fouling from bore wear/ring failure) effectively retarding the timing and creating heat related issues which bore wear is one. Additional side thrust is not going to be welcome when everything else is stretched to the absolute limit.
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Just had another look at typical piston damage (links) and low and behold if the damage isn't at 90deg to the gudgeon pin axis - exactly what might be expected from side thrust.
Thanks Gotcha - You are well named!
Incidentally, you were spot on re plug location - opposite sides.
F.
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Quote:
Originally Posted by Meshman View Post
Far from stupid! You have highlighted something that hadn't occurred to me. The increase in side thrust may well be an issue here.
The common theory is that the problem begins with bore wear which then leads to valve burning, usually on one cylinder. I am convinced that the problem originates with the loss of effective ignition from one plug, (possibly by oil fouling from bore wear/ring failure) effectively retarding the timing and creating heat related issues which bore wear is one. Additional side thrust is not going to be welcome when everything else is stretched to the absolute limit.
Fudgesmart - Smart Servicing, Engine Rebuilding and Maintenance
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Just had another look at typical piston damage (links) and low and behold if the damage isn't at 90deg to the gudgeon pin axis - exactly what might be expected from side thrust.
Thanks Gotcha - You are well named!
Incidentally, you were spot on re plug location - opposite sides.
F.

It was just a theory, but your pictures support it. If the side thrust is the cause, the damage can only happen on the sides of the piston upright to the gudgeon pin. On the sides of the pinholes the piston cant cross in the bore, but on the opposite sides it has more play to cross in the bore and the wear should occur there.

It looks like the coil pack failure is for Smart something like cambelt failure for Alfa. Unexpected quiet death...
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Originally Posted by gotcha View Post

It looks like the coil pack failure is for Smart something like cambelt failure for Alfa. Unexpected quiet death...
I got a chat today with a guy who rebuilds smart engines for a living and between us we seem to have gotten a little bit closer to the cause of the problems. All twin plug motors are susceptible to damage if one plug in a cylinder fails. This in the smart is mainly caused by oil fouling when the oil starts getting past the rings.
There are two very cheap to buy tools required to service an early model smart. A plug lead removal tool. Pull on the leads without it and if damage occurs unnoticed burned exhaust valves ensue. The bigger problem lies with an oil control ring that is marginal and easily gums, after which all the problems arrive in full measure. It shouldn't gum but...
The early smart doesn't have a drain plug on the sump. Instead a vacuum extractor pump is used. For the want of it some engines don't get their oil changed as they should. On a 100hp/litre motor this is an invite to trouble - and sure enough. The real weakness for these cars is poor maintenance for the want of basic tools. Compounded by being bought for their cheapness of running: easy 50mpg, group 1 insurance, £35 road tax (want one yet?) and save a bit more by skimping on servicing... When these people realise that they have trouble they sell it on and the ensuing failure is plastered all over some forum or other.
However these engines can be identified by heavy varnish deposits on the engine internals and should be avoided. The pics in the earlier links was of an engine that failed because it ran dry of oil and is clean. The ones without proper maintenance are filthy inside.
The valve failure is intrinsic to the twin plug layout, hence my questions on this site. If the cylinder didn't keep firing with the other plug the ex valve wouldn't burn.
Anyway thanks to all who replied and offered their knowledge - I am extremely grateful.
In summary Gotcha, yes a lot of engines do have an inherent weakness. The smarts biggest failing it seems is the lack of a sump plug!
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Quote:
Originally Posted by Cuore_Sportivo_155 View Post
the big and small plug are both fired simultaneously, from different coils.
On the Euro3 TS, both plugs are on the same coil.
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Yes, I've heard that, which suggests that sequential ignition could be used.... makes sense, because the camshaft sensor is there...
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