I appreciate I have posted this within another thread, but for those who may not go into the Revo thread I thought it was worth sectionalising it out. It can then be built upon as a separate topic as we learn more through further investigations / case studies etc. If you've read this post already in the Revo thread, the content has changed so scroll on to the next post down!
I spoke with NGK about some of the misfire issues / plug dilemmas that owners of standard / modified cars have been experiencing, and they have kindly provided a list of considerations for us to think about with respect to selection the optimum plug.
PLEASE NOTE: The following information does not constitute a formal recommendation from NGK/NTK UK Ltd other than where OEM standards and running conditions are quoted.
NGK/NTK's state that a car should always be run with the appropriate spec plug as specified by the manufacturer. Should the owner choose to modify their vehicle and subsequently insert an alternative plug choice NGK/NTK UK accept no responsibility or subsequent liability for failures or damage as a result of these changes. They do not provide a formal recommendation for alternative plug specification for tuned engines. Any guidance provided to the general public by NGK/NTK UK to assist owners in selecting an alternative plug choice for lightly or heavily modified vehicles is done so as a gesture of goodwill, and as part of the process of trial and error by the owner. The owners can choose to follow this advice but does so entirely at their own risk.
Put simply; leave the car as it was intended, and if you do anything else, you are (quite rightly) on your own!
1) What is a spark plug?
Before we go into any detail about the plugs, I'm just going to insert a labelled diagram of the plug shamelessly stolen from NGK's website to help inform the uninititiated!
A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.
Spark plugs have an insulated central electrode which is connected by a heavily insulated wire to an ignition coil or magneto circuit on the outside, forming, with a grounded terminal on the base of the plug, a spark gap inside the cylinder.
Reciprocating internal combustion engines can be divided into spark-ignition engines, which require spark plugs to initiate combustion, and compression-ignition engines (diesel engines), which compress the air and then inject diesel fuel into the heated compressed air mixture where it autoignites.
2) OEM specification : NGK Plug part number (Audi S3 2.0TFSi 2007>)
Recommended NGK plug (as supplied to VAG Group): PFR7S8EG
PDF Document Press Release 2010: LINK
Under normal operating conditions this plug is perfectly adequate, so if you run with no modifications and want to purchase a plug in the aftermarket arena that best suits the manufacturers original specification this is the one to buy. It has a copper core, and a platinum electrode, and is an exact match for the manufacturers spec.
3) Running modified : NGK Plug part number (Audi S3 2.0TFSi 2007>)
A suggested option NGK plug which you could use if you felt so inclined(! - legals covered):
BKR7EIX (Stage 1) - Same heat setting as OEM
BKR8EIX (Stage 2+) - One stage cooler than OEM plugs, should run better with increased combustion temps and boost levels
R7434-8 - This plug comes in 8 / 9 / 10 heat settings but 8 would probably be the best to run on modified vehicle - THESE PLUGS CANNOT BE GAPPED DUE TO THE CONSTRUCTION AND DESIGN OF THE PLATINUM BAR.
Engines have a particular resonance when they are running, and due to differences in the construction of plugs and the materials that go into them, there is no guarantee that a race plug will be more robust than a road plug, or more attuned to the engine resonance, it is designed to operate at WOT (Wide Open Throttle) for extended periods, so unless your car is used this way it may be of limited benefit).
4) Why iridium plugs?
Iridium plugs have a slower wearing tip, and can in some applications take more abuse than it's platinum cousin. Some manufacturers claim that due to conductivity of material and strength they will show better wear rates, stronger sparks, and put less strain on the coil packs, but what's important in modified applications is consistency of performance, and durability due to increased intensity of operating parameters.
In some respects the platinum plug is a better plug for day to day use as it's ground is better protected with it's copper core than the BKR plug, but for outright strength and durability the BKR plug is likely to be the overall best choice.
5) Why go a stage cooler?
When modified turbocharged cars run more boost, the temperatures within the piston chamber will increase significantly. This puts far greater exposure onto the plug, and very often by running either a cooler plug, or changing the plug gap (trial and error), it is likely you will be able to achieve better consistent performance where less pre-ignition occurs.
A cooler plug is visually identifiable from a warmer plug as the insulator is thicker (see below '6' heat plug on the right, '8' plug on the left).
It is generally accepted that a cooler plug is likely to cause less damage than a hotter plug. A cooler plug will just foul up, whereas the hotter plugs can cause catastrophic engine failure if they are to over heat. Which would you prefer, a failure due due a foulled plug or a rebuild?!
Obviously the aim is to get the best performing plug, so if you do a lot of track work then an 8 is likely to be the safer bet, whereas if you tend to use your S2+ car as a shopping car / school bus for local journeys in the main then 7's may be adequate.
As engines become more advanced and have leaner burn cycles the plugs seem to be getting colder and colder, and so I'm told some Fiats are now running '9' plugs (I'd imagine the new Multi Air technology LINK engines which give upto 20% more power for the same CC)
6) Plug gapping
Plug gapping can be a mechanism to fine tune the running of a car with turbo modifications. As the plug tips are generally precious metals and very delicate it is strongly recommended that GREAT CARE is taken when gapping plugs. If you are unsure of what you are doing, get someone qualified to do it for you (dropping a ground electrode into your chamber is never going to be a good idea, and if you damage the tip when you gap it the plug is basically a write off anyway). Only consumer plugs can be gapped, and the general convention of gapping might see a tuned car dropping from 0.8mm to 0.6mm, or an NA to forced induction conversion going from 1.1m to 0.9mm.
OEM Audi recommended gap for stock spec plugs in OEM engine set up: 0.8mm
7) My plugs have a red tinge to them, should I be concerned?
NGK have noted that more recently the insulators on plugs have a more reddish tinge to them than they have in the past. The thinking behind this to date is that as this cleans off, it is likely to be down to additives within the fuels supplied to forecourts and is not a cause for concern.
8) So to summarise
If you choose to stray from the OEM manufacturer recommendation as part of an upgrade or modification path, always engage with a qualified tuning agent to ensure you get the best advice or support. It appears there are a range of options available to us, but as none of these plugs have been put through up to 4 years back to back intensive testing, none of the alternative recommendations can be guaranteed to offer a defined improvement in running or reliability and it is very much a case of suck it and see. Lets all hope the advent of corona plugs is not too far away!!!
9) And finally...
A huge thank you goes out to Mick at NGK/NTK UK for coming back to me so quickly with such useful insight which at least means we have a clear list of possible part numbers to test out (at our own risk!), the definitive OEM part number, the recommended gap, and some very useful pointers are food for thought. Top mark guys, great customer service.