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Ignition: Coils, HT leads and spark plugs

Read the Frequently Asked Questions that have been accumulated over time.
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Ignition: Coils, HT leads and spark plugs

Post by Naz » Thu Feb 10, 2011 1:41 pm

Definitions (in layman terms)

Coil – produces energy for the spark to ignite the fuel/air mixture.
HT Lead – the lead which carries this charge to the spark plugs.
Spark Plugs – these are placed on the outer rim of the engine on the edge of the combustion chamber producing a spark to ignite the fuel when the charge is carried from the coil via the HT Lead.

Conclusion
After much discussion, it has been found that the original coils and HT leads provided as standard by Mazda tend to deteriorate after about 30k miles (regardless of age of vehicle). Some have managed to get Mazda to replace the coils under warranty as well as the HT leads. Mazda have upgraded the specification of the coils since the original fitment. The upgraded coils can be identified by a "B" and more recently by a "C" in the part number embossed on the coil and these are the ones that should be supplied by any trusted source.

Spark plugs will be changed regularly with the service, so long as the correct plugs are used and there is no outside interference (flooding etc) these are nothing to worry about. However, if the coils need replacing then the plugs will have become fouled with deposits and it would be false economy not to replace these at the same time.

How do I know my coils are dead?

If you drive the car regularly it’s really quite difficult to tell as they will deteriorate slowly and you might not notice the change in power output. The best way to check is to take them off and have a look at the undersides. If the bottom of the coil has white spots this is a sign they are on the way out, or are gone completely. There is still some speculation as to whether the coils actually short to the mounts but it is agreed that the white spots are due to overheating and breakdown of the potting compound, which will result in (at least) internal shorting of the output coil and reduced spark energy. Mazda will argue the white spots are nothing to worry about and will test your coils for resistance and if within their parameters they will do nothing about it. Through the experience of the club members, white spots mean get them changed.
Other symptoms may include increased fuel usage and lack of power under hard acceleration.
A single leading or trailing coil could fail completely and that rotor will still produce (reduced) power, unlike a piston engine where loss of a coil will immediately result in complete loss of power from the associated cylinder

Why should I change my coils?

In short, if there is reduced ignition energy being delivered to the spark plug, there is unburnt fuel left in the combustion chamber. This can cause two problems.
1 – the unburnt fuel makes it’s way through the exhaust port into the catalytic converter where the heat ignites the fuel. This excessive pressure and heat breaks down the internal honeycomb of the catalytic converter. Once the honeycomb deteriorates your emissions get higher (may not pass MOT), your fuel efficiency will get worse (incorrect readings from O2 sensor in exhaust system), may get CEL on dash as the system recognises incorrect gasses flowing through the exhaust system. More so the honeycomb can collapse on itself causing a blockage in the exhaust system meaning your car won’t accelerate properly. This will more than probably also cause a rattling noise from the catalytic converter and you may find if you don’t do anything about it the catalytic converter will be glowing red after a long drive, to the point it is dangerous and could cause a fire.
2 – the unburnt fuel from lack of ignition washes away the lubrication on the rotor housings. This causes damage to the rotor housings as well as the rotor tips and could eventually mean a new engine - ~£3,500 rebuild cost.

Coils should always be replaced in complete sets as the obvious failure of one or more is the indication of imminent failure of the rest.

Why should I change my HT Leads?

Poor starting and lumpy tickover, especially when hot in traffic, can also be signs of ignition system issues, normally the coils themselves but can also be tracking on the plug insulators due to poor contacts or water ingress in HT leads; these are the main issues with the OEM leads.

The position of the spark plugs, low on the nearside of the engine, make them exposed to road splash which can be a particular issue in the winter when the roads are salted. A good water-tight seal on the HT leads are therefore critical.

If you are changing your coils, for the little extra in cost change to thicker more durable HT Leads. You want to be able to carry the most amount of charge from the coil to the plug. Poor insulation on the leads will mean some charge is lost to any earthing points on the way to the plug, and thin poor copper wire in cheap HT Leads will mean it won’t carry as much charge (a little like trying to get a articulated lorry through a pinhole, you’ll only get some of it through).

Recommendations for parts:
Coils
You don’t have to get uprated coils, you can get standard coils readily available from dealers in the UK, traders on this site (who will give full members discounts) or from some US e-bayers.
For uprated coils you can get genuine uprated Mazda coils (from traders on here), or a more expensive competition set up by a firm in the USA but beware of cheap copies marketed as "OEM spec".

HT leads
Most opt for Magnecor leads, either the blue (8.0mm) or the red (8.5mm) will do. The red is the KV85 version and is for more competition performance therefore also more expensive. The Blue is more than adequate and slightly easier to fit. Leads are also available freely online or from traders on this forum.

Spark plugs
The RX8 uses two different types of plugs, leading and trailing. The trailing only has one option, the leading has two main options dependant on type of driving:
Trailing - NGK RE9B-T
Leading - NGK RE8C-L (standard Mazda issue)
Leading - NGK RE7C-L (referred to as "hotter plugs"; better short distance town driving. can also help with some hot start issues - no issues using on long distance either).

Note the "C" in both the Leading plug options, they are an improved design from NGK and weren't available 3 years ago so your car may have RE8B-L which foul more easily.

Be aware of some eBay sellers clearing old stock, as you don't want to save a few pennies only to be putting old design plugs back in.

You may see "Racing plugs" being advertised for the RX-8, these are not suitable for road use as they run colder and therefore make flooding and carbon build-up more likely in road conditions. Only suitable for all out track days or drifting events.

If you are changing plugs yourself make sure you put the right plug in the correct place. The engine block labels the correct chamber with L (leading) and T (trailing).

You may read that one plug is slightly longer than the other and if done incorrectly you will knock your apex seal on one of the incorrect plugs - a very expensive mistake probably costing you an engine rebuild - but this is incorrect.

However, the plugs aren't exposed directly to the combustion chamber but fit in counterbored holes with an aperture through to the chamber. The trailing plug has a smaller aperture to prevent inter-chamber leakage. Mismatch leading and trailing plugs causes sealing issues around the leading plug as the tip will bottom out. The tip designs of the latest versions of these plugs are also different to match their specific roles, so it is still critical that you get them the right way round.

Original article by Naz with additions by warp0il [10/02/11]
Last edited by warpc0il on Thu Jun 02, 2011 6:11 pm, edited 3 times in total.
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Re: Ignition: Coils, HT leads and spark plugs

Post by warpc0il » Sun Dec 09, 2012 1:35 pm

Note that Mazda dealers only perform a "bench check" on suspect coils.
This is done at room temperature and using low voltage, so if a coil fails the check you can be sure that it's dead.

However, if a coil passes the test then it doesn't mean that it would still work in the temperatures in the engine bay and at high voltage. There are many examples of useless coils being passed as "good" by dealers.

Dealers will also only replace the coil that has "failed", rather than the complete set, which is a false economy and will cause further problems in the very near future.

Independent garages will usually source replacement coils from their local motor factors.
These "OEM quality" coils will fail much sooner than the genuine Mazda parts.

There have been examples of Mazda dealers supplying non-genuine parts, some even charging genuine prices.
This isn't helped by fake clone coils being produced in China, and available @ $10 each, including all the expected markings and coming in genuine-looking boxes. It's possible that some of these may be finding their way into the legitimate supply chain.

An great alternative to genuine coils are the kits produced by Black Halo Racing (BHR) and Rotary Revs.
These use the coils specified for the Dodge Viper with loom adaptors, custom mounting brackets and special HT leads produced by Magnacor. Many owners (myself included) have taken this option, despite the initial cost, as there has yet to be a single instance of coil failures with these kits.
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Re: Ignition: Coils, HT leads and spark plugs

Post by warpc0il » Thu Jun 13, 2013 11:06 am

The Blueprint coils, as sold by Euro Car Spares and others, aren't pretending to be OEM but aren't up to the job either.

The failure mode of these coils is somewhat different in that they will not show the classic "white spots", as the potting compound is different.

Club Members can view the rogues gallery of fake and failed coils here
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Re: Ignition: Coils, HT leads and spark plugs

Post by warpc0il » Thu Jul 18, 2019 10:58 pm

More detailed background on Mazda rotary-engine spark plugs here
https://www.rx8ownersclub.co.uk/forum/v ... 25&t=80097
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Re: Ignition: Coils, HT leads and spark plugs

Post by warpc0il » Mon Jan 13, 2020 12:54 am

The MkII version of the Ryan Rotary custom coil sets have three practical advantages over other non-Mazda alternatives
- they work without issues on all RX-8 versions, including JDM with triggering a CEL
- they don't require any changes to the PCM mapping for "dwell"*
- they have the same HT connections as the Mazda coils, so the HT leads are compatible

See review here
viewtopic.php?f=205&t=80124#p1224810

*you will have to have the dwell map change reversed if yours has been "mini-mapped" for D585 coils
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