Spark plugs through the ages...

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PeteH
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Spark plugs through the ages...

Post by PeteH » Thu Jul 18, 2019 2:14 pm

Rotary spark plugs a odd things. They have to withstand flooding when doused in petrol during cold starts, avoid fouling when the engine runs rich and has oil injected into the chamber, and avoid burning out when they get hot (because they don't ever see cold intake air). In addition they have to create a reasonable seal as the apex seals pass over them, so that compressed gasses don't escape from the working chamber (this is particularly complex because the spark plug can't extend into the chamber, but cannot be too recessed either, and Mazda hide the trailing plug behind a very small hole).

All rotaries (at least since the FB) have had four spark plugs, but only the Renesis has had different leading and trailing plugs. Before the RX-8 all four plugs were the same.

Mazda have stuck to a very small number of recommended rotary-only plugs. Over the years the plugs have evolved. I have used NGK plug part numbers, because they are the usual "go to" plug for us. The timeline is as follows:

FB: BR8ET
FC Series 4: BR9ET
FC Turbo: BR10ET
FC Series 5: BUR9EQ
FD: BUR9EQP
RX-8: RE8C-L (leading), RE9B-T (trailing)

The RE7C-L is also listed as an option for the RX-8 if the car is used for lots of cold starts and the plug is prone to fouling.

NGK codes are easy to decipher, so we can trace the technological developments.

Firstly, the RX-8 plugs are so unique that NGK didn't use their own coding system!
- RE means rotary engine
- 7, 8, 9 is the heat range, from hot to cold
- B & C are the revision numbers
- L & T refer to leading and trailing

All the pre-RX-8 plugs follow the normal NGK system:
B: 14mm thread diameter
U: surface discharge type (no obvious ground electrode, because it sparks across to the body of the plug)
R: resistor plug
8, 9, 10: heat range, from hot to cold
E: 19mm thread length
T: 3 point ground electrode
Q: 4 point ground electrode
P: Platinum centre electrode

So, at the start of the RX-7 Mazda had a fairly conventional 14mm plug, but with an unusual 3 point ground electrode, and they gradually made the plug colder as the engines got more powerful. For the Series 5 Mazda introduced the surface discharge type plug with the four "castellations". For the FD the design stayed similar, but the centre electrode went to Platinum. For the RX-8 the plugs were specials, based around an iridium centre electrode. The leading was a similar design to the FD style of plug (4 castellations) but also with a ground electrode, whereas the trailing was a more conventional single ground electrode design.

I believe any RX-7 plug should fit an RX-8, which is why the BUR9EQP is sometimes recommended for boosted Renesis engines (but don't quote me on that).

Three point ground electrode (FB and early FC):
BR8ET.jpg
BR8ET.jpg (35.67 KiB) Viewed 1781 times
Four point surface discharge (Late FC and FD):
BUR9EQP.jpg
BUR9EQP.jpg (8.18 KiB) Viewed 1781 times
Renesis leading:
RE8C-L End.jpg
RE8C-L End.jpg (44.03 KiB) Viewed 1781 times
Renesis trailing:
RE9B-T End.jpg
RE9B-T End.jpg (27.32 KiB) Viewed 1781 times

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Ainmhidh
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Re: Spark plugs through the ages...

Post by Ainmhidh » Thu Jul 18, 2019 2:20 pm

PeteH wrote:
Thu Jul 18, 2019 2:14 pm
... based around an indium centre electrode...
indium (49) or iridium (77) Pete?
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Re: Spark plugs through the ages...

Post by warpc0il » Thu Jul 18, 2019 3:39 pm

I know some people say that the RE7C-L is fine even for track days but I can't believe that a plug specifically designed to deal with sooting caused by excessive low revs - pootling around town, especially with auto 'boxes that change up too soon - can be appropriate for track use.

Mazda made the RE7C-L a standard fit for the R3 but there were a bunch of other engine changes and it probably says more about how they expect the average R3 to be driven, most of the time.

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Re: Spark plugs through the ages...

Post by PeteH » Thu Jul 18, 2019 3:57 pm

Ainmhidh wrote:
Thu Jul 18, 2019 2:20 pm
PeteH wrote:
Thu Jul 18, 2019 2:14 pm
... based around an indium centre electrode...
indium (49) or iridium (77) Pete?
Corrected :thumleft:
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Ainmhidh (Thu Jul 18, 2019 3:59 pm)

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Re: Spark plugs through the ages...

Post by delta0 » Thu Jul 18, 2019 10:53 pm

Rotary engines have surface discharge spark plugs which is why they are unique from other engines.
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Re: Spark plugs through the ages...

Post by warpc0il » Thu Jul 18, 2019 10:57 pm

I'm moving this to the FAQ section, before it just gets buried among the general discussions.
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