The difference between the two might seem trivial, but I think there's a big difference at least for my case.

After some googling, I have noticed that the RX-8 leading spark plug with the code "RE7C-L" is for hotter climates and optimized for hot-starts whereas the RE8C-L is for colder climates and so makes better cold starts (info: Mazda RX-8 Spark Plugs - boards.ie)

And on a search on ebay I noticed that RE8C-L plugs are much more expensive than RE7C-L (at least twice as expensive) and that they are usually sold one by one, not as 4 piece set (2x leading, 2x trailing)

And to my suprise, I have noticed that one ebay seller has set the RE7C-L plugs only for the 2008-2011 RX-8's (!) here is the ad: JDM Mazda RX8 NGK Laser Iridium Power Spark Plugs RE7CL Trailing RE9BT Leading | eBay

If you select 2006 Mazda RX-8 on this ad, it says "This part is not compatible with 2006 Mazda RX-8 Base Coupe 4-Door 1.3L 1308CC R2 GAS Naturally Aspirated." I'm surprised of that because I was thinking of ordering.

Then I have taken a closer look at the plugs I have replaced last April, there on them were written "RE8C-L" (Currently I have RE7C-L on the engine)

I remember I 've never had difficult cold starts with the RX8C-L plugs but had numerous difficulties in cold starts with the RE7C-L plugs. I became aware of all of these as I started to consider replacing the spark plugs because the plugs were the last suspect in my cold-start problems.

Do you think RE8C-L vs RE7C-L would really make such a difference? (My RX-8 2006, engine at 22k miles)

 

The 8 plug is colder and will foul easier than the 7 plug all being equal. So for a daily driver that gets a lot of stop and go..you could notice the difference. Mazda has a 6 plug for cold weather and short trips that they used to recommend

 

Thank you for the comment. Well I've got to tell you that RE8C-L makes a BIG difference over RE7C-L, because today I have reinstalled the RE8C-L plugs and the car started to work extremely smoothly, just like it did until I changed them last April.

I can even claim that the RE7C-L "sucks" (so you say it in English?) at least in my case. And the engine works extremely smooth though the RX8C-L plugs I have inserted were used for a year and for about 10k miles, they are that good. It looks like, the design of RE8C was better. The engine no longer vibrates in idle at the traffic lights and it starts perfectly fine in cold starts.

 

Likely there is something wrong with the RE7C-L plugs....

Usually the colder plugs will idle worse than the hotter plugs all things being equal

The advantage of the colder plug is when you are really pushing it.....not at idle

 

Are the RE8C plugs more comparable to the Denso versions available?

 

Thank you for the comment.

When I first bought my RX-8 last year, it had RE8C-L on it and it almost never had issues starting cold with those plugs on. Until I replaced them with RE7C-L thinking that they were better because they were newer version and more common so I never gave a second thought that all the cold start problems I had might be caused by those leading plugs, so far.

After I reinstalled the old RE8C plugs, I haven't noticed anything negative compared with the RE8C. On the contrary, they had positive aspects over RE8C: They start very well in cold starts and they idle better.

With the RE7Cs, the exhaust was making a small explosion sound on cold starts and the engine was shaking at a noticable level at idle, so much so that I was thinking of replacing the engine mounts. Now it became more clear that the explosion sound was caused by inability of the plugs to burn the fuel mixture on cold. And even on normal operating temperatures the RE7C is worse than RE8C, because the engine was shaking at idle, again the plugs must have been working inefficiently at normal operating temperature.

 

NGK RE8C plugs are sometimes advertised as Copper Core and the Denso as Iridium Racing

In my opinion the Denso versions are more comparable to the RE7C because they are advertised as Iridium, too.

 

Aaa yes the spark plug dilemma.

I know this is an old-ish thread but it still deserves correct information for all those who come across it searching for the information presented so far.

So here is the score of the rotary RX-n plugs dilemma.

Standard NGK Iridium plugs off the shelf if you were to go and purchase them would be:

NGK RE9B-T (T = trailing plugs)

NGK RE7C-L (L = Leading plugs)

How they work is that the leading plugs fire the mixture first which begins the rotor turning in the right direction while the trailing plug fires a fraction of a second later, this creates two flame fronts which forces the combustion in the desired direction in the rotors combustion cavity.
When one of the plugs fails to ignite the mixture the car feels like a misfire and stumble so both plugs are very important especially at low revs due to the lack of motion due to the flywheel at slow rpm.
At High RPM the flywheel has enough motion to force the rotors round if one of the plugs fail to ignite.

Denso vs NGK.

This is a simple situation, for racing applications or where you are driving at high RPM for long periods of time then you use a Denso plug or a NGK Racing plug.

Denso racing plugs are:

IRT01-31 as Trailing plug

IRL0127 As leading plug

Note the IR(T) and IR(L) in the start lettering.

If we compare the heat ranges of these plugs we can see in fact the Trailing IRT01-31 against the RE9B-T we can see that the Denso IRT01-(31) is equivalent to a NGK heat range of 10 so it is a much colder plug and hence used for racing or driving style of constant high RPM.
Same rules apply for the Leading plugs where heat range Denso 27 is equivalent to NGK heat range 9, that is a massive heat change from RE7C-L up to 9 so expect plug fouling if you are not driving at high speed for long periods.

So as we can see it depends on your style of driving so you have to ask yourself do you roll in 5th gear allot when doing 60mph+ which will be around 4-5K RPM or do you use 6th gear and roll at 3-4k RPM or even further do you even do much highway driving at all and potter round town/city with short bursts of high revs.

The higher the RPM the higher the combustion chamber temperature and the more heat you need to get rid of. Lower RPM driving means less heat and a cold plug such as a Denso racing would foul quickly.

If you actually grasped all that then we can move on to plug examination after you chose which one to install or examine the ones already in your RX.

So if you take out the trailing plug and it is black and assuming you have no engine damage then this is a plug too cold for your driving and climate.

If the plug porcelain is a chocolate tanned colour then this is the correct heat range for your driving and climate.

If the Porcelain is white without a tanning chocolate colour then the plug is too hot and is what we call blistered.

Inspect all plugs the same leading and trailing to ascertain the effect of your plug heat range.

The problem is we DON'T have many choices regarding heat ranges for our rotary engines.

The only NGK Trailing plugs available for everyday use is the RE9B-T and the choice of either RE7C-L or RE8C-L for leading.

The only option for the Trailing plug is the racing plugs which are colder heat ranges and allow for more heat to be transferred away from the combustion chamber so if you are in the UK and you need a colder plug you may need change these out every year from Denso to NGK and back to Denso as the weather changes or your driving habit changes.

Many think the turn off procedure of holding the RPM at 3K RPM before shut-off is actually to just clear oil and fuel out the housing but in fact it serves this purpose and to get the plug upto it's cleaning temperature and stop it fouling up before you shut-off the engine.

The leading plug is the important plug when starting the car hot or cold as if this expands too much and exceeds the spark gaping it won't start well or if its cold and fouled it again wont start well. This is why a rotary floods allot as the computer constantly pumps in fuel and the plugs cant ignite the mixture, yes low compression could be an issue as can a fouled plug as can pre-ignition because if the fuel mixture fires to early it will try push the rotor back the opposite direction especially when the trailing plug fires and the leading plug has misfired.

Now there is a range of NGK racing plugs which are the same as the Denso heat range, these are:

R7440B-10T

RE8A-L
(A) instead of (C) which is the plug that does not have the castellated groves cut in as to allow for better heat transfer. Regardless you would probably be better off still using the RE8C-L.

The castellation groves are custom to NGK and Denso plugs don't as far as i know have these. The groves are there to allow for carbon build up to be washed away better when up to operating temperature.

You have to make your choice based on the diagnostic data you find out about from your engine and or way you drive to choose the correct heat range and it is a bit annoying as the lack of knowledge for choosing a heat range plug is not well known so much so that NGK and Denso do not manufacture a large heat range for this engine.
So if you need a plug hotter than NGK 7 you are out of luck, you will need play with the ECU mapping or fuel type and mixture which is a art in itself and well beyond the scope of this thread.

 

wow,

that is out of nowhere. great post!

beers