$P€€DD€VIL

There are millions of sparkplug threads asking the same stuff about using the same plugs leading and trailing etc.
But I couldn't find out what I want to know.
Everyone says BUR9EQ leading and trailing works fine for normal driving and better than the oem setup for track and for spirited driving.
That's what I drive in my TII since 4 years.
Before , it had 7EQ and 9EQ ,..but can't really remember how it was and I never calculated the mileage.

I wanted to order new ones now, from mazdatrix (i can't buy them localy and just try out what's better, they're either not available or ridiculous expensive).
But I'm not sure what I should buy,..I'll buy alot of them so the choice should be good.
Mazdatrix says that for normal driving the hot plugs (6 and 8) are recommended.

8$/gallon is the gas here in austria, and my car is pretty thirsty, the best mileage I can get when I never get in to boost and drive like a grandma is 15MPG.
It's a rotary, so it is thirsty, I know... but I don't want to waste fuel with sparkplugs that are most of the time in the wrong heat range.
I do drive spirited sometimes on mountainroads and planning to do time attacks on the track every 2 months or so, but 90% of the time I just drive normaly.

So I thought 8EQ+6EQ should be better most of the time, and give me better gas mileage?
Or would that be unsafe for the engine when I really push the car up and down a mountain for an hour or on the track?
Would it be better to use 8EQ+7EQ maybe? As some kind of in-between solution?
My car is a stock port s4 TII and runs 7PSI and has a racingbeat turboback with the stock cat.converter modified to 3" . Soon with a Bonez converter and a bit more boost.

BTW I never drive the car in the winter. The temperature here in austria can be compared to the east coast USA I think.

So what would you recommend?
And has someone noticed a difference in milegae with cold/hot sparkplugs?

j9fd3s

iTrader: (3)

heat range of the plugs doesn't make any difference in mileage, unless its so cold the engine is actually misfiring.

beefhole

iTrader: (29)

You can use 7/9 for your normal driving. Buy an extra pair of 9 and use all 9's when you are at the track for a little "colder" running.

nycgps

iTrader: (4)

different heat range plug will give you "slightly better/worst" mpg, depends rather if its hotter/colder.

hotter plugs (6/8) will give you a tiny bit better in mpg but the main benefit is your plugs will foul "less" for city driving (stop & go traffic,), which means longer plug life. Mazda documented that for the Rx-8. but you will lose a bit of performance which I don't think it matters for city driving. and you should be ok to go with 6L9T plugs.

$P€€DD€VIL

6 and 9 sounds like a very strange combo, ..7&8 was what I was thinking about.
But I found eight new 9EQ in my basement, I think I'll try 7/9,...I'll look if I can get 7's locally somewhere, and test them and if they're good then buy alot of them in the US, and if they're too cold I'll buy 6 and 8's ....but it seems like 6 and 8 are discontinued?
I can only find them @ mazdatrix

nycgps

iTrader: (4)

7/9 is just "stock" combo. nothing special about it.

6/9 is not strange either, it is recommended by Mazda for "City stop & go traffic", if you don't drive in the city all that much you can just stick with 7/9.

8 is not needed and Mazda discontinued the 8 trailing plugs for rx-8 couple years ago.

I know this is FC and not FE, but the engine is 90%+ the same so it's safe to say it works the same.