Why does my RX7 need two different types of spark plugs? I know this seems like such a noob question, but I honestly don't know what the difference is.

 

You have 2 heat ranges of plugs.
The plugs are called Leading and Trailing.
the leading is a hotter plug as it fires and burns off the main amount of air/fuel.The trailing plug fires to burn the residual off.
Spark Plugs most commonly used are NGK Bur7eq for LEADING plugs,and NGK Bur9eq for the TRAILING plugs.
If you are confused about the Placement of the plugs,the Good way to remember where they go on the engine is that L(leading) is LOWER spark plug holes and T(trailing) is TOP spark plug holes.
Mike,The way I got started was to Cruise the Sec Gen Specific section,almost nightly.Armed with a coffee,lol.
The specific section and the archives are like a Bank vault of info.If it wasn't for that,I would be driving a Cavalier!...oh no!..haha!
Good luck,STYX.

 

As stated above, you have leading and trailing plugs. They fire with essentially the same energy but differ in heat range. With a rotary, it's particulary important to get the proper heat range plugs. Too hot and you risk pre-ignition. With a rotary, that can cost you an engine. Too cold and it will foul and not perform. NGK is the OEM spark plug manufacturer and IMO, for a stock or near-stock engine, you can't really do any better.
Easy to understand tech page ----> NGK Spark Plugs USA

 

what are benefits, if any, to using all one or the other?

 

Colder plugs are used in high-horsepower applications to reduce the amount of heat retained. This can reduce the risk of pre-ignition due to a hot-spot. For your average stock to mildly modified engine, there is no need to use anything other than the standard 7EQ and 9EQ plugs.

The NGK link above discusses this.

 

On a stock, or near stock engine, none. The only time you should change from the factory suggested heat ranges is if you're highly modified, and usually also forced induction. Otherwise, on a rotary, you want to be damn careful going cheap or simple.

The shape of the rotary combustion chamber is long. It's why they put leading and trailing plugs per chamber and it also exposes the plug to the heat of the combustion event longer than a piston engine. Too cool and the plug fouls, too hot and it can pre-detonate. And pre-detonation is NOT tolerated by the hard seals of a rotary.

Edit: RotaryRocket beat me to it.

 

Just use the table here:
Spark Plugs

 

I'm sorry, I don't want to un-say what you said but, aren't the BUR7EQ the Leading spark plugs and the BUR9EQ the Trailing ones?

And another thing: As far as I know, the Trailing spark plugs are the colder ones to prevent pre-detonation, and the Leading ones the hotter ones to burn the rest of the fuel, but I may be wrong here...

 

^ Yes, 7EQ is leading and 9EQ is trailing. The position of the trailing plugs further up in the housing means they are the first to be "seen" in the compression stroke. If they retain to much heat, you could get pre-ignition (before the leading plugs fire) really early in the cycle. It could also be a catalyst for detonation (after the plugs fire, but before all gas burns).

 

So to continue the noobness.....wouldn't I wanna go cold all around if I'm running advaced timing?

 

Impossible to answer without vehicle information.

Then you would need to answer the following question first: Why is you timing advanced and what are you considering "advanced"?

 

Ok so my set up is a 13b bridgy running weber 48s headers, ect.....

 

And advanced by ignition

 

run all 9's with the carb/bridge.

 

Thanks for the help......right now I'm running oem suggested plugs, with the 9's would my rotor behave any different?

 

less prone to ignition breakup in the higher rev ranges. bridges like fuel, spark likes stoich mixtures, they never cross in reality so stronger ignition is required.

 

Thx again

 

QUOTE=RotaryRocket88;11228877]^ Yes, 7EQ is leading and 9EQ is trailing. The position of the trailing plugs further up in the housing means they are the first to be "seen" in the compression stroke. If they retain to much heat, you could get pre-ignition (before the leading plugs fire) really early in the cycle. It could also be a catalyst for detonation (after the plugs fire, but before all gas burns).[ sorry wrong post[

 

Holy Poopola!!.

Thanks man!.I edited my Original post and also took out the Bad info Quote in your Reply,so ALL info is now correct.
7's are Leading and 9's are Trailing...like when counting,7's before 9's.

 

On my 2001 i'm going with 4x 9EQP....

change spark plug more times--->OK
Detonation risk-->NO thx^^

On stock car you don't like to go colder then 9