rx7lives

My apologies if this is a reprint but I thought I had asked this question but now I can't find it.

My question, I'm using direct fire on the leading plugs using an FC leading coil that has two connections and both fire at the same time. I'm also using an MSD for the leading plugs.

Now for the question, why can't we do the same thing for the trailing plugs, use one FC coil and use it to fire both plugs at the same time as it does on the leading coil and eliminate the distributor cap and rotor or just use the rotor to keep dirt out of the dist?

Thanks.

rotaryjunkee

Excellent question, I would like to know also. I'm subscribed

t_g_farrell

iTrader: (1)

Because the trailing fires later in the combustion cycle, you can't do a wasted spark (or multiple
spark) solution like you can for the leadings. This would cause one of the trailing (at least) to fire
too early in the next combustion and lead to preignition which would hurt performance and also
may actually cause damage to the engine. So you have to maintian the single spark and the
only way to do that is with the dizzy or maybe a standalone computer.

Search on DLIDFIS and you will see this subject come up pretty often.

j9fd3s

iTrader: (3)

yes, on the wasted spark setup, one leading sparkplug fires into the compression stroke, and the other sparkplug fires in the exhaust stroke.

on the trailing side one sparkplug fires into the compression stroke, and the other rotor is in the intake stroke.

rx7lives

If this is the case and I don't have any reason to not believe you, then we should be able to do the same thing on the trailing ones as on the leading plugs. However, the FC trailing coils are two separate coils in one package and there is a wire that tells which coil to fire. Now, if we could get a reference on the rotors' position, we could use that to turn on the correct coil. The problem is the eccentric shaft rotates three times for each full rotation of the rotor. I assume that Mazda uses the crank position sensor to keep track of where the rotors are and to fire the correct trailing coil. As was pointed out to me when I bought a CAS, it doesn't have mechanical or vacuum advance, I don't even know how it interfaces with the coils, does it go to the coils directly or, more likely, it goes to the ECU to control spark timing and advance? The bottom line is it was worthless on an FB unless you wanted to use the FC wiring harness and ECU, fun...


If the active rotor is in the compression stroke, that is the place where it fires the trailing plug milliseconds after the leading plug fires. Since the other rotor is 180° out of phase, that puts its Apex seal somewhere around both plugs. I could see a problem if it is carrying the intake charge on one side and the spent exhaust on the other and the trailing plug fires just as the apex seal crosses the trailing plug's threshold so it would possibly pre-ignite the fuel/air mixture or worse, it could ignite the fuel/air mixture at such a place that the explosion forces the rotor to attempt to go backwards.

I'm hoping someone smarter than me (not exactly a hard target to hit) can figure it out. If we could find a reference point as to where each trailing rotor is in relation to the trailing plug, we could enable the correct coil. This means we only need a FC trailing coil (or two high performance ones that are wired so they turn on at the correct point). If we can get the reference point, I can design a simple circuit to use the stock ignitors (I use the stock ones with my MSD since it doesn't have nearly the load as the coils have. This would mean we could get by with just two MSDs and two coils whichever you prefer but if you don't use the FC ones, you'll need two unless we can fire the trailing plugs like the leading ones.

Ideally, we find out that we can fire the trailing plugs the same way we do direct fire for the leading plugs. i don't have much hope for this though as Mazda used a single two terminal coil that fired both plugs when it was activated on the leading plugs but used two coils (in one plastic or Bakelite case) to fire the trailing plugs and this is what makes me think if we fired the trailing plugs at the same time, we would get one rotor firing correctly and one that is trying to ignite a mixture that isn't in the correct place.

t_g_farrell

iTrader: (1)

Thats why you have to use the dizzy for the trailing, it separates the spark to
each rotor while signalling a single coil. You would need some sort of different
setup to get 2 distinct signals per trailing coil to fire the individual coils correctly.
I can see 2 ways to do this; modified dizzy somehow or a cas and standalone
computer to negotiate the spark signal.

Just don't use trailing. I don't have it and it runs great

j9fd3s

iTrader: (3)

the CAS just tells the ECU where in the cycle and how many rpms the engine is turning. the ECU fires the coils.

for the rest you're overthinking it. the reason the FC coil works better is because its bigger, not because of the waste spark.

secondly, the trailing doesn't provide tons of power, and thus doesn't really need an upgrade

rx7lives

1) I'm using the FC coil not because I think it's better; I'm using it because it's convenient to have to only hook the MSD to one coil and it has two outputs that I run directly to the leading spark plugs.

2) I'm in California and they do a physical inspection during the biannual smog test. You could be outputting pure unicorn farts and they'd still fail you if your timing is off a degree. I wouldn't be surprised if they re-instituted the 'safety' inspections they had in the '70s where they'd pull you over and cite you for performance parts, exhaust, green tree shaped air purifier, they'd nail you for everything. I had a two stroke bike that had expansion chambers, I'd get a ticket, put the stock pipes back on and get it signed off and then put the chambers back on. Rinse, repeat.

3) I'd just like to hook up the trailing plugs like the leading ones, that way I could eliminate the distributor rotor and cap. Currently, I've got the trailing plugs set up like stock.

4) If it turns out that firing the 'wasted' trailing spark ignites the intake charge too soon, I could see why we're stuck using a coil or adapting the FC CAS and ECU which is way too much work for so little in return. I'll just put an MSD on an aftermarket coil and use the dist to make sure the right plug fires at the right time.

5) Does anyone know whether the injectors in a GSLSE are sequentialy fired or not? If they're sequential, I could pull a signal off of each injector and use it to tell the FC trailing plug to fire the correct coil.