Direct Fire Trailing
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Joined: May 2008
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From: San Jose, California
Direct Fire Trailing
I have read a number of ways to convert the FB to direct fire ignition, but they all seem to only convert for the leading plugs. I was wondering why this is? Is it just because people just dont think its worth the cost of an extra coil/igniter or is there really something wrong with doing this? I would really like to eliminate the distributor cap/rotor all together considering they cost about $50 for a set.
In order to do this you would need what's called a Crank Angle Sensor(CAS for short). The problem with this is you need a computer to control the CAS. At least that's my undertanding but if I'm wrong someone will chime in I hope
Thread Starter
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Joined: May 2008
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From: San Jose, California
Doesnt the distributor already have the CAS built in and that is what people are using for the leading plugs? I was under the impression that were was a magnetic pick up for both leading and trailing plugs which were hooked to a pair of ignitors in the stock setup. I was just thinking that you could have each pickup output being split to two igniters each which would each fire a coil. Four coils, four spark plugs, leading and trailing ignition separated by the offest between the to pickups in the distributor. Am I missing something?
Last edited by vipernicus42; May 31, 2008 at 11:32 AM.
Not sure exactly what injectors you're talking about? The carb'd RX-7's use a distributor to distribute the spark to the correct plugs while the fuel injected RX-7's use a CAS that is electronically controlled by the ECU which a carby wont have
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Joined: May 2008
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From: San Jose, California
Wow, thats what happens when I dont get enough sleep. Replace the word injector with spark plug.... sorry about that....
and yes, this is for a carbed engine. Isnt the magnetic pickup a CAS of sorts?
and yes, this is for a carbed engine. Isnt the magnetic pickup a CAS of sorts?
Joined: May 2002
Posts: 5,972
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From: Ottawa, Soviet Canuckistan
Okay.... the reason that direct fire is usually done for leading only is because you can fire both leading plugs at the same time and not cause any problems. One will fire at the appropriate time and the other will fire into the exhaust stroke, basically doing nothing (called "wasted spark")
If you did that with the Trailing though, one would fire at the right time, and the other would fire into the compression stroke WAY earlier than it's supposed to. This would ignite the air/fuel mix too early and send that rotor spinning backwards (or trying to). This would be very bad, and you'd probably blow an apex seal at the worst, or lose a ton of power at the best.
So if you want to do direct-fire trailing, you need to rig them both separately. Now depending on which direct fire system you're using this could take a lot more effort and money. For example, with an MSD box, you only need one MSD box to do both the leading, but if you wanted leading *and* trailing you'd need three of them. That's bloody expensive. For the 2nd Gen coil setup, the only way to do it is to replace your distributor with a Crank Angle Sensor (CAS) and get a computer like a Megasquirt, Haltech, etc... to use that signal from the CAS to figure out when to fire the coils. The Megasquirt is the cheapest option of the aftermarket computers, but again it's a lot more cost compared to just rigging up a 2nd gen leading coil and leaving the trailing stock.
The reason people don't work with the trailing much is because the trailing is not there for power. The Trailing are basically there for Emissions control to finish burning the mixture. Someone on here posted dyno graphs where he ran back-to-back, one with his ignition stock and the other where he disconnected his trailing coil completely and ran leading-only. The graphs were so close that the difference (1-2hp) could be explained as easily by the margin of error for the dyno as it could by the removal of the trailing! Hardly worth spending 300+ dollars to get the trailing working on direct fire.
So if you're swimming in money, go ahead and setup direct fire on all three (Leading-All, T1, T2) and have fun. Otherwise, it makes more sense to just do Direct-Fire on leading and leave the trailing alone.
Jon
If you did that with the Trailing though, one would fire at the right time, and the other would fire into the compression stroke WAY earlier than it's supposed to. This would ignite the air/fuel mix too early and send that rotor spinning backwards (or trying to). This would be very bad, and you'd probably blow an apex seal at the worst, or lose a ton of power at the best.
So if you want to do direct-fire trailing, you need to rig them both separately. Now depending on which direct fire system you're using this could take a lot more effort and money. For example, with an MSD box, you only need one MSD box to do both the leading, but if you wanted leading *and* trailing you'd need three of them. That's bloody expensive. For the 2nd Gen coil setup, the only way to do it is to replace your distributor with a Crank Angle Sensor (CAS) and get a computer like a Megasquirt, Haltech, etc... to use that signal from the CAS to figure out when to fire the coils. The Megasquirt is the cheapest option of the aftermarket computers, but again it's a lot more cost compared to just rigging up a 2nd gen leading coil and leaving the trailing stock.
The reason people don't work with the trailing much is because the trailing is not there for power. The Trailing are basically there for Emissions control to finish burning the mixture. Someone on here posted dyno graphs where he ran back-to-back, one with his ignition stock and the other where he disconnected his trailing coil completely and ran leading-only. The graphs were so close that the difference (1-2hp) could be explained as easily by the margin of error for the dyno as it could by the removal of the trailing! Hardly worth spending 300+ dollars to get the trailing working on direct fire.
So if you're swimming in money, go ahead and setup direct fire on all three (Leading-All, T1, T2) and have fun. Otherwise, it makes more sense to just do Direct-Fire on leading and leave the trailing alone.
Jon
Joined: May 2002
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From: Ottawa, Soviet Canuckistan
I changed your post so that "injectors" was changed to "plugs"....
But no a CAS and your distributor's magnetic pickups are very different. The inside of a CAS looks like this:
https://www.rx7club.com/showthread.p...ht=CAS+Picture
AS you can see there are two sets of teeth. The top set that has 2, and the bottom gear which has many more. The computer uses these signals to know what angle your e-shaft is at and thus when to fire the plugs based off its ignition map.
The distributor doesn't tell the coil what angle the engine is at, it just says "I've got the signal, fire it now".
Jon
But no a CAS and your distributor's magnetic pickups are very different. The inside of a CAS looks like this:
https://www.rx7club.com/showthread.p...ht=CAS+Picture
AS you can see there are two sets of teeth. The top set that has 2, and the bottom gear which has many more. The computer uses these signals to know what angle your e-shaft is at and thus when to fire the plugs based off its ignition map.
The distributor doesn't tell the coil what angle the engine is at, it just says "I've got the signal, fire it now".
Jon
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Joined: May 2008
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From: San Jose, California
Ok, thanks for setting me strait. You basicly got to the meat of my question when you started talking about wasted spark. I should know not to think of a magnetic pickup as a CAS, not exactly sure why I did... I have setup a MS on another car and I know that it is way too much work to get the trailing spark working on direct fire.
Anyway, thanks alot for helping me out!
Anyway, thanks alot for helping me out!
I hate drum brakes
Joined: Oct 2006
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From: West Palm Beach, Florida
Add another modified reluctor wheel, with two tabs removed, and two magnetic pickups, spaced 90 degrees apart *timed appropriately*, and an FC trailing coil with integral ingiters.
Joined: Mar 2001
Posts: 31,833
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From: https://www2.mazda.com/en/100th/
Ok, thanks for setting me strait. You basicly got to the meat of my question when you started talking about wasted spark. I should know not to think of a magnetic pickup as a CAS, not exactly sure why I did... I have setup a MS on another car and I know that it is way too much work to get the trailing spark working on direct fire.
Anyway, thanks alot for helping me out!
Anyway, thanks alot for helping me out!
the hardware looks the same except the dizzy has 4 teeth, one for each rotor, for 2 engine revolutions (dizzy/cas spin at half speed), and one magnetic pickup for leading, and one for trailing.
the cas has a 2 tooth wheel, and a 36 (i think) tooth wheel. one is a "home" signal, tells the ecu when the engine has completed a cycle, and the other is a "trigger" giving rpm.
84 SE
Joined: Sep 2005
Posts: 1,004
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From: Summit Hill, PA
Okay.... the reason that direct fire is usually done for leading only is because you can fire both leading plugs at the same time and not cause any problems. One will fire at the appropriate time and the other will fire into the exhaust stroke, basically doing nothing (called "wasted spark")
If you did that with the Trailing though, one would fire at the right time, and the other would fire into the compression stroke WAY earlier than it's supposed to. This would ignite the air/fuel mix too early and send that rotor spinning backwards (or trying to). This would be very bad, and you'd probably blow an apex seal at the worst, or lose a ton of power at the best.
So if you want to do direct-fire trailing, you need to rig them both separately. Now depending on which direct fire system you're using this could take a lot more effort and money. For example, with an MSD box, you only need one MSD box to do both the leading, but if you wanted leading *and* trailing you'd need three of them. That's bloody expensive. For the 2nd Gen coil setup, the only way to do it is to replace your distributor with a Crank Angle Sensor (CAS) and get a computer like a Megasquirt, Haltech, etc... to use that signal from the CAS to figure out when to fire the coils. The Megasquirt is the cheapest option of the aftermarket computers, but again it's a lot more cost compared to just rigging up a 2nd gen leading coil and leaving the trailing stock.
The reason people don't work with the trailing much is because the trailing is not there for power. The Trailing are basically there for Emissions control to finish burning the mixture. Someone on here posted dyno graphs where he ran back-to-back, one with his ignition stock and the other where he disconnected his trailing coil completely and ran leading-only. The graphs were so close that the difference (1-2hp) could be explained as easily by the margin of error for the dyno as it could by the removal of the trailing! Hardly worth spending 300+ dollars to get the trailing working on direct fire.
So if you're swimming in money, go ahead and setup direct fire on all three (Leading-All, T1, T2) and have fun. Otherwise, it makes more sense to just do Direct-Fire on leading and leave the trailing alone.
Jon
If you did that with the Trailing though, one would fire at the right time, and the other would fire into the compression stroke WAY earlier than it's supposed to. This would ignite the air/fuel mix too early and send that rotor spinning backwards (or trying to). This would be very bad, and you'd probably blow an apex seal at the worst, or lose a ton of power at the best.
So if you want to do direct-fire trailing, you need to rig them both separately. Now depending on which direct fire system you're using this could take a lot more effort and money. For example, with an MSD box, you only need one MSD box to do both the leading, but if you wanted leading *and* trailing you'd need three of them. That's bloody expensive. For the 2nd Gen coil setup, the only way to do it is to replace your distributor with a Crank Angle Sensor (CAS) and get a computer like a Megasquirt, Haltech, etc... to use that signal from the CAS to figure out when to fire the coils. The Megasquirt is the cheapest option of the aftermarket computers, but again it's a lot more cost compared to just rigging up a 2nd gen leading coil and leaving the trailing stock.
The reason people don't work with the trailing much is because the trailing is not there for power. The Trailing are basically there for Emissions control to finish burning the mixture. Someone on here posted dyno graphs where he ran back-to-back, one with his ignition stock and the other where he disconnected his trailing coil completely and ran leading-only. The graphs were so close that the difference (1-2hp) could be explained as easily by the margin of error for the dyno as it could by the removal of the trailing! Hardly worth spending 300+ dollars to get the trailing working on direct fire.
So if you're swimming in money, go ahead and setup direct fire on all three (Leading-All, T1, T2) and have fun. Otherwise, it makes more sense to just do Direct-Fire on leading and leave the trailing alone.
Jon
You don't get a better explanation than this :-) Good job Viper. I totally agree with you.
Joined: Jul 2002
Posts: 7,301
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From: District of Columbia
search search search
there was this thread where a guy modified a dizzy to be able to direct fire the trailing just like you do the one billion ways you DF the leading. what he did was stacked a second set of magnetic pick ups and a second rotor thing that triggers the pickups, but the important thing was it had 2 of the 4 points ground off so that you would be able to direct fire the trailing without the worry of wasted spark.
there was this thread where a guy modified a dizzy to be able to direct fire the trailing just like you do the one billion ways you DF the leading. what he did was stacked a second set of magnetic pick ups and a second rotor thing that triggers the pickups, but the important thing was it had 2 of the 4 points ground off so that you would be able to direct fire the trailing without the worry of wasted spark.
Where there's a will, there's a way. Make your own!
Hmm, something compatible with most rotary engines... It would have to bolt on the front plate at a power steering stud and on the rear plate on the starter long bolt hole. That could work.
Hmm, something compatible with most rotary engines... It would have to bolt on the front plate at a power steering stud and on the rear plate on the starter long bolt hole. That could work.
I wanna go fast
Joined: May 2006
Posts: 378
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From: North Bend, WA
somewhere in the second gen section i saw someone who made a bracket to fit the ps/ac studs and then had 4 separate coils. I will try and find it... i know it wasnt what drivefast7 was asking about but it will probably give you an idea
Joined: Jul 2002
Posts: 7,301
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From: District of Columbia
Word. I love when threads get ideas spawning and thought going.
Damn I searched for 2 hours trying to find that thread with the modded dizzy. Jeff do you know where it is or still have the pix. My computer crashed so i have lost everything...
Damn I searched for 2 hours trying to find that thread with the modded dizzy. Jeff do you know where it is or still have the pix. My computer crashed so i have lost everything...
I think I found the thread. It's only been about two months since it was last posted to. It's not what I was thinking about, but I think it's what you were thinking about. Enjoy.
https://www.rx7club.com/forum/showthread.php?t=604087
https://www.rx7club.com/forum/showthread.php?t=604087
That would be the way 2 go. Now, what ignition coil would plug directly onto a spark plug?
Senior Member
Joined: Sep 2005
Posts: 252
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From: Cleveland, TN
It was asked earlier in this thread but never answered why couldnt you fire trailing the same time as leading or maybe 1-5 deg behind? Would this not keep the wasted spark in a safe place on the other rotor?
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