mwatson184

I'm planning on running a modified second gen cas for a nice locked out 0 split direct fire setup (incorporating an MSD DIS-2). I need to know if the signal from the second gen magnetic pickups will be able to fire a first gen ignitor. The first gen ignitors will then be the signal for the DIS-2 (front rotor signal/rear rotor signal). I know i can use the second gen ignitors, but I have a few j109's kicking around.

Jeff20b I know you will hate this setup because of the lack of a wasted spark and the 0 degree l/t split so you don't need to tell me about that

-Marques

mwatson184

I made two threads, reply to this one.

bump

IanS

iTrader: (4)

I think it is good to try out something a bit different. Just be careful to MAKE SURE that the trailing is NOT firing before the leading. I am sure you have researched this topic, but just in case... Good luck with the project and let us know how it goes, and what your experiences were.

mwatson184

It will be firing out of 2 separate msd dis coils (dual tower coils). One for the font rotor, one for the rear. L/T will be firing at the exact same time. I will set the timing pretty conservatively and tune the carb pretty fat so detonation due to too much advance or leanness will not be a problem.

anybody know?

Marques

Jeff20B

iTrader: (13)

How are you planning to mod the CAS?. A J-109 can only handle so many teeth whizzing past the sensor per revolution. A DIS coil firing both plugs in a rotor housing is a bad idea. peejay can explain why a lot better than I can.

mwatson184

I am grinding off 22 of the 24 teeth on the bottom, and relocating one of the pikups. The CAS rotates at half the speed of the motor, so I will have a pickup for the top two teeth, and a pickup for the bottom two teeth. every 0 and 180 degree's of cas rotation (360 eccentric rotation) the top pickup will get its signal for the front rotor, and every 90 and 270 of cas rotation the rear will get it's signal for the rear rotor.

-Marques

nevarmore

Please try to explain it, peejay is MIA due to a jackass roommate "forgetting" to pay the internet bills.

White_FC

I'm no ignition guru but as I understand it if you use a two post coil setup in a single chamber the spark energy will tend to focuss on one of the plugs and not the other due to all sorts of reasons but basically one will end up with a lower resistance than the other (pressure in the chamber.. **** like that).
This is not good, especially if it concentrates on the trailing plug and not the leading.


I could be way off, but i dont think so..

mwatson184

Interesting. That might keep me from trying it out...but probably not. I like all the features the dis-2 has, mainly the 2 step rev limiter and the boost/nitrous retard. I could get that out of the stock dizzy and 2 msd 6a's (or btms), 2 msd 2 step modules, and a couple of other things. The main point is, in stock form you have to double up on everything, and that frustrates me. I just want a simple, bare bones locked out ignition with a two step, and I think i'm going to give this a try. If it doesn't work out i'll go back to a dizzy and sell everything.

I've been thinking about this whole ignition thing for the past couple days. I've gotta say, ignition is one of the only dislikes I have about rotaries.

Just so you guys know, I'm building up a bridge ported, T66, holley blow through car, that is what this little project will make it's debut on.

-Marques

mwatson184

bump, any answer to the origional question?

renns

Marques,

The voltage required to jump the spark is a function of (among other things) the temperature and pressure of the gases in the spark gap. At higher pressures and lower temperatures (end of compression stroke), the required voltage is high. At the end of the power stroke (wasted spark time in a rotary) the pressure is low and temperature is high, requiring much less voltage to jump the gap. The total voltage required is the sum of these two, as the voltage generated by the coil must be high enough to jump both gaps simultaneously in order to fire.

What you propose is to fire both plugs in one chamber, requiring two simultaneous high-voltage sparks. This will require a higher than normal secondary voltage. That is not to say it won't work, though. It depends on the excess capacity in the ignition system. As for magnetic pickup (VR sensor) compatibility, I'd expect the 2nd gen units will work at least as well as the 1st gen parts.

If you're into math, here's an empirical relationship between spark voltage, pressure, temperature, and plug gap as presented in SAE paper #200-01-0245:

V = 4.3 + 136*(P/T) + 324*(P/T)*gap

V = KV required to jump the gap
P = absolute pressure in bars
T = temperature in Kelvin
gap = plug gap in mm

Roger.

mwatson184

Awesome info Renns, that will keep me thinking for a bit. In your opinion, do you think that the MSD DIS2 has the capacity to fire both coils into one chamber, or should I go back to the drawing board?

Marques

peejay

But it won't work.

Note that the OEM's with dual plug per cylinder systems still use separate coils for each plug. A nice example because it is contemporary is the new-Hemi. Two plugs per cylinder, eight coils, and each coil is double-ended, one to the plug directly beneath and one across to the cylinder 360 degrees out of phase.

There isn't a double ended coil made that has enough secondary to fire two plugs in the same cylinder. If there was, it'd be a sweet setup, but there isn't so you need separate coils leading and trailing.

mwatson184

Yep, I did some research and found out that my origional plan won't work. I have a new idea (using second gen cas on a carbed car) for something that will work. The best part is wasted spark will be kept, as with an l/t split, and timing will be locked. That will be perfect for my goals, maybe not for everyone elses. I completely took apart a CAS and have started modifying it. I'll put some pictures up a little later with an explaination. I don't feel like I will be able to exlain without the pictures.

Marques

Mr BiG G

marques...any updates? i was wondering, why not go with a COP(Coil on Plug) style setup if your'e using a CAS, wouldn't that be much better? you could mod the CAS to have 4 teeth...im not too sure about this...

G

mwatson184

^that is exactly what I am doing now. I have ground off all but four theeth on the bottom set of 24, and left the 2 teeth on the top. I am adding one ignition pickup on the top 90 degrees from the origional one. The Leading will fire both plugs every 90 degree's of cas rotation (180 eccentric rotation) and the trailing (one pickup for each coil/plug) will fire every 0 and 180 T1, and 90 and 270 T2 with a fixed 15 degree L/T split between leading and trailing.

Like I said, hard for me to explain with words. I'm leaving to go on a job in less than a week, when I get back my car will be my main concern (with money to blow on it) and I will try to get this thing up and running. If it works then great, if it doesn't i'll just throw in a normal locked dizzy.

BTW, this will be on a second gen, but I like you guys waaaay better than the second gen people. They all get on me about carbing my car etc....

-Marques

Mr BiG G

where exactly is the CAS located on the 2nd gen and where are u planning on putting it on the 12A? and what do u do with the hole the distributor goes into? ahhh so many questions....ever since i learnt how ignition systems work at college...i've always wanted to do a coil on plug setup for the 12A but i always thought it would need an ignition computer...

-G