should have 2 dwell settings ????

fritoss007 · Post by **fritoss007** » Tue Aug 22, 2006 5:25 am

here is my problem...

when the car is cold...i should run something like 4~4.2ms of dwell....this the car is going fine no missfire....

but when the car when hot ( takes arround 7~10 minutes of smooth driving ) it goes on 3 cylinder.....

to fix the problem i have to wait that car (the sparkplugs? or the coils? or the vb921? ) get colder to get back my 4 cylinder...and there is no way the car will stay on 4 cylindre until i go to max 2.1ms of dwell...if i go lower that 1.9ms the car is not going fine !!!

but when i leave the car to 1.9~2.1ms of dwell i have to drive for 10 minutes to get the going fine...otherways the car is really hard missfiring !!

here is the description of my settings..
-msI ver.3
-29q
-4 vb921 each with a 680Ohms resistor that drives 4 MSD SS blaster coils in wastespark mode

hobieboy · Post by **hobieboy** » Tue Aug 22, 2006 8:17 am

Can it be the fact that the VB921 is not being biased correctly?

In my case (6 VB921 remote mount though), I have to add a bus driver between the CPU output & the VB921 or the VB921 won't "open" enough to charge the coils properly.

fritoss007 · Post by **fritoss007** » Tue Aug 22, 2006 8:23 am

what you mean by "bus driver" ??

probably not the guy that drives the yellow school bus ???

:)

hobieboy · Post by **hobieboy** » Tue Aug 22, 2006 9:54 am

a 74hc125

fritoss007 · Post by **fritoss007** » Tue Aug 22, 2006 10:13 am

sorry but i'm really a newbie.....i'm working in computer for 11 years now...and doing some tuning for 5 years...but electronic is almost new for me !!!

so this would be really nice from you if you could just explain me what a 74hc125 will do in my setup.....

and i'm sorry too for my english, cause i'm coming from quebec, canada and i'm a french speaking person !!!

thanks so much !!

hobieboy · Post by **hobieboy** » Tue Aug 22, 2006 1:22 pm

Honestly not sure if this would apply in your case, but for me, connecting the VB921's directly to the CPU pin and the LED resulted in too little bias current for the VB921; hence I could use the VB921 to light a LED test light but not charge a coil.

I used the 74hc125 to "boost" the CPU signal then drive the VB921 with success. 74HC125 is a TTL line/bus driver chip.

fritoss007 · Post by **fritoss007** » Tue Aug 22, 2006 2:07 pm

thanks hobieboy.....i'll be looking to install that "bus driver" if this may fix my problem...be sure i'll try it !!

ps silky....email moi directement a jerome@micro-centre.com

fritoss007 · Post by **fritoss007** » Wed Aug 23, 2006 5:25 am

would please tell me what "minimum discharge time" is for ??

nobody can tell me how to set it...

Post by **jsmcortina** » Wed Aug 23, 2006 5:29 am

This setting only really comes into play on V8 distributor installs where the desired dwell exceeds the available time. It defines a minimum time that the coil has to be off to allow it to fire.
Typically it is set to 0.1ms and will in practice allow more time than that.

James

fritoss007 · Post by **fritoss007** » Wed Aug 23, 2006 5:44 am

thanks jsmcortina

you read about my problem ? have any ideas ?

Fastest95PGT · Post by **Fastest95PGT** » Wed Aug 23, 2006 5:46 am

This also comes into play on my V6 single coil distributor setup as well.

Here's an example.
5000rpm * 3 (ignition events per revolution) = 15000 ignition events per minute
15000 / 60 (seconds) = 250 ignition events per second
1s / 250 = 4ms per ignition event (for charge AND discharge time)

Since my running dwell is set at 4ms, the minimum discharge definitely comes into play. In fact, on the dyno, I had to increase my minimum discharge to .5 from .1 to avoid sputtering under boost. Now, does that mean that my running dwell could be 3.5ms instead of 4ms?.. not quite as the car doesn't do perform so well in the lower rpm with a 3.5ms running dwell.

fritoss007 · Post by **fritoss007** » Wed Aug 23, 2006 8:48 am

i'm having a 4 cyl. waste spark setup...so :

max 8000RPM * 2 spark events = 16 000sprk/min
16000sprk / 60 = 266 prk/sec
1s / 266 = 3.75 ms

my engine is doing fine arroud 4.0ms when it is cold....but if i leave that 4.0ms be sure that in 15 minutes the vb921 will get really hot and the engine will begin to run on 3 cyl.

to fix the problem i have to set the dwell to 2.1ms max...but this dwell make the car missfire so much when it is cold !!!

another question ?

do i have 2 or 4 spark event because of the wastespark ?

max 8000RPM * 4sprk events = 32 000sprk/min
32 000 / 60 = 533sprk/sec
1s / 533 = 1.87ms

???

Fastest95PGT · Post by **Fastest95PGT** » Wed Aug 23, 2006 8:56 am

You have two spark events per revolution of the motor on a wasted spark 4 cylinder setup. However, wasted spark triggers only one individual coil pack every revolution.

Your math is slightly off though. My example used a single coil. You essentially have two coils and are only firing two cylinders. So your "spark event per revolution" value would be 1.

So the math would be...
8000rpm * 1 spark event = 8000 sparks/min
8000 / 60s = 133 sparks/second
1/133 = 7.5ms (charge and discharge time)

Remember, in the case of multiple coils, one can be charging while the other is discharging.

Side note, if you were COP (4 coils) then you would cut the rpm in half because you only spark one coil once per two revolutions.

fritoss007 · Post by **fritoss007** » Wed Aug 23, 2006 10:52 am

maybe my math is not as good as your......not maybe but surely....

but be surfe that if i put 7.5ms of dwell...that engine will not love that...

PS i'm using LED17 and LED19 as sprk A&B connected to 4 vb921 and 4 msd blaster ss coils !

Fastest95PGT · Post by **Fastest95PGT** » Wed Aug 23, 2006 10:59 am

When I said 7.5 for charge and discharge time,... that was meant to be taken as the MAXIMUM time between ignition events per individual coil!

Obviously if you put 7.5 in there you'd probably burn out something (VB921 or coil) and cause who knows what other damage.

fritoss007 · Post by **fritoss007** » Wed Aug 23, 2006 11:50 am

ok that is fine...thanks for the advice....

but i'm still needing help for my little problem ?

I use 4 680Ohms as bias resistor for my 4 vb921...is that OK ??

Fastest95PGT · Post by **Fastest95PGT** » Wed Aug 23, 2006 11:56 am

I don't run any VB921's so I cannot comment other than I believe 680's what you're supposed to use on the pair.

fritoss007 · Post by **fritoss007** » Wed Aug 23, 2006 12:33 pm

I guess so...and 330Ohms for single ( 1 sprk out to 1 vb961 )