VB921 and 1.5ohm coil.

BottleFed70 · Post by **BottleFed70** » Sat Jan 21, 2006 5:02 pm

Hi,

Can anyone tell me if the vb921 can drive a 1.5ohm coil without the use of a ballast resistor?

This will be for my 5.8L V8

Thanks!

natesully · Post by **natesully** » Sat Jan 21, 2006 10:15 pm

Well, I drove some 1.6ohm bike coils with 4 VB's in wasted spark. I had bad interference causing resets like this, but when not reseting it worked really well. Around 4,000+ the VB's would start to get a little warm, not scary hot just warm, so I would bolt them to a heatsink. They did not get too hot considering each one had an extra spark to fire.

sparkandfuel.com · Post by **sparkandfuel.com** » Sun Jan 22, 2006 9:16 am

we ran a set of 2Ω coils for a good while, 4 banger motor, each coilpack had control of 2 cylinders. it got warm but did not get crazy hot. we also had the extra vb921 on the r38 (big to-220 resistor on the heatsink), used a longer screw to hold both the resistor and the vb921 on top of eachother on the big heatsink bar, with thermal grease.

i dont think it sounded like it was a great idea to mount the vb921 like that, but it ended up working a-ok. and it seemeed to stave of the heat effectivly.

we use 30watt resistors with a mouning hole in them, not the hole-less ones from the BOM.

sparkandfuel.com · Post by **sparkandfuel.com** » Tue Jan 24, 2006 2:19 pm

jeez, i killed the thread. dead!
i was hoping on some more input form alternative sources for some even more in depth reasonings.

BottleFed70 · Post by **BottleFed70** » Tue Jan 24, 2006 4:03 pm

sparkandfuel.com wrote:jeez, i killed the thread. dead!
i was hoping on some more input form alternative sources for some even more in depth reasonings.

I was also hoping for a little more feedback.

1.5ohm = 8amps @ 12V or 10amps @ 15V,

From what I can tell from the data sheet the VB921 is only good for 7.5amps. But I can't tell if it'll just clamp the current to 7.5amps(which would be purfect...almost act like the ballast resistor), or if the VB921 will end up burning out.

natesully · Post by **natesully** » Tue Jan 24, 2006 6:43 pm

BottleFed70 wrote:
sparkandfuel.com wrote:jeez, i killed the thread. dead!
i was hoping on some more input form alternative sources for some even more in depth reasonings.
I was also hoping for a little more feedback.

1.5ohm = 8amps @ 12V or 10amps @ 15V,

From what I can tell from the data sheet the VB921 is only good for 7.5amps. But I can't tell if it'll just clamp the current to 7.5amps(which would be purfect...almost act like the ballast resistor), or if the VB921 will end up burning out.

1.5ohm != 8amps @12V. The coil has to charge and build a magnetic field, and it has a higher impedence as it does so, as per lenz's law. If your dwell is short enough, it may never reach 8 amps. That is why my VB's were not on fire (although the small resistance could be causing all those resets).

jsmcortina · Post by **jsmcortina** » Wed Jan 25, 2006 6:09 am

It is easy to get confused by the resistance of a coil and not appreciate that it is an inductor. With the right dwell you will saturate the coil for a good strong spark without drawing a huge current.
I recall Ford EDIS coils are a similar resistance and they run fine on my car with little heat from the VB921.

James

BottleFed70 · Post by **BottleFed70** » Wed Jan 25, 2006 12:18 pm

jsmcortina wrote:It is easy to get confused by the resistance of a coil and not appreciate that it is an inductor. With the right dwell you will saturate the coil for a good strong spark without drawing a huge current.
I recall Ford EDIS coils are a similar resistance and they run fine on my car with little heat from the VB921.

James

That's good news!

I plan on having my MSII V3 with the build in VB921 driving the coil.

I havn't played with the spark settings much yet, just getting things ready. Can you adjust the dwell using the MSII?

mops · Post by **mops** » Sun Feb 05, 2006 3:42 pm

yes, ms2 has fully configurable dwell.

however i got problems with coil on my ms2 setup. i get ocasional misses. my stock coil is 1.1 ohm, i tried 2ohm coild and it works worse

(i.e more misfires, car not driveable).

i'm trying to work out whether ms can drive 1.1 ohm coil. many ppl put ms2 on e30's and have no problems running stock coil.... i dunno "huh" is wrong with my setup... but i'm working on it and will update once it's worked out. right now i'm making proper ground from ecu to MS2.