Capacitive Discharge or not?
Discussion
There seems to be two schools of thought out there, some say CD and some say dont bother unless you're hitting 8krpms+
Spoke to Motec and they advise that when cylinder pressures are running very high then you need more volts to get a good arc, so would this point to a CD setup being better?
But then a CD setup doesnt have the current a direct-coil setup can push (talking MSD HVC2).
A larger spark gap and more current = more complete combustion (better initial ionisation followed by better arc leading to a stronger flame front)
What you techies think?
My engineers reckon a direct-coil setup is the way to go, simpler and less flaffing around, but then CD setups with their multi-spark abilities sound quite interesting, but of course considering as the RPM's go up the multiple sparks reduce per cycle eventually leading to a single spark setup anyway.
My setup:
flatplane 6.0 V8
28psi blower
max rpm 6500
16 injector staged fuelling system
So chaps, CD or not? and what is your technical reasoning...
Spoke to Motec and they advise that when cylinder pressures are running very high then you need more volts to get a good arc, so would this point to a CD setup being better?
But then a CD setup doesnt have the current a direct-coil setup can push (talking MSD HVC2).
A larger spark gap and more current = more complete combustion (better initial ionisation followed by better arc leading to a stronger flame front)
What you techies think?
My engineers reckon a direct-coil setup is the way to go, simpler and less flaffing around, but then CD setups with their multi-spark abilities sound quite interesting, but of course considering as the RPM's go up the multiple sparks reduce per cycle eventually leading to a single spark setup anyway.
My setup:
flatplane 6.0 V8
28psi blower
max rpm 6500
16 injector staged fuelling system
So chaps, CD or not? and what is your technical reasoning...
I would have thought that with your charge density/rev limit that ignition issues would not be that high on the agenda.
I've just had a peruse of my supercharger bibles and I can't find a reference of anyone getting too excited about ignitions. I've seen some funny setups on drag engines but I think theory dictates a very similar dyno result.
What octane/advance/boost at what RPMs are you running (max)?
I would have thought there may have been some advantage detonation wise of running surface discharge plugs and the associated gear to fire them (minimum) from a purely hotspot minimisation point of view.
Purely my opinion.
I've just had a peruse of my supercharger bibles and I can't find a reference of anyone getting too excited about ignitions. I've seen some funny setups on drag engines but I think theory dictates a very similar dyno result.
What octane/advance/boost at what RPMs are you running (max)?
I would have thought there may have been some advantage detonation wise of running surface discharge plugs and the associated gear to fire them (minimum) from a purely hotspot minimisation point of view.
Purely my opinion.
At the relativly low speeds you're running a std coil per plug will be fine, it is possible to get multiple sparks with this set-up also up to a limited Rpm, but might need some code re-writing in your ECU. Usually if you light the mixture off well enough in the first place it doesn't need any gimmicks.
Matt
Matt
My engineers said theres no need for CD setup so I'm inclined that way, no problem there.
But...
Looking at write-ups like this one, makes me wander:
The ChevyHighPerformance mag wrote:
"CD ignitions are virtually a necessity when building supercharged, turbocharged, or nitrous-oxide–injected engines. These engines create tremendous cylinder pressures that increase the resistance that the ignition faces when lighting the spark. Generally, higher cylinder pressures require more voltage to initiate the spark. Violent misfires under maximum, high-rpm load in these situations are often caused by an inadequate ignition system that cannot fire the spark plug because of the resistance caused by the increased cylinder pressure."
www.chevyhiperformance.com/techarticles/45618/
But...
Looking at write-ups like this one, makes me wander:
The ChevyHighPerformance mag wrote:
"CD ignitions are virtually a necessity when building supercharged, turbocharged, or nitrous-oxide–injected engines. These engines create tremendous cylinder pressures that increase the resistance that the ignition faces when lighting the spark. Generally, higher cylinder pressures require more voltage to initiate the spark. Violent misfires under maximum, high-rpm load in these situations are often caused by an inadequate ignition system that cannot fire the spark plug because of the resistance caused by the increased cylinder pressure."
www.chevyhiperformance.com/techarticles/45618/
Looking into all the technical bla bla... it makes common sense to go with the Motec CDI/8 to plug directly into the Motec M800 via CAN. Oh what the heck is only money, but if its good enough to be used on aircraft, it'll be safe enough for me.
Been talking to a few of the big-hp boys, and they all run hybrid CD systems, they also concur with the theory that 28psi will need a solid spark at anything above 6000rpm and CD is a SAFETY margin you cant be without when running that amount of boost.
Makes sense why the likes of MSD etc all produce CD hybrid systems for top fuellers, safety margin means I get to keep my wallet safe for a bit longer maybe?
Been talking to a few of the big-hp boys, and they all run hybrid CD systems, they also concur with the theory that 28psi will need a solid spark at anything above 6000rpm and CD is a SAFETY margin you cant be without when running that amount of boost.
Makes sense why the likes of MSD etc all produce CD hybrid systems for top fuellers, safety margin means I get to keep my wallet safe for a bit longer maybe?
MSD DIS-4 amplifier can run 4 coils, or if you wire them differently, 8 seperate coils, firing wasted spark.
It can fire std coils, or CD specific coils.
Or AEM do an 8 or 10 channel CD amplifier.
If you are running a lot of boost, which you no doubt will, then a CDI setup is probably a good idea, if only for peace of mind.
It can fire std coils, or CD specific coils.
Or AEM do an 8 or 10 channel CD amplifier.
If you are running a lot of boost, which you no doubt will, then a CDI setup is probably a good idea, if only for peace of mind.
stevieturbo said:
MSD DIS-4 amplifier can run 4 coils, or if you wire them differently, 8 seperate coils, firing wasted spark.
It can fire std coils, or CD specific coils.
Or AEM do an 8 or 10 channel CD amplifier.
If you are running a lot of boost, which you no doubt will, then a CDI setup is probably a good idea, if only for peace of mind.
Hey there stevie, yup exactly my thoughts, peace of mind.
I want to stay away from wasted spark - another peace of mind thing...
I think the CDI/8 is not bad for the money, it works directly with the CAN bus on the Motec and is fully configurable (settings galore to play with to get it running perfecto on the emmissions side, which is always a plus) so all I need now, is to decide which coils to go for, definitely will be goin coil per plug, so 8 coils, but which one is reliable, solid for drag-racing and great for street use?
Been looking at the MSD HVC2 coils, what you think?
Incorrigible said:
SXS, you are a complete lunatic
well done
And a very warm welcome to PH
>> Just checked out your web site, any more recent piccies of the engine
>> Edited by Incorrigible on Tuesday 26th July 17:38
I've been on these forums for a good 6 years now, keep forgetting passwords and signing up again,
Thanks for the welcome!
Pictures? Soon, very soon!
I think that the biggest issue with coil based ignition systems relates to coil charging times. As the RPM's go up there's less dwell time for the coil to gain the appropriate energy.
If you have a coil for each plug, that problem goes away.
Capacitive discharge systems are able to deliver far less energy to the spark plug, but they are very fast, the charging time is very short indeed. This makes them more suitable for fast runnng engines.
If you have a coil for each plug, that problem goes away.
Capacitive discharge systems are able to deliver far less energy to the spark plug, but they are very fast, the charging time is very short indeed. This makes them more suitable for fast runnng engines.
dilbert said:
I think that the biggest issue with coil based ignition systems relates to coil charging times. As the RPM's go up there's less dwell time for the coil to gain the appropriate energy.
If you have a coil for each plug, that problem goes away.
Capacitive discharge systems are able to deliver far less energy to the spark plug, but they are very fast, the charging time is very short indeed. This makes them more suitable for fast runnng engines.
Thats why you can chain up low resistance high voltage and ampage coils to a CD controller, consistency and reliability is key, especially if the CD communicates with the ECU, which in Motec's case it does, and it can take appropriate actions to prevent pending disaster VERY quickly.... the crank sensor and cam sensor feeds go into the ecu, the CD gets a stream of data telling it which coil to fire, and the coil gets a big dump of charge very very quickly from the CD cap, resulting in a nice big bang, and if time allows, a few more bangs very quickly.... thats the beauty of CD.... its like having an intelligent brain handling the ignition and working hand in hand with the ecu, well in Motec's CDI/8 case that is...
From what I understand, theres even scope for a back-up coil to click in if required.
You have nothing to worry about firing wasted spark.
Your flat plane engine fires 2 cylinders at a time anyway, so essentially must run wasted spark.
Or looking at it like mine. It simply fires 2 coils/cylinders at a time.
I do this for true wasted spark, as a necessity due to lack of cam sensor. Your engine does it, as it must by design.
On Subarus, from my experience, they can handle about 1.7bar before spark becomes dodgy.
MSD DIS-4 will work fine, and either use coil packs ( 2 x 4 ), or ( 4 x 2 ), or if you have 8 seperate coils, wire them together in 4 pairs, each pair in series.
Not sure which coil to go for. I'd just go for whatever packages neatest. Or indeed you can use the DIS with standard coils, and we have just done this with Subaru using a DIS-2, firing wasted, wired in series as i described.
>> Edited by stevieturbo on Tuesday 26th July 19:12
Your flat plane engine fires 2 cylinders at a time anyway, so essentially must run wasted spark.
Or looking at it like mine. It simply fires 2 coils/cylinders at a time.
I do this for true wasted spark, as a necessity due to lack of cam sensor. Your engine does it, as it must by design.
On Subarus, from my experience, they can handle about 1.7bar before spark becomes dodgy.
MSD DIS-4 will work fine, and either use coil packs ( 2 x 4 ), or ( 4 x 2 ), or if you have 8 seperate coils, wire them together in 4 pairs, each pair in series.
Not sure which coil to go for. I'd just go for whatever packages neatest. Or indeed you can use the DIS with standard coils, and we have just done this with Subaru using a DIS-2, firing wasted, wired in series as i described.
>> Edited by stevieturbo on Tuesday 26th July 19:12
-SXS- said:
I think that the biggest issue with coil based ignition systems relates to coil charging times. As the RPM's go up there's less dwell time for the coil to gain the appropriate energy.
If you have a coil for each plug, that problem goes away.
Capacitive discharge systems are able to deliver far less energy to the spark plug, but they are very fast, the charging time is very short indeed. This makes them more suitable for fast runnng engines.
Thats why you can chain up low resistance high voltage and ampage coils to a CD controller, consistency and reliability is key, especially if the CD communicates with the ECU, which in Motec's case it does, and it can take appropriate actions to prevent pending disaster VERY quickly.... the crank sensor and cam sensor feeds go into the ecu, the CD gets a stream of data telling it which coil to fire, and the coil gets a big dump of charge very very quickly from the CD cap, resulting in a nice big bang, and if time allows, a few more bangs very quickly.... thats the beauty of CD.... its like having an intelligent brain handling the ignition and working hand in hand with the ecu, well in Motec's CDI/8 case that is...
From what I understand, theres even scope for a back-up coil to click in if required.
I'd not really thought about it that way, but yes, a fast spark is also an accurate spark.
stevieturbo said:
You have nothing to worry about firing wasted spark.
Your flat plane engine fires 2 cylinders at a time anyway, so essentially must run wasted spark.
Or looking at it like mine. It simply fires 2 coils/cylinders at a time.
I do this for true wasted spark, as a necessity due to lack of cam sensor. Your engine does it, as it must by design.
On Subarus, from my experience, they can handle about 1.7bar before spark becomes dodgy.
MSD DIS-4 will work fine, and either use coil packs ( 2 x 4 ), or ( 4 x 2 ), or if you have 8 seperate coils, wire them together in 4 pairs, each pair in series.
Not sure which coil to go for. I'd just go for whatever packages neatest. Or indeed you can use the DIS with standard coils, and we have just done this with Subaru using a DIS-2, firing wasted, wired in series as i described.
>> Edited by stevieturbo on Tuesday 26th July 19:12
The CDI/8 just fires the relevant coils when required - all in parallel, so regardless of it being a flatplane and having a firing order thats a mirror image on either bank, the relevant coils will spark up simultaneously as and when required, when I said I want to stay away from wasted spark, I meant in its true form 'wasted'... the CDI/8 is fully programmable via the Motec software thus the firing/fuelling can effectively be configured at map level in the ECU, with an added advantage of tweaking the CDI/8 further...
Hi,
I'm running Nology cd plug leads on my 5 lt v8 griff. Anyone else out there ran these leads or have any thoughts on these compared with 'normal' plug leads/ignition.
From the blurb i fell for, supposed to give out a much shorter and consequently more powerful spark??
They seem to work fine, can't tell either way now if better or worse than the normal leads i had before.
I'm running Nology cd plug leads on my 5 lt v8 griff. Anyone else out there ran these leads or have any thoughts on these compared with 'normal' plug leads/ignition.
From the blurb i fell for, supposed to give out a much shorter and consequently more powerful spark??
They seem to work fine, can't tell either way now if better or worse than the normal leads i had before.
All the mixture needs is to be ignited. All this blurb about fancy ignition systems making more power is sales pitch.If there is some problems, thay maybe require a better spark, then perhaps a bit more power can be found, but thats highlighting other problems, rather than an ignition problem
Most factory ignition systems in modern cars are very very good, and can see engines easily make in excess of 200bhp/litre ( blown )
Have you ever seen the plug leads on some Toyotas ?? Ive seen some making over 500bhp, with std leads, that look like they are about 4-5mm. So tiny. Yet they still work perfectly.
Most factory ignition systems in modern cars are very very good, and can see engines easily make in excess of 200bhp/litre ( blown )
Have you ever seen the plug leads on some Toyotas ?? Ive seen some making over 500bhp, with std leads, that look like they are about 4-5mm. So tiny. Yet they still work perfectly.
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