Ignition systems
06-28-2005, 10:15 AM
#2
Nordschleife Master
The question you ask is a very open ended question. I would have to ask what problem are you trying to solve by moving to a distributorless system.
The answer to your question is yes. You could for instance install a stand alone ignition system such as an Electromotive HPX which is a wasted spark distributorless system. However, you:
- Are trading ECU control over the ignition for a 3d MAP on a standalone setup via a laptop
- Are trading (in my opinion) a better inductive ignition system for a mid 80s GM inductive ignition system.
- Will still need to retain the distributor because it contains the cam position sensor required by the ECU to operate the engine.
There are also some other options from companies like AEM which allow a single ignition output (from the ECU) to be sent to a direct ignition controller. However, you'd need to determine if you wanted to go wasted spark which would require 6 coils or 12 coils if you wanted to go sequential direct ignition. Due to the dual plug design, ignition systems can get expensive and make packaging difficult.
I have found that the Bosch ignition system in the 964/993 systems works Ok, however, in a turbocharged application such as the 993tt above 1bar of boost it has difficulty firing a richer mixture required to keep the engine cool at the higher boost pressures. Furthermore, I have seen improvements on the order of 10-20% power by switching from a factory inductive ignition system to a high quality racing CDI system on a high compression or turbocharged engine where a factory style ignition system is functioning, but highly stressed. I don't believe you'd see that much on a stock engine, but you may see 5-8%. A better ignition system will allow the plug gap to be wider creating a larger flame kernl which increases flame propagation speed.
There are plug in CDI systems that will work with the factory computer and replace the factory ignitiors.
On my RSR engine I'm retaining the twin distributor and using a pair racing quality digital CDI boxes with a pair of CDI specific coils (not to be confused with a factory inductive coil). This should allow for a .060-.080" or more plug gap.
The answer to your question is yes. You could for instance install a stand alone ignition system such as an Electromotive HPX which is a wasted spark distributorless system. However, you:
- Are trading ECU control over the ignition for a 3d MAP on a standalone setup via a laptop
- Are trading (in my opinion) a better inductive ignition system for a mid 80s GM inductive ignition system.
- Will still need to retain the distributor because it contains the cam position sensor required by the ECU to operate the engine.
There are also some other options from companies like AEM which allow a single ignition output (from the ECU) to be sent to a direct ignition controller. However, you'd need to determine if you wanted to go wasted spark which would require 6 coils or 12 coils if you wanted to go sequential direct ignition. Due to the dual plug design, ignition systems can get expensive and make packaging difficult.
I have found that the Bosch ignition system in the 964/993 systems works Ok, however, in a turbocharged application such as the 993tt above 1bar of boost it has difficulty firing a richer mixture required to keep the engine cool at the higher boost pressures. Furthermore, I have seen improvements on the order of 10-20% power by switching from a factory inductive ignition system to a high quality racing CDI system on a high compression or turbocharged engine where a factory style ignition system is functioning, but highly stressed. I don't believe you'd see that much on a stock engine, but you may see 5-8%. A better ignition system will allow the plug gap to be wider creating a larger flame kernl which increases flame propagation speed.
There are plug in CDI systems that will work with the factory computer and replace the factory ignitiors.
On my RSR engine I'm retaining the twin distributor and using a pair racing quality digital CDI boxes with a pair of CDI specific coils (not to be confused with a factory inductive coil). This should allow for a .060-.080" or more plug gap.
Last edited by Geoffrey; 06-28-2005 at 11:25 AM.
06-28-2005, 12:49 PM
#3
"Furthermore, I have seen improvements on the order of 10-20% power by switching from a factory inductive ignition system to a high quality racing CDI system on a high compression or turbocharged engine where a factory style ignition system is functioning," -Geoffrey -
The key factors for any ignition system are:
1. spark energy
2. spark voltage
3. spark rise time
CDI spark systems had merit over inductive discharge ignitions previously because
of the spark rise time. With major improvements in semiconductor power devices,
inductive discharge systems can now provide all the necessary factors for a spark
system.
CDI systems do provide some advantages from the standpoint of ignition coil design,
i.e. much smaller coil design. CDI systems have the disadvantage of less reliability because
of the additional component count. Improvements of 10-20% are very questionable.
The key factors for any ignition system are:
1. spark energy
2. spark voltage
3. spark rise time
CDI spark systems had merit over inductive discharge ignitions previously because
of the spark rise time. With major improvements in semiconductor power devices,
inductive discharge systems can now provide all the necessary factors for a spark
system.
CDI systems do provide some advantages from the standpoint of ignition coil design,
i.e. much smaller coil design. CDI systems have the disadvantage of less reliability because
of the additional component count. Improvements of 10-20% are very questionable.
06-28-2005, 02:04 PM
#4
Nordschleife Master
I avoided comparing inductive ignitions to CDI directly because they both have their strengths and weaknesses as you've pointed out. In fact, 1/2 of the F1 cars run inductive and the other 1/2 run CDI so go figure.
To clarify what I said was that a 10-20% improvement has been seen on dyno testing between a FACTORY inductive system when highly stressed or is operating outside its performance envelope and a RACING QUALITY digital CDI system.
This can be seen when you are using the factory Bosch system found in a 964 or 993 engine and the engine is operated at 8000rpm. The ignition system does not have the performance capability or frequency in the stock distributor environment. The Bosch coil has a dwell time of about 3.1ms to be fully charged. At 6500rpm the engine makes 1 crankshaft revolution in 9.2ms and there are 3 ignition events that occur in 1 crankshaft revolution, so there is 3.07ms of time to charge and fire the coil. We know that the factory coil requires 3.1ms of dwell, so at 6500rpm the igniton system is at capacity. Above 6500 the coil is not getting the full charge and therefore the spark energy will fall off as the dwell is reduced. Now, in a factory car where the peak cylinder pressure occurs at 5200rpm there is sufficent spark energy and at redline of 6700rpm, even though there is not full spark energy, it is probably sufficient for the reduced cylinder pressure.
If you take that same ignition system and install it on a racing engine where peak torque and HP are higher up in the rev range, then the factory ignition system is insufficient and performance will fall off. It is in these scenarios where a Racing designed ignition system will allow for performance gains, whether inductive or CDI.
BTW, coil saturation can be measured with a current probe and an oscilloscope to determine the coil's capability.
An ignitor is a simple switching device where a transistor allows current to flow to the coil where it is stored. The inductive ignition performance is highly dependant on the quality an characteristics of the coils as well as the switching speed of the transistor. A CDI ignition is a much more complex system due to the storage of energy in a capacitor rather than in the coil itself.
To clarify what I said was that a 10-20% improvement has been seen on dyno testing between a FACTORY inductive system when highly stressed or is operating outside its performance envelope and a RACING QUALITY digital CDI system.
This can be seen when you are using the factory Bosch system found in a 964 or 993 engine and the engine is operated at 8000rpm. The ignition system does not have the performance capability or frequency in the stock distributor environment. The Bosch coil has a dwell time of about 3.1ms to be fully charged. At 6500rpm the engine makes 1 crankshaft revolution in 9.2ms and there are 3 ignition events that occur in 1 crankshaft revolution, so there is 3.07ms of time to charge and fire the coil. We know that the factory coil requires 3.1ms of dwell, so at 6500rpm the igniton system is at capacity. Above 6500 the coil is not getting the full charge and therefore the spark energy will fall off as the dwell is reduced. Now, in a factory car where the peak cylinder pressure occurs at 5200rpm there is sufficent spark energy and at redline of 6700rpm, even though there is not full spark energy, it is probably sufficient for the reduced cylinder pressure.
If you take that same ignition system and install it on a racing engine where peak torque and HP are higher up in the rev range, then the factory ignition system is insufficient and performance will fall off. It is in these scenarios where a Racing designed ignition system will allow for performance gains, whether inductive or CDI.
BTW, coil saturation can be measured with a current probe and an oscilloscope to determine the coil's capability.
An ignitor is a simple switching device where a transistor allows current to flow to the coil where it is stored. The inductive ignition performance is highly dependant on the quality an characteristics of the coils as well as the switching speed of the transistor. A CDI ignition is a much more complex system due to the storage of energy in a capacitor rather than in the coil itself.
06-28-2005, 04:22 PM
#5
"A CDI ignition is a much more complex system due to the storage of energy in a capacitor rather than in the coil itself." - Geoffery -
The 3ms (or less) max time for the inductive energy storage is also a problem
for a CDI system whether one uses resonance or inductive capacitive charging.
The inductive system is less difficult to further enhance as:
Time= inductance / resistance
Energy = inductance X current squared
So, to reduce dwell time one reduces the coil inductance and increases the current.
It's not as simple with a CDI system to gain RPMs as with an inductive system.
The 3ms (or less) max time for the inductive energy storage is also a problem
for a CDI system whether one uses resonance or inductive capacitive charging.
The inductive system is less difficult to further enhance as:
Time= inductance / resistance
Energy = inductance X current squared
So, to reduce dwell time one reduces the coil inductance and increases the current.
It's not as simple with a CDI system to gain RPMs as with an inductive system.
06-28-2005, 04:34 PM
#6
Nordschleife Master
Yes you can use coils with differece primary resistance or inductance, however, the dwell is controlled by ECU in the case of the 964/993 (dumb ignitors if you will) and is not changable except via aftermarket chip.
I disagree that a CDI system is not a simple to gain RPMs as an inductive system. It takes less than 1ms to fully charge a capactior and the coil is simply used to "step up" the voltage so it takes less time. Furthermore, the better digital CDI systems utilize two capacitors that alternate firing to assist in the higher frequency capability.
I disagree that a CDI system is not a simple to gain RPMs as an inductive system. It takes less than 1ms to fully charge a capactior and the coil is simply used to "step up" the voltage so it takes less time. Furthermore, the better digital CDI systems utilize two capacitors that alternate firing to assist in the higher frequency capability.
06-28-2005, 05:48 PM
#7
"the dwell is controlled by ECU in the case of the 964/993 (dumb ignitors if you will) and is not changable except via aftermarket chip." - Geoffery -
So! If I can develop the proper energy in <= to the dwell time by reducing the
inductance and increasing the current, I've increased the effective RPMs of
the ignition system. The ECM doesn't have to change anything!
"Furthermore, the better digital CDI systems utilize two capacitors that alternate firing to assist in the higher frequency capability." - Geoffery -
As I said, it's not as easy, i.e. because of complexity, to modify a CDI system for increased RPMs,
as you've confirmed.
Key Relationship:
V = L dI/ dT where;
V is the battery voltage
L is primary inductance of the coil
dI is the change in current
dT is the dwell time
then
dI = V X dT / L
So, to increase the current with a constant dwell time, increase
the voltage or decrease the inductance.
So! If I can develop the proper energy in <= to the dwell time by reducing the
inductance and increasing the current, I've increased the effective RPMs of
the ignition system. The ECM doesn't have to change anything!
"Furthermore, the better digital CDI systems utilize two capacitors that alternate firing to assist in the higher frequency capability." - Geoffery -
As I said, it's not as easy, i.e. because of complexity, to modify a CDI system for increased RPMs,
as you've confirmed.
Key Relationship:
V = L dI/ dT where;
V is the battery voltage
L is primary inductance of the coil
dI is the change in current
dT is the dwell time
then
dI = V X dT / L
So, to increase the current with a constant dwell time, increase
the voltage or decrease the inductance.
Last edited by Lorenfb; 06-28-2005 at 06:06 PM.
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06-28-2005, 06:21 PM
#8
Nordschleife Master
Loren,
Yes, that is true except that the ECU will deliver 3ms of dwell regardless of whether or not your replacement coil needs 3ms and you'll be overcharging the coil which will burn it out. You also need to be careful not to run the coil at 100% duty cycle because that will burn it out as well. If you could install a coil that requires less dwell and reprogram the ECU for less dwell, then that would be an improvement and allow a higher RPM.
This is also why a high quality racing CDI unit costs $400 for a single channel unit compared to $150 for a MSD 6A.
Yes, that is true except that the ECU will deliver 3ms of dwell regardless of whether or not your replacement coil needs 3ms and you'll be overcharging the coil which will burn it out. You also need to be careful not to run the coil at 100% duty cycle because that will burn it out as well. If you could install a coil that requires less dwell and reprogram the ECU for less dwell, then that would be an improvement and allow a higher RPM.
This is also why a high quality racing CDI unit costs $400 for a single channel unit compared to $150 for a MSD 6A.
06-28-2005, 10:47 PM
#9
Nordschleife Master
Loren,
Since you introduced Math, lets talk about the physics involved in the two types of ingition systems.
CDI Ignition operating principles:
In a CDI igntion, the 12v battery power is stepped up by an inverting powersupply to about 450v and stored in the Capacitor. This occurs in much less than 1ms of time and about 105mJ of energy is available. The formula for CDI ignition is W (energy in Joules) = 1/2C (capacitance in Fareds) * Vsquared (voltage). Since the energy is not stored in the coil, the coil only needs to have low resistance to step up through the secondary windings. These coils are designed to handle the high energy level (105mJ or so)
Inductive Ignition operating principles:
Unlike the CDI ignition, the energy is stored in the primary windings of the coil. The formula for Inductive ignition is W (energy in Joules) = 1/2L (primary inductance (Henries) * Isquared (Amps). Most inductive coils have a maximum spark energy of 63mJ and require 3-3.5ms of charge time (Dwell). You can increase the energy by increasing the primary current or primary inductance, however, the time required to charge the coil (Dwell) will increase.
So, can you use inductive coils in a CDI or vice versa?
All formulas point to decreased energy capabily in Inductive ignition or increased energy choking and absorption in CDI, so perhaps not. In addition, most inductive coils can handle only about 65mJ of of continuous energy before running the risk of overheating if the 105mJ of a CDI ignition is applied.
The generally accepted maximum required amount of energy required in most applications is about 100mJ. An interesting point is that coil on plug applications may need as little as 50mJ.
Since you introduced Math, lets talk about the physics involved in the two types of ingition systems.
CDI Ignition operating principles:
In a CDI igntion, the 12v battery power is stepped up by an inverting powersupply to about 450v and stored in the Capacitor. This occurs in much less than 1ms of time and about 105mJ of energy is available. The formula for CDI ignition is W (energy in Joules) = 1/2C (capacitance in Fareds) * Vsquared (voltage). Since the energy is not stored in the coil, the coil only needs to have low resistance to step up through the secondary windings. These coils are designed to handle the high energy level (105mJ or so)
Inductive Ignition operating principles:
Unlike the CDI ignition, the energy is stored in the primary windings of the coil. The formula for Inductive ignition is W (energy in Joules) = 1/2L (primary inductance (Henries) * Isquared (Amps). Most inductive coils have a maximum spark energy of 63mJ and require 3-3.5ms of charge time (Dwell). You can increase the energy by increasing the primary current or primary inductance, however, the time required to charge the coil (Dwell) will increase.
So, can you use inductive coils in a CDI or vice versa?
All formulas point to decreased energy capabily in Inductive ignition or increased energy choking and absorption in CDI, so perhaps not. In addition, most inductive coils can handle only about 65mJ of of continuous energy before running the risk of overheating if the 105mJ of a CDI ignition is applied.
The generally accepted maximum required amount of energy required in most applications is about 100mJ. An interesting point is that coil on plug applications may need as little as 50mJ.
06-29-2005, 01:36 AM
#11
"Yes, that is true except that the ECU will deliver 3ms of dwell regardless of whether or not your replacement coil needs 3ms and you'll be overcharging the coil which will burn it out. " - Geoffrey -
Not true!
Many inductive system. e.g. Motronic systems, have a current limiting capability.
The major real problem is the coil core saturation and the resistive heat (to a lesser extent).
"Most inductive coils have a maximum spark energy of 63mJ and require 3-3.5ms of charge time (Dwell). You can increase the energy by increasing the primary current or primary inductance, however, the time required to charge the coil (Dwell) will increase." - Geoffrey
Not totally true!
1. The minimum energy required is generally accepted to be greater than 50mj.
Energy greater than that is wasted & contributes little to nothing to further ignite the
charge.
2. Since V = L dI / dT, then dT = L X dI / V, for V = 12, L = 2 mH, I = 10amps and;
dT (dwell time) = 1.6 msec, Energy = 100 mjoules
For a 6 cylinder, a spark occurs every 3.3 ms at 6000 RPMs.
For a 6 cylinder, a spark occurs every 2.2 ms at 9000 RPMs.
Thus a dwell time of 1.6 ms is adequate for an inductive system.
As mentioned on this thread, F1 engines which reach 15000+ RPMs use
inductive systems. Obviously, the above example would be inadequate
for a F1 engine.
"Since the energy is not stored in the coil, the coil only needs to have low resistance to step up through the secondary windings. These coils are designed to handle the high energy level (105mJ or so) " - Geoffrey -
No, the real reason (not resistance) is that CDI systems DON"T require
large inductances, as they DON"T need to store the energy in the magnetic field
as do inductive systems. CDI coils ONLY transfer energy & as such can be small.
Little/no energy loss occurs when using an inductive type coil in a CDI system.
"So, can you use inductive coils in a CDI or vice versa?
All formulas point to decreased energy capabily in Inductive ignition or increased energy choking and absorption in CDI, so perhaps not. "
- Geoffrey -
A very common confusion!
Most/all inductive coils can be used in a CDI system. The key here is the turns ratio
limitation (determines the secondary voltage). When used in a CDI system ,
the coil only transfers energy and as such has no first order problem with energies > 100mj.
The other case of using a CDI coil, e.g. 911SC type, in an inductive system, is not possible
because of the very low inductance, i.e. inadeqate energy storage.
"In addition, most inductive coils can handle only about 65mJ of of continuous energy before running the risk of overheating if the 105mJ of a CDI ignition is applied." - Geoffrey -
Not true!
The heat (energy loss) in a coil is a function of the I squared R losses times duty cycle (RMS power).
Since the energy transfer in a CDI system occurs in microseconds, the RMS power (heat)
is very low compared to an inductive system. Thus, overheating is NOT a problem for most/any coils
in a CDI system.
Check out Ignition Systems on the Technical page at www.systemsc.com/technical.htm for additional info.
Not true!
Many inductive system. e.g. Motronic systems, have a current limiting capability.
The major real problem is the coil core saturation and the resistive heat (to a lesser extent).
"Most inductive coils have a maximum spark energy of 63mJ and require 3-3.5ms of charge time (Dwell). You can increase the energy by increasing the primary current or primary inductance, however, the time required to charge the coil (Dwell) will increase." - Geoffrey
Not totally true!
1. The minimum energy required is generally accepted to be greater than 50mj.
Energy greater than that is wasted & contributes little to nothing to further ignite the
charge.
2. Since V = L dI / dT, then dT = L X dI / V, for V = 12, L = 2 mH, I = 10amps and;
dT (dwell time) = 1.6 msec, Energy = 100 mjoules
For a 6 cylinder, a spark occurs every 3.3 ms at 6000 RPMs.
For a 6 cylinder, a spark occurs every 2.2 ms at 9000 RPMs.
Thus a dwell time of 1.6 ms is adequate for an inductive system.
As mentioned on this thread, F1 engines which reach 15000+ RPMs use
inductive systems. Obviously, the above example would be inadequate
for a F1 engine.
"Since the energy is not stored in the coil, the coil only needs to have low resistance to step up through the secondary windings. These coils are designed to handle the high energy level (105mJ or so) " - Geoffrey -
No, the real reason (not resistance) is that CDI systems DON"T require
large inductances, as they DON"T need to store the energy in the magnetic field
as do inductive systems. CDI coils ONLY transfer energy & as such can be small.
Little/no energy loss occurs when using an inductive type coil in a CDI system.
"So, can you use inductive coils in a CDI or vice versa?
All formulas point to decreased energy capabily in Inductive ignition or increased energy choking and absorption in CDI, so perhaps not. "
- Geoffrey -
A very common confusion!
Most/all inductive coils can be used in a CDI system. The key here is the turns ratio
limitation (determines the secondary voltage). When used in a CDI system ,
the coil only transfers energy and as such has no first order problem with energies > 100mj.
The other case of using a CDI coil, e.g. 911SC type, in an inductive system, is not possible
because of the very low inductance, i.e. inadeqate energy storage.
"In addition, most inductive coils can handle only about 65mJ of of continuous energy before running the risk of overheating if the 105mJ of a CDI ignition is applied." - Geoffrey -
Not true!
The heat (energy loss) in a coil is a function of the I squared R losses times duty cycle (RMS power).
Since the energy transfer in a CDI system occurs in microseconds, the RMS power (heat)
is very low compared to an inductive system. Thus, overheating is NOT a problem for most/any coils
in a CDI system.
Check out Ignition Systems on the Technical page at www.systemsc.com/technical.htm for additional info.
Last edited by Lorenfb; 06-29-2005 at 02:39 AM.
06-29-2005, 05:46 AM
#13
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Originally Posted by RJohn
Can anyone tell me if there is a "distributorless" electronic ignition system conversion/update for the 964 n/a engines?
I have run the 964 system at up to 390bhp n/a and am running the 993 system in my RSR at 415bhp n/a with no problems whatsoever, as is headbanging Christer (hi mate) who runs a stock system on his 964 at almost 360bhp.
06-29-2005, 09:55 AM
#15
Burning Brakes
Originally Posted by Lorenfb
... A very common confusion! ...
