Tech: The 944 / 951 Ignition System.
02-10-2013, 05:23 AM
#1
Addict
Rennlist Member
Rennlist
Small Business Partner
Rennlist Member
Rennlist
Small Business Partner
Thread Starter
Tech: The 944 / 951 Ignition System.
The automotive ignition system is commonly one of the least understood parts of the powertrain. Unlike mechanical operation, it can be difficult to visualize exactly what is required to successfully fire the spark-plugs.
To make things more difficult, doing any quantitative testing of the ignition system requires specialized tools that very few people posses.
That said, I have been doing a lot of ignition system, and ignition coil testing lately, and (as always) I would like to share what has been learned.
To get started, let's go over the ignition system.
The ignition coil:
This is the Bosch "black coil", which is the OEM unit for both the 944 and 951. The purpose of the ignition coil is two-fold. Firstly, the coil has a primary and secondary windings. By having many more turns on the secondary winding than the primary, the coil acts as a "step-up" transformer. This steps-up the voltage to many thousands of volts - enough to fire the spark-plug. The secondary purpose of the coil is to store the energy needed to actually fire the spark-plug, and to keep the spark existing for a short time (typically less than 1/1000th of a second).
In the 944 & 951 (and nearly any electronically-controlled ignition), the coil is supplied with a constant +12 volts whenever the ignition is turned on. The DME controls the "ground-path", by a large transistor (the silver colored can-style transistor in the DME). To operate the ignition coil, it first must be charged, which the DME does by turning on the transistor, providing a ground for the coil - completing the circuit. Then, once the coil is charged, the DME turns-off the transistor, which opens the circuit causing the coil to fire. (yes there are more details, but they are not necessary for this thread - feel free to search/learn more about coils/inductors)
This digital (the transistor) control gives the DME very precise control of when the actual ignition event (spark) happens.
When the DME turns on the transistor controlling the ignition coil, current (amps) starts moving through the coil. Due to the property of coils, they resist changes in current flow (amps). So, initially when the transistor is first turned on, there is very little current flow, but as time increases, the current flow rises. Generally as long as the coil is not saturated, this current flow rise is a linear relationship with time (and voltage). Measuring the current rise is not an easy thing to do (especially without altering the charging circuit). However, it can be done, and here is a graph of the stock ignition coil being charged:
The cyan/blue trace (CH2) is the digital logic which turns on the transistor for the ignition coil. The important data is how long the logic pulse is "low", which is the length of time that the DME is charging the ignition coil for. Here each horizontal division (block) is 1mS (milli-seconds), the DME is charging the coil for 9.78mS. The yellow trace (CH1) is the coil current. Each vertical division is roughly 1.5amps worth of current.
So, as you can see, when the DME first turns on the transistor to charge the coil, there is very little current flow. But, as time increases, the current through the ignition coil starts to rise. After a few milli-seconds, the current reaches a peak and will not rise anymore - about 9 amps in this case. This is actually due to the internal current-limiting circuitry inside the DME. (more on this in a bit)
As one might imagine, all things remaining equal, more current means more ignition energy. So, it is ideal that the coil receives as much current as possible, to result in as powerful of a spark as possible. However, lots of current through a transistor, wires and coil can heat-up and generally be hard on the components. Look again at the graph, peak current is reached well in advance of the total charging-time. In-fact, peak current is reached nearly 3mS before the end of the charge-cycle. During these final 3mS, the coil is not gaining any energy, rather it is needlessly conducting current, which can heat-up the coil, wires, and DME transistor... So, what should we do? This:
Now the coil is still receiving the same peak current flow (9amps), but it spends no time holding at this peak current. If you inspect the blue trace data we see that the charge time is now 6.46mS (down 3.1mS from the previous graph). This is an ideal charging setup - we hit the maximum energy possible, and minimize the impact to the coil, wires, and DME transistor.
Now that we understand basic coil charging, and understand that it takes roughly 6.5mS to fully charge the factory ignition coil, the next thing to think about is how much time is actually available to charge the ignition coil. In the 944, one ignition coil must fire all 4-cylinders, which means there needs to be two ignition events per revolution. Therefore, as RPMs go up, the available time to actually charge the coils becomes shorter.
Here is a graph of the possible coil charge time, with actual ignition event time account for:
What should be peculiar is the fact that the possible charge time after roughly 4000rpm is less than the time needed in the earlier graph to reach peak current. Well, because there is less time available to charge the coil, the DME must reduce the amount of time it charges the coil for. By 6500rpm there is only ~3.6mS of time available to charge the coil:
A quick count of the divisions shows that at this shorter charge time, the coil is only reaching ~5.25 amps of current, which is nearly 4 amps less than the possible peak! Furthermore, coils store energy according to the calculation:
1/2 * Inductance * Current^2
Yes, that is current-squared. So, the amount of current is very important for the final coil energy value. Given our current data, we can see that at 6500rpm, our coil is down to ~34% of its peak possible value! Is it any wonder why we must run tight spark-plug gaps in order to prevent mis-fires.
So, what is the solution? Lets try a different ignition coil. The next coil I tested is the MSD Blaster coil:
This coil has nearly identical physical dimensions to the factory Bosch coil, which makes it an easy swap. Testing the blaster coil, we see this graph:
For this coil, we see that it takes 4.8mS to reach a peak current of ~9amps. This is a significantly shorter amount of time than the stock Bosch coils time of 6.5mS. So by using this coil it has time to fully charge until ~5200rpm - which is a significant improvement over the Bosch coils 4000RPM. Even though this coil charges faster than the Bosch unit, by 6500rpm it is also down on potential energy:
Since this coil charges faster, even at the short charge-time of 6500rpm, it has more current flow than the Bosch coil. We see it has ~7.5amps of current, which means this coil is only down to ~70% of its peak possible energy - a HUGE improvement over the factory Bosch coil (which was down to ~34% energy at the same RPM).
There must be a catch, right? Well, yes there is. Because the MSD coil charges faster, that means it needs less charge time to hit peak current. Remember the first graph, where the coil hit peak charge, and then maintained it for a significant amount of time? Well, that is exactly what will happen if we install the MSD coil without changing the charge time (known as dwell). The DME transistor, wires, and coil are fairly robust. So one could install the MSD coil without any other changes, and benefit from improved ignition energy, but this is needlessly hard on these components. What needs to be done is to change the charge-time data in the DME software (easy for us to do, and is user-available in the DME Tuner software).
Even though the MSD Blaster coil is a huge improvement, I was not satisfied with the loss of ignition energy as RPMs increase. So, after a bit of research, I picked-up another MSD coil to test. This is the most powerful, inductive coil that MDS offers; the HVC-II:
This coil is obviously not the same physical dimensions of the factory coil. However, it utilizes newer coil design, and is not difficult to retrofit for use in the 944 / 951. Testing this coil:
Immediately, we see that this coil charges extremely fast! Even with a little bit of hold-time at the end of the charge cycle, this coil hits peak current in a short 3.3mS! This is an improvement of 1.5mS over the MSD Blaster coil, and nearly half the time of the factory Bosch unit! Furthermore, this coil will not run short on charge time until 7000RPM. This means that there is no ignition energy loss for the entire rev-range of the 944 / 951!
This is an awesome coil!
But, like the problem mentioned earlier with the MSD Blaster coil, this coil charges very fast, and if the ignition dwell map is not adjusted to account for the fast charging, the DME transistor, wires, and coil will suffer. Further, this coil is much more likely to damage the DME transistor or wires than the MSD Blaster coil - the dwell map MUST be changed to account for this coil.
Let me say this again:
DO NOT RUN THIS COIL WITHOUT CHANGING THE IGNITION DWELL MAP TO ACCOUNT FOR THE FAST CHARGE TIME.
Seriously - don't do it, you will kill the DME transistor (just a matter of time).
Now let us finish-up by revisiting the energy equation earlier:
1/2 * Inductance * Current^2
We now know current, but what about the inductance? Like current, measuring coil inductance requires a special tool - lucky that I have such a tool.
Inductance is, essentially, the energy storage potential of the coil. More inductance does mean more potential energy. And though it would be tempting to make a coil with as much inductance as possible, inductance also adds to the overall coil impedance; higher impedance means slower charging. So, it is a bit of a juggling act of coil inductance vs charging rate.
That said, the stock Bosch coil has an inductance of 5.85 mH (milli-Henries). Now we can complete the calculation for peak energy (at peak current):
1/2 * 0.00585 * 9^2 = 237mJ (milli Joules)
This is actually quite a bit of energy! But remember that by 6500RPM, the Bosch coil's current has been significantly reduced:
1/2 * 0.00585 * 5.25^2 = 81mJ
Wow! That is a huge reduction in coil energy from the peak potential...
The MSD Blaster coil has an inductance of 4.36 mH, which is less than the Bosch unit, but since the MSD Blaster charges quite a bit faster, will there be an overall improvement?
First the peak energy (at peak current):
1/2 * 0.00436 * 9^2 = 177mJ
Interesting that the MSD coil actually has less energy at peak current, but lets see it as RPMs increase; here at 6500rpm:
1/2 * 0.00436 * 7.5^2 = 123mJ
Even though this coil doesn't have quite as much peak current energy as stock, it maintains a much higher level of energy as RPMs increase. By 6500rpm, the MSD Blaster coil is 50% more powerful than the stock Bosch unit.
Finally, the MSD HVC-II coil. This coil has an inductance of 6.23, which for how fast it charges, is a lot of potential! So, the coil at peak energy (peak current):
1/2 * 0.00623 * 9^2 = 252mJ*
WOW!!! This is by far the most powerful coil tested. AND remember that this coil does not lose any energy for the entire rev-range. So by comparison, this coil at 6500rpm has twice as much energy as the MSD Blaster coil, and three times as much energy as the factory coil !!!
*The coil was starting to saturate, so it will most likely have a little less energy than the linear calculation predicts*
To make things more difficult, doing any quantitative testing of the ignition system requires specialized tools that very few people posses.
That said, I have been doing a lot of ignition system, and ignition coil testing lately, and (as always) I would like to share what has been learned.
To get started, let's go over the ignition system.
The ignition coil:
This is the Bosch "black coil", which is the OEM unit for both the 944 and 951. The purpose of the ignition coil is two-fold. Firstly, the coil has a primary and secondary windings. By having many more turns on the secondary winding than the primary, the coil acts as a "step-up" transformer. This steps-up the voltage to many thousands of volts - enough to fire the spark-plug. The secondary purpose of the coil is to store the energy needed to actually fire the spark-plug, and to keep the spark existing for a short time (typically less than 1/1000th of a second).
In the 944 & 951 (and nearly any electronically-controlled ignition), the coil is supplied with a constant +12 volts whenever the ignition is turned on. The DME controls the "ground-path", by a large transistor (the silver colored can-style transistor in the DME). To operate the ignition coil, it first must be charged, which the DME does by turning on the transistor, providing a ground for the coil - completing the circuit. Then, once the coil is charged, the DME turns-off the transistor, which opens the circuit causing the coil to fire. (yes there are more details, but they are not necessary for this thread - feel free to search/learn more about coils/inductors)
This digital (the transistor) control gives the DME very precise control of when the actual ignition event (spark) happens.
When the DME turns on the transistor controlling the ignition coil, current (amps) starts moving through the coil. Due to the property of coils, they resist changes in current flow (amps). So, initially when the transistor is first turned on, there is very little current flow, but as time increases, the current flow rises. Generally as long as the coil is not saturated, this current flow rise is a linear relationship with time (and voltage). Measuring the current rise is not an easy thing to do (especially without altering the charging circuit). However, it can be done, and here is a graph of the stock ignition coil being charged:
The cyan/blue trace (CH2) is the digital logic which turns on the transistor for the ignition coil. The important data is how long the logic pulse is "low", which is the length of time that the DME is charging the ignition coil for. Here each horizontal division (block) is 1mS (milli-seconds), the DME is charging the coil for 9.78mS. The yellow trace (CH1) is the coil current. Each vertical division is roughly 1.5amps worth of current.
So, as you can see, when the DME first turns on the transistor to charge the coil, there is very little current flow. But, as time increases, the current through the ignition coil starts to rise. After a few milli-seconds, the current reaches a peak and will not rise anymore - about 9 amps in this case. This is actually due to the internal current-limiting circuitry inside the DME. (more on this in a bit)
As one might imagine, all things remaining equal, more current means more ignition energy. So, it is ideal that the coil receives as much current as possible, to result in as powerful of a spark as possible. However, lots of current through a transistor, wires and coil can heat-up and generally be hard on the components. Look again at the graph, peak current is reached well in advance of the total charging-time. In-fact, peak current is reached nearly 3mS before the end of the charge-cycle. During these final 3mS, the coil is not gaining any energy, rather it is needlessly conducting current, which can heat-up the coil, wires, and DME transistor... So, what should we do? This:
Now the coil is still receiving the same peak current flow (9amps), but it spends no time holding at this peak current. If you inspect the blue trace data we see that the charge time is now 6.46mS (down 3.1mS from the previous graph). This is an ideal charging setup - we hit the maximum energy possible, and minimize the impact to the coil, wires, and DME transistor.
Now that we understand basic coil charging, and understand that it takes roughly 6.5mS to fully charge the factory ignition coil, the next thing to think about is how much time is actually available to charge the ignition coil. In the 944, one ignition coil must fire all 4-cylinders, which means there needs to be two ignition events per revolution. Therefore, as RPMs go up, the available time to actually charge the coils becomes shorter.
Here is a graph of the possible coil charge time, with actual ignition event time account for:
What should be peculiar is the fact that the possible charge time after roughly 4000rpm is less than the time needed in the earlier graph to reach peak current. Well, because there is less time available to charge the coil, the DME must reduce the amount of time it charges the coil for. By 6500rpm there is only ~3.6mS of time available to charge the coil:
A quick count of the divisions shows that at this shorter charge time, the coil is only reaching ~5.25 amps of current, which is nearly 4 amps less than the possible peak! Furthermore, coils store energy according to the calculation:
1/2 * Inductance * Current^2
Yes, that is current-squared. So, the amount of current is very important for the final coil energy value. Given our current data, we can see that at 6500rpm, our coil is down to ~34% of its peak possible value! Is it any wonder why we must run tight spark-plug gaps in order to prevent mis-fires.
So, what is the solution? Lets try a different ignition coil. The next coil I tested is the MSD Blaster coil:
This coil has nearly identical physical dimensions to the factory Bosch coil, which makes it an easy swap. Testing the blaster coil, we see this graph:
For this coil, we see that it takes 4.8mS to reach a peak current of ~9amps. This is a significantly shorter amount of time than the stock Bosch coils time of 6.5mS. So by using this coil it has time to fully charge until ~5200rpm - which is a significant improvement over the Bosch coils 4000RPM. Even though this coil charges faster than the Bosch unit, by 6500rpm it is also down on potential energy:
Since this coil charges faster, even at the short charge-time of 6500rpm, it has more current flow than the Bosch coil. We see it has ~7.5amps of current, which means this coil is only down to ~70% of its peak possible energy - a HUGE improvement over the factory Bosch coil (which was down to ~34% energy at the same RPM).
There must be a catch, right? Well, yes there is. Because the MSD coil charges faster, that means it needs less charge time to hit peak current. Remember the first graph, where the coil hit peak charge, and then maintained it for a significant amount of time? Well, that is exactly what will happen if we install the MSD coil without changing the charge time (known as dwell). The DME transistor, wires, and coil are fairly robust. So one could install the MSD coil without any other changes, and benefit from improved ignition energy, but this is needlessly hard on these components. What needs to be done is to change the charge-time data in the DME software (easy for us to do, and is user-available in the DME Tuner software).
Even though the MSD Blaster coil is a huge improvement, I was not satisfied with the loss of ignition energy as RPMs increase. So, after a bit of research, I picked-up another MSD coil to test. This is the most powerful, inductive coil that MDS offers; the HVC-II:
This coil is obviously not the same physical dimensions of the factory coil. However, it utilizes newer coil design, and is not difficult to retrofit for use in the 944 / 951. Testing this coil:
Immediately, we see that this coil charges extremely fast! Even with a little bit of hold-time at the end of the charge cycle, this coil hits peak current in a short 3.3mS! This is an improvement of 1.5mS over the MSD Blaster coil, and nearly half the time of the factory Bosch unit! Furthermore, this coil will not run short on charge time until 7000RPM. This means that there is no ignition energy loss for the entire rev-range of the 944 / 951!
This is an awesome coil!
But, like the problem mentioned earlier with the MSD Blaster coil, this coil charges very fast, and if the ignition dwell map is not adjusted to account for the fast charging, the DME transistor, wires, and coil will suffer. Further, this coil is much more likely to damage the DME transistor or wires than the MSD Blaster coil - the dwell map MUST be changed to account for this coil.
Let me say this again:
DO NOT RUN THIS COIL WITHOUT CHANGING THE IGNITION DWELL MAP TO ACCOUNT FOR THE FAST CHARGE TIME.
Seriously - don't do it, you will kill the DME transistor (just a matter of time).
Now let us finish-up by revisiting the energy equation earlier:
1/2 * Inductance * Current^2
We now know current, but what about the inductance? Like current, measuring coil inductance requires a special tool - lucky that I have such a tool.
Inductance is, essentially, the energy storage potential of the coil. More inductance does mean more potential energy. And though it would be tempting to make a coil with as much inductance as possible, inductance also adds to the overall coil impedance; higher impedance means slower charging. So, it is a bit of a juggling act of coil inductance vs charging rate.
That said, the stock Bosch coil has an inductance of 5.85 mH (milli-Henries). Now we can complete the calculation for peak energy (at peak current):
1/2 * 0.00585 * 9^2 = 237mJ (milli Joules)
This is actually quite a bit of energy! But remember that by 6500RPM, the Bosch coil's current has been significantly reduced:
1/2 * 0.00585 * 5.25^2 = 81mJ
Wow! That is a huge reduction in coil energy from the peak potential...
The MSD Blaster coil has an inductance of 4.36 mH, which is less than the Bosch unit, but since the MSD Blaster charges quite a bit faster, will there be an overall improvement?
First the peak energy (at peak current):
1/2 * 0.00436 * 9^2 = 177mJ
Interesting that the MSD coil actually has less energy at peak current, but lets see it as RPMs increase; here at 6500rpm:
1/2 * 0.00436 * 7.5^2 = 123mJ
Even though this coil doesn't have quite as much peak current energy as stock, it maintains a much higher level of energy as RPMs increase. By 6500rpm, the MSD Blaster coil is 50% more powerful than the stock Bosch unit.
Finally, the MSD HVC-II coil. This coil has an inductance of 6.23, which for how fast it charges, is a lot of potential! So, the coil at peak energy (peak current):
1/2 * 0.00623 * 9^2 = 252mJ*
WOW!!! This is by far the most powerful coil tested. AND remember that this coil does not lose any energy for the entire rev-range. So by comparison, this coil at 6500rpm has twice as much energy as the MSD Blaster coil, and three times as much energy as the factory coil !!!
*The coil was starting to saturate, so it will most likely have a little less energy than the linear calculation predicts*
The following users liked this post:
Kilby256 (03-04-2024)
02-10-2013, 05:24 AM
#2
Addict
Rennlist Member
Rennlist
Small Business Partner
Rennlist Member
Rennlist
Small Business Partner
Thread Starter
Just a quick note - all coil testing was done at 13.8volts.
Additionally, soon I will be testing some of the common wasted-spark coil-packs, and coil-near-plug options (such as the LS2/Truck coil).
Additionally, soon I will be testing some of the common wasted-spark coil-packs, and coil-near-plug options (such as the LS2/Truck coil).
02-10-2013, 06:24 AM
#3
Three Wheelin'
Joshua
Thanks or the write up, very interesting. So does the higher coil energy result in any measurable increase in spark duration and combustion efficiency? I always wondered whether these MSD kits really made a difference. It's not like a twin spark head in the 911 derivatives with the dual distributors, which do make some difference in power output.
Thanks or the write up, very interesting. So does the higher coil energy result in any measurable increase in spark duration and combustion efficiency? I always wondered whether these MSD kits really made a difference. It's not like a twin spark head in the 911 derivatives with the dual distributors, which do make some difference in power output.
02-10-2013, 06:45 AM
#4
Drifting
Join Date: Aug 2009
Location: Bangkok, Thailand, Milpitas, CA & Weeki Wachee, FL
Posts: 2,239
Likes: 0
Received 2 Likes
on
1 Post
Does this mean the Rogue Tuning penultimate ignition kit is going to be available soon? You know I have been waiting a long time for this! 
02-10-2013, 09:23 AM
#7
Rennlist Member
Great information! Thanks to Joshua I have my ignition setup covered for the Ultra Stroker Build. The HVC-II Coil is going to allow me to run the timing I want to keep the motor very crisp. 
He once again gives an outstanding solution that is very reasonable from a cost standpoint.
I must say that Joshua's knowledge goes FAR BEYOND just tuning. He has some great insight into many areas concerning our cars. Not only has he made significant contributions to the platform, he has a genuine passion for the 951.
He once again gives an outstanding solution that is very reasonable from a cost standpoint.I must say that Joshua's knowledge goes FAR BEYOND just tuning. He has some great insight into many areas concerning our cars. Not only has he made significant contributions to the platform, he has a genuine passion for the 951.
Last edited by refresh951; 02-10-2013 at 06:45 PM.
Trending Topics
02-10-2013, 11:16 AM
#8
Basic Sponsor
Rennlist
Site Sponsor
Rennlist
Site Sponsor
question how do I change the ignition dwell map to account for the HVC II col?
02-10-2013, 11:21 AM
#9
Drifting
Join Date: Aug 2009
Location: Bangkok, Thailand, Milpitas, CA & Weeki Wachee, FL
Posts: 2,239
Likes: 0
Received 2 Likes
on
1 Post
Josh:
Since I have not seen my car in so long, I am curious about the 23+ year old wire that supplies the coil with it's power. Would it be valuable/advisable to upgrade this to a better grade of OFC and possibly a larger AWG? Of course, it would be better still to upgrade both the supply and the ground if this is deemed advantageous. While we are discussing upgrades, I would be very interested in your opinion on the plug wires. I am sure there is a limit to their validity, or better stated at what point are we wasting our money on fancy wires. I see some rediculously large wires being installed on JDM cars that can't possible need a conductor that large. On the other hand, I am sure the 23+ year old OEM design can be improved upon.
Since I have not seen my car in so long, I am curious about the 23+ year old wire that supplies the coil with it's power. Would it be valuable/advisable to upgrade this to a better grade of OFC and possibly a larger AWG? Of course, it would be better still to upgrade both the supply and the ground if this is deemed advantageous. While we are discussing upgrades, I would be very interested in your opinion on the plug wires. I am sure there is a limit to their validity, or better stated at what point are we wasting our money on fancy wires. I see some rediculously large wires being installed on JDM cars that can't possible need a conductor that large. On the other hand, I am sure the 23+ year old OEM design can be improved upon.
02-10-2013, 11:27 AM
#11
Basic Sponsor
Rennlist
Site Sponsor
Rennlist
Site Sponsor
02-10-2013, 12:29 PM
#15
Rennlist Member
I know the dwell map can be changed with the DME Tuner Package Rogue has. Also, Summit sells the coil at a great price:
http://www.summitracing.com/parts/msd-8253
Or Jegs even better:
http://www.jegs.com/i/MSD/121/8253/10002/-1
Last edited by refresh951; 02-10-2013 at 10:53 PM.