Posted by Austin on Feb 28th 2021
The following is an analysis between the secondary spark energy & thermals produced by various ignition coils. Secondary spark energy is the measurement of energy released from the ignition coil during the discharge event and represents energy delivered across the spark plug gap. My test setup was configured in compliance with the SAE 973-202011 standards with a modification to the measurement of secondary spark current. The modification being the use of a shunt resistor instead of a current clamp for increased accuracy of secondary spark current measurements. It is important to note that comparisons cannot be directly made between my tests and tests performed by others as conditions and component tolerances will vary. Much like a dyno, accurate comparisons can only be achieved when testing is performed with the same equipment and procedures (although I do welcome others to repeat my tests).
All tests were performed at a power supply regulated voltage of 14V with a lead acid battery inline for buffering. Dwell time was controlled by a waveform generator (simulated 8000 RPM) that is controlling a ISL9V5036S3ST IGBT as the coils tested are all “dumb” coils without igniters. The ISL9V5036S3ST is the same IGBT that is used in MSD80 DME’s and was picked to compare thermal data between the coils tested and the stock N54/N55 ignition coils. The thermal study was conducted with the IGBT in free air at room temperature (67F) with no thermal compound or heatsinking. This was done to better see the impact of certain coils and dwell times on the IGBT without having to sample data for an hour. Thermal tests concluded once the IGBT reached 160F or 260 seconds of testing. IGBT temperature was sampled every 20 seconds with a FLIR thermal imaging camera while ignition coil temperature was sampled every 20 seconds with two thermocouples.
Before getting into the data I want to mention I've found conflicting information regarding the PR coil. V8bait did a nice Primary Energy comparison between different ignition coils where it was stated the PR coil saturated at 3.7 ms and that they are rated for 118 mJ with a 3 ms dwell. I've tested these numbers and have concluded that there must have been an accident when they were published. At 3 ms dwell I get around 93 mJ from the PR coils, however at 3.7 ms I get 117 mJ. In addition I've verified the delivered dwell with an oscilloscope on my car with the PR Coil option selected. Recorded dwell times were between 4.5 ms and 4.8 ms depending on RPM which is much higher than the stated 3.7 ms. For that reason I have chosen to include PR dwell times at 3.0 ms, 3.7 ms and 4.8 ms.
There's a lot of data here to go through so we'll start off with a TLDR version before diving into it.
TLDR Version (Updated with addition of Delphi S55 & Eldor B58 01/28/2021) (Updated with addition of Audi R8, Eldor S55 & Bosch S54 02/28/2021):
It’s no wonder there’s many high power B58’s running stock ignition coils without issue. The stock B58 coils are potent and produce significantly more energy than the OE N54/N55 coils (twice as much to be exact) and can match PR’s coils in secondary spark energy with a longer spark duration. Another consideration is the impact on IGBT temperature. The stock N54/N55 coils hit the 260 second test duration with a maximum temperature of 114-118 F. The long dwell time used by the PR coils increases the thermal load put on the IGBT with temperatures reaching 161.1 F in only 40 seconds with a 4.8 ms dwell and 164.3 F in 100 seconds with a 3.7 ms dwell. The B58 coils on the other hand provide a happy medium with IGBT temperatures reaching 160 F in 220 seconds.
I like data and colorful graphs version :
Inductive Secondary Spark Energy data as captured on an oscilloscope. Light blue/teal is dwell signal from the waveform generator, pink is secondary current measured at the output of the ignition coil after discharging through a zener diode load. Dark blue is primary coil current measured with an inductive current clamp. Inductive Secondary Spark Energy (mJ) is being calculated as (Spark Duration / 2) * Peak Secondary Current * Zener Diode Voltage. Spark duration is being divided by two as in most cases the decrease in secondary current is linear at a rate of 1/2. This simplification allows us to avoid having to represent the secondary current curve using calculus. Of note, the PR coils tested displayed a non-linear decrease in secondary current which will become apparent in a below graph where the B58 coil produces more energy over 3/4 of the spark event.
Bosch N54/N55 @ 2.5ms
Delphi S55 @ 2.5 ms
Eldor S55 @ 2.5 ms
Bosch S54 @ 2.5 ms
Audi R8 @ 2.5 ms
Precision Raceworks N54/N55 @ 3.0 ms
Precision Raceworks N54/N55 @ 3.7 ms
Precision Raceworks N54/N55 @ 4.8 ms
Bosch B58 @ 3.2 ms
Eldor B58 @ 3.2 ms
B58 (red) @ 3.2 ms vs PR N54/N55 (blue) @ 3.7 ms
As shown above the Bosch B58 coil @ 3.2 ms dwell produces a higher peak output current, higher average current and longer spark duration than the PR coil @ 3.7 ms dwell; which is what V8Bait stated was saturation for these however it appears PR is actually using a 4.8 ms max dwell @ 14V. So what does that look like in comparison?
Overlapping the secondary current for each coil allows us to visualize the difference in peak output and also energy under the curve. The PR coil (blue) with a 4.8 ms dwell generated the highest peak output energy but that energy could not be maintained and quickly decreases. The B58 (red) coil has a more linear decrease in secondary current and is able to generate a peak output energy within 12% of the PR coil (The Eldor B58 coil actually beats the PR coil for peak output energy - 01/28/2021) but is able to maintain a higher average current over 3/4 of the spark event with an increased spark duration over the PR coil.
Adding in the Eldor B58 Coil from testing on 01/28/2021
The Eldor B58 coil produces a very large initial spark current, even higher than the PR N54/N55 coils but quickly drops to output energy inline with the Bosch B58 coil as seen below.
IGBT Temperatures sampled via FLIR thermal imaging camera.
IGBT Temperatures graphed for comparison with other coils/dwell times.
As mentioned earlier the long dwell time on the PR coils (far left grey and yellow) creates a higher thermal load on the IGBT's than other coils at shorter dwells (even if they require higher peak current to charge the primary side of the coil). At higher dwell times the IGBT has less off-time to dissipate the heat. The B58 coil's require a shorter dwell time at higher peak current but perform better in my testing. It's important to remember the conditions of the IGBT thermal test, the IGBT is in free air without thermal compound or heatsinking (via the DME case). This test does NOT represent IGBT temperature inside the DME during normal conditions; however the temperature trend above would continue with the PR coils generating the highest IGBT temperatures of the coils tested.
Lastly I wanted to capture temperatures of the ignition coils to see if a certain designed performed better than the other. The results were as expected with all coils following a general trend with only a few stand outs. The two standouts are from the stock N54/N55 coils at 3ms dwell and the B58 coils at 3.5ms dwell. These dwell times are above the point where the coil's core is beginning to become magnetically saturated. When the core starts to enter saturation the impedance decreases resulting in a dramatic increase in current until the coil reaches full saturation and current flatlines.
In conclusion: The B58 coil is a light, low profile coil that produces substantial spark energy over a long spark duration. The B58 coil's shorter dwell time places less load on ignition IGBT's while matching the Precision Raceworks coil in total energy delivered over the spark event.
The results of this research lead me to develop a billet aluminum bracket & components for mounting the B58 coils on the N54 & N55/S55 engines. The bracket is CNC machined in the USA from 6061 aluminum and mounts the B58 coils securely in the stock location providing ample clearance for top mount setups. Our B58 coil conversion kit (including coils) comes in at 3.5 lbs less than the Precision Raceworks kit.
To learn more about B58 coil conversion kit click here.