Frequently Asked Questions: Dwell
Choose from the most frequently asked questions below to find the answers to Dwell questions. Use the navigation menu to the right to link to other frequently asked questions categories.
Q: What Is Dwell? (Electronic Ignition Engines)
A: Dwell is the amount of time required to charge an inductive coil to its maximum energy level. In terms of modern engine control, dwell is defined in milliseconds. Typical ID coils will have a dwell between 2ms and 5ms.
Electronic ignition is much better at regulating dwell than distributor based engines. Return to Top.
Q: What Is Dwell? (Distributor Ignition Engines)
A: In older distributor based engines a cam in the distributor opened and closed a set of points. The points applied battery voltage to the coil. When the points were closed, the coil charged. When the points opened, the coil discharged thru the distributor to the spark plug. The “dwell angle” was the number of cam shaft degrees that the points were closed. The “dwell angle” is fixed but the “dwell time” varies with rpm. When rpm increases, dwell in terms of time decreases. At higher rpm a distributor based inductive ignition system would not have enough dwell “time” to fully charge the coil. This would result in a weaker spark at high rpm and could lead to less than optimal engine performance. Hence the birth of CD ignition systems that could charge quickly at high rpm.
Modern Inductive Discharge ignition systems have overcome the decreasing dwell problem by using a coil per cylinder or coil on plug. Return to Top.
Q: Is Dwell Important To Engine Performance?
A: Dwell is very important to engine performance. When tuning engines most people focus on ignition timing and don’t pay enough attention to dwell. It is a set and forget approach. Dwell controls the amount of energy stored in the ID ignition coil. Too little dwell and the coil will be undercharged resulting in a weaker spark. Too much dwell and the coil will overheat. An overheating coil will breakdown easier, won’t store as much energy and will result in a weaker spark. Return to Top.
Q: Does Dwell Time Change With Battery Voltage?
A: Yes, the required dwell time is dependent upon battery voltage. If battery voltage increases, the dwell time can be reduced. If battery voltage decreases, the dwell time needs to increase. Many ECUs have a trim table that will adjust dwell based on battery voltage. If your ECU does not adjust dwell based on battery voltage two things can happen. 1) If battery voltage decreases, the amount of energy in the coil will decrease and result in a weaker spark 2) If the battery voltage increases, the coil will overheat – you won’t achieve a more powerful spark.
The chart on the right is an example of an inductive coil charging time. You can see that the dwell time increases significantly below 12V. When fully charged this coil will produce a 0.9ms duration spark with 25 mJ spark energy, 26 kV breakdown voltage and 62 mA peak current. Return to Top.
Q: How Can I Determine How Much Dwell Time My Coil Needs?
A: Don’t guess. Make sure you know exactly how much dwell your coil needs to optimize performance.
The manufacturer should publish a data sheet for your coil that provides dwell vs voltage.
If you can’t find the data sheet you can measure dwell with an oscilloscope. Connect the oscilloscope to the primary side of the coil and measure the time it takes to charge at different voltages. You need to measure current, not voltage.(Caution: Be very careful when discharging the coil)
Need help – we offer a coil mapping service. Send us your coil and we will map it for a nominal fee (about $50 per coil + shipping and taxes) contact: firstname.lastname@example.org.
Q: Can I Increase My Inductive Spark Performance By Increasing Dwell
A: No. You can’t charge an ignition coil beyond 100% of its energy capacity. If a coil has a dwell time of 2ms at 12V and you increased dwell to 5ms at 12V this would just cause your coil to overheat. An overheating coil is more prone to internal breakdown and will result in a weaker spark. Return to Top.
Q: What Is Better A Coil With A 2ms Dwell Time Or A Coil With A 5ms Dwell Time?
A: Depends. Dwell is a measure of the time it takes a coil to charge to 100% of its energy capacity. The amount of energy a coil can store is a function of its construction.(number of turns, wire diameter, core material etc) A coil that takes 5ms to charge might be able to store more energy than a 2ms coil or it might just be less efficient. Compare the manufacturer’s specifications to determine the characteristics of each coil. Return to Top.
Q: Do Capacitive Discharge Ignitions Require Dwell?
A: Not in the same sense as an ID ignition. A CD system charges a capacitor not a coil. Charging a capacitor at 400V is a much faster process than charging a 12V coil; however, it still takes time. It might take 500 microseconds to 1 millisecond to charge a capacitor whereas a coil could take 2 to 5 ms to charge. Most CD systems probably pre-charge the capacitor and wait for a trigger event from the ECU. Some CD systems might use the ECU dwell output to determine when to start charging the capacitor. Check with the CD manufacturer to determine how the ignition system is triggered. One thing to be aware of is the charge time between CD sparks in a multi spark CD system. Some CD systems fire multiple sparks below a specific rpm. The capacitor requires time to charge between each spark and this can result in a retarded ignition event. Return to Top.
Q: Does SPDi Require Dwell?
A: SPDi prefers to see a fixed dwell signal from the ECU. It does not really matter what the dwell is set to but we recommend 2ms for consistency. SPDi watches for the charge signal from the ECU on each channel. SPDi takes approximately 500 micro seconds to charge. SPDi fires the spark when the charge signal from the ECU ends (same as an inductive spark). There is no measureable latency between when SPDi fires the spark and when the OEM inductive system fires a spark. Return to Top.
Q: How Do I Program The Dwell On My ECU When Using SPDi?
A: We recommend that you set the dwell to a fixed value. 2ms is fine. Set all dwell trims to zero. Return to Top.
Q: How Does SPDi Interpret The Dwell From The ECU?
A: SPDi looks for the charge signal from the ECU. SPDi estimates the dwell period and begins charging about 500 micro seconds before the ignition event. When the charge signal is removed (at the ignition timing point) SPDi fires the spark. (same as an inductive spark) There is no measureable latency between when SPDi fires the spark and when the OEM inductive system fires a spark. Return to Top.
Q: I Have Tried Measuring The Dwell Of My System But It Does Not Seem To Be Stable?
A: We have seen this on some OEM ECUs. We don’t know why some manufactures are using this technique but we assume it is to control spark energy and maybe influence the catalytic converter. If the spark is weak and does not burn all the fuel in the mixture maybe this might help catalytic converter temperature control or efficiency. We don’t really know but if you do we would be happy to hear from you. Return to Top.