Why do we like the VAG Smart Coil for the Corvair?
Here is the full mathematical breakdown for a single VAG smart coil operating across the entire RPM range on a Corvair engine.
The maximum available dwell window drops sharply as engine speeds rise. However, because the VAG coil fully saturates in just 1.6 to 2.2 ms, it maintains a 100% saturated spark all the way up to 6,000 RPM—though it hits a thermal limit at the top end due to a 100% duty cycle.
The Performance Math (6,000 to 1,000 RPM)
| Engine Speed | Total Time Window | Spark Burn Duration | Max Available Dwell | VAG Spark Status | Duty Cycle & Heat |
|---|---|---|---|---|---|
| 6,000 RPM | 3.33 ms | 1.20 ms | 2.13 ms | 100% Saturated | ~100% (Extremely Hot / Limit) |
| 5,000 RPM | 4.00 ms | 1.20 ms | 2.80 ms | 100% Saturated | ~80% (Runs Warm) |
| 4,000 RPM | 5.00 ms | 1.20 ms | 3.80 ms | 100% Saturated | ~64% (Safe / Efficient) |
| 3,000 RPM | 6.66 ms | 1.20 ms | 5.46 ms | 100% Saturated | ~48% (Cool & Stable) |
| 2,000 RPM | 10.00 ms | 1.20 ms | 8.80 ms | 100% Saturated | ~32% (Excellent) |
| 1,000 RPM (Idle) | 20.00 ms | 1.20 ms | 18.80 ms | 100% Saturated | ~16% (Stone Cold) |
Key Takeaways From the Math
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- The Crucial Crossover at 6,000 RPM: At 6,000 RPM, the total window is 3.33 ms. Subtracting 1.20 ms for the actual spark arc leaves you with 2.13 ms of charging time. Because a VAG Smart coil commands a standard running dwell of 2.0 ms, it just fits. It delivers a full-power spark, whereas the GM D585 (which demands 3.5 ms+) drops to a fraction of its power.
- The Duty Cycle Bottleneck: Look closely at the transition from 4,000 RPM to 6,000 RPM. At 4,000 RPM and below, the coil spends a significant portion of its time resting (turned off), allowing the internal driver to shed heat. At 6,000 RPM, the duty cycle hits 100%, meaning the coil is permanently working with zero downtime.
If this engine regularly passes 5,500–6,000 RPM, You may want to consider a CD Ignition Unit which our system
Why do we NOT use the GM-585 Truck Coil? In fairness, these coils are Coil-Near-Plug (CNP) – one (1) per cylinder and it’s saturation rate is not even a design consideration, but many people use them and on a 4 Cylinder VW engine, I think they are perfect.
The GM D585 Performance Math (6,000 to 1,000 RPM)
| Engine Speed | Total Time Window | Spark Burn Duration | Max Available Dwell | D585 Coil Saturation % | Spark Output Behavior |
|---|---|---|---|---|---|
| 6,000 RPM | 3.33 ms | 1.20 ms | 2.13 ms | ~60% (Severely Starved) | High risk of severe spark blowout under load. |
| 5,000 RPM | 4.00 ms | 1.20 ms | 2.80 ms | ~80% (Under-Saturated) | Weakened spark; prone to misfires under wide-open throttle. |
| 4,000 RPM | 5.00 ms | 1.20 ms | 3.80 ms | 100% Saturated | Optimal performance point; right at the threshold. |
| 3,000 RPM | 6.66 ms | 1.20 ms | 5.46 ms | 100% Saturated | Full energy, stable spark delivery. |
| 2,000 RPM | 10.00 ms | 1.20 ms | 8.80 ms | 100% Saturated | Full energy, long internal cooling window. |
| 1,000 RPM (Idle) | 20.00 ms | 1.20 ms | 18.80 ms | 100% Saturated | Full energy, stone cold operation. |
Crucial Math Realities for the D585 Setup
- The 4,000 RPM Crossover Point: At 4,000 RPM, the max available dwell time left is 3.80 ms. Since the D585 achieves solid, clean performance right at 3.5 ms, this is the highest engine speed where the coil can still completely saturate. Anything past 4,000 RPM forces the coil to fire prematurely before its internal magnetic field has completely built up. [1, 2]
- The Auto-Fire Risk (Low RPM Danger): Look at 1,000 and 2,000 RPM. The available windows are massive (8.80 ms to 18.80 ms). If you are using a programmable ECU and your ignition dwell map accidentally allows the dwell to track with the available window past 4.5 ms, the D585’s internal smart logic circuits will assume a systemic error has occurred. It will execute an emergency auto-discharge (“auto-fire”) early, causing devastating pre-ignition or severe engine kickback. [1, 2, 3]
- Thermal Duty Cycle Strain: Because the coil requires so much time to charge relative to the total window size, its duty cycle punches through the safe 50-60% zone immediately after 3,500 RPM. By 5,000 RPM, the coil has zero rest time to dissipate heat, leading to rapid component degradation if run at high RPMs for prolonged periods.
The Traditional GM Round Coil:
Here is the complete mathematical breakdown for a classic GM 12V traditional round canister coil operating as the single, centralized coil on a Corvair engine.
Because a traditional round coil has a high internal primary resistance (1.5 to 3.0 Ohms), it charges very slowly. It takes a lengthy 3.5 to 4.5 milliseconds (ms) of dwell time to fully saturate its magnetic field. As engine speed climbs, the shrinking available time window starves the coil of charge time, causing spark energy to drop linearly. A important point to remember is that this coil will work above 6k RPMs, just at a reduced about of spark. Is that spark reduction enough to make a difference…a lot of other factors influence that.
The GM Traditional Round Coil Performance Math (6,000 to 1,000 RPM)
| Engine Speed | Total Time Window | Spark Burn Duration | Max Available Dwell | Coil Saturation % | Spark Output Behavior |
|---|---|---|---|---|---|
| 6,000 RPM | 3.33 ms | 1.20 ms | 2.13 ms | ~53% (Severely Weakened) | Massive voltage drop; highly prone to spark blowout under heavy load. |
| 5,000 RPM | 4.00 ms | 1.20 ms | 2.80 ms | ~70% (Under-Saturated) | Noticeable spark degradation; struggles to clear rich or high-compression mixtures. |
| 4,000 RPM | 5.00 ms | 1.20 ms | 3.80 ms | ~95% (Near Saturation) | Solid, reliable spark; right at the ragged edge of performance. |
| 3,000 RPM | 6.66 ms | 1.20 ms | 5.46 ms | 100% Saturated | Full nominal energy (~30 mJ); excellent, stable combustion. |
| 2,000 RPM | 10.00 ms | 1.20 ms | 8.80 ms | 100% Saturated | Full nominal energy; plenty of internal cooling time. |
| 1,000 RPM (Idle) | 20.00 ms | 1.20 ms | 18.80 ms | 100% Saturated | Full nominal energy; coil runs completely cool. |
Crucial Math Realities for the Traditional Round Coil
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- The 3,800 RPM “Wall”: At 3,800 RPM, the max available charging window drops below 4.0 ms. Because this old-school inductive setup requires a long soak time to build up its magnetic field, its spark power starts dropping continuously from this point onward. By 6,000 RPM, it has lost nearly half its potential energy.
- The Thermal Advantage (Oil-Filled Cooling): While its high-RPM electrical performance is weak compared to modern smart coils, the traditional “beer can” coil handles the thermal stress of a single-coil layout exceptionally well. Because its metal casing is filled with transformer oil, it sheds internal heat efficiently. Unlike modern plastic coils, it can run continuously at high duty cycles without immediate electronic failure.
- Overcoming the Math via CDI: The only way to bypass these strict inductive time constraints with a traditional round coil is to add an external CDI (Capacitive Discharge Ignition) box, such as an MSD 6AL. A CDI box sends a massive 400V+ spike into the primary side instantly, forcing the coil to completely saturate and fire in less than 0.2 ms. This restores 100% spark energy all the way to 6,000 RPM and beyond.