here is a lot of useful info I found while shopping around for ignition coils this week, please feel free to add more info on I may have left out.
Here are the coils that you will currently see when you go shopping:
ACDelco D576 - most expensive; direct OE replacements for FI cars. These coils have blue tabs on the bottom side. Can be used as a "mod" upgrade as long as all three coils are the same.
ACDelco D555 - Direct OE replacements. Puts out slightly less spark than D576 (voltage and amperage are different). Costs less. These coils do not have any blue on the bottom. Not recommended for FI, will work but decreases performance and may cause longer-term issues, especially if blue and non-blue are mixed on same car.
ACDelco E530C - a "one size fits all" replacement; suitable for L36 engines in any car. Appears to be pretty much the same as D555, but some feel that it is slightly inferior. As with D555, it is not recommended for FI.
Delphi GN10123 - Same as E530C. Possibly made in same factory. Just a different brand name on the box. As with E530C and D555, is not recommended for FI. Delphi used to be a part of General Motors, and still makes many parts for AC Delco.
Airtex/Wells C849 - aftermarket equivalent of E530C (sold under "Duralast" brand at AutoZone). Felt by many to be of lesser quality than Delphi/ACDelco. Coils that are not Delphi or AC Delco are not recommended, unless you do not plan on keeping the car for long. Junkyard parts preferred over Airtex/Wells.
Standard Motor Products DR39T - very low-priced. Commonly found on eBay, also on Rock Auto as "Economy option." Coils that are not Delphi or AC Delco are not recommended, unless you do not plan on keeping the car for long. Junkyard parts preferred over SMP.
MSD 8224 - These are comparable to D576 and are suitable for FI. Considered by some to be a "mod" due to their red color and association with racing. Some feel that they are slightly better than D576 and others say they are slightly less good. Either way, most seem to agree that they are pretty close to D576, however, so the differences may be negligible. Prices are similar to D576, if not a little lower. As with D576, can be used as a "mod" upgrade.
Junkyard coil prices vary widely. These used coils will typically be AC Delco D555 or D576 (definitely are if they still have white printed numbers on top).
A note about the white printed numbers on the tops of the original coils: such coils are original coils installed at the factory. Other than the numbers, all three coils are exactly the same. The numbers were only printed for convenience, when changing out spark plug wires. Replacement coils do not have these numbers, so that they can be used in any position. You can write your own numbers on top, if you wish, using a Sharpie marker.
credit: to Ken_W, ChibiBlackSheep and various other forums etc.
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