In order to build a trigger shell correctly, you will need to provide the placement of the first magnet. This is NOT the degree of timing you want to run but rather the number at which the pick-up is pointing to when the TDC pointer is at the firing degree.

If you want to run 32° of timing, first rotate the engine to put the 32° mark at the TDC pointer. Then, move the pick-up to the center of its travel (so you will have room for adjustment either way) and indicate the number the pick-up is pointing to. This will be the placement of the first magnet. When the magnet comes around and meets the pick-up, the engine will fire at 32°.

Most EFI set ups require the signal to fire between 50° and 60°. (This may also be your “crank reference” number in your EFI software) This allows full travel within the timing map. In these cases, you will need to rotate the engine so that your specified number is at the TDC pointer. Where you tell us the pick-up is now pointing is where we will put the 1st magnet.

Most people use the commonly available pick-up brackets and run similar total timing numbers. ATI offers three stocking trigger shells for Chevrolet based engines on the most commonly requested marks.