Hey Dan it's only money
The MSD is a aftermarket ignition which is more powerful and accurate then the stock unit. The 6A is just a performance igniton module, the 6AL and other models have soft touch rev limiter so you can raise the stock rev limiter from 6250 to basically what ever you need. The MSD coils fire a hotter spark whci helps burn the unspent fuel more efficently. Basically I feel you should upgrade as things go bad or if they have a ton of miles and are wearing out. A built performance motor will benifit from the hotter spark, and the rev limiter. I just picked up a 6A from my friend for $20.00, I am going to add on the rev limiters which will costy about another $80.00, the coils run between $30.00-$45.00.
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My Ride Check Out My Site 91GT, Holley SysteMAX II Kit-Heads 2.02" x 1.60",Full MAC exhaust 1-5/8" Long tubes, 2-1/2" O.R H-Pipe, 2 1/2" cat back, 12" K&N Filter, RNH PERFORMANCE Ram Air, Ron Davis Radiator, Full Suspension,3:73's, Welds, Best on new motor-12.60 @ 108, no traction 1.98