The truth is there is no difference except for the fact that all 63- numbers are pending C.A.R.B. approval. They have not been approved and therefor are for race only. All 57- numbers are approved for street use and come with the C.A.R.B. sticker for emissions. The two units above, 1536 are idenical except for the C.A.R.B. sticker. I have had them side by side in our shop and all measurements are the same no matter what K & N tells you. the 63 number has not been improved in any way.
Robbie
AtRo Racing Products