Naming schemes and their turbocharged engine translations:
AMG 65 — 6.0L x 2.0 = 12.0L (because it can handle up to 885 lb-ft with stock internals)
AMG 63 — 4.0L x 1.5/2.0 = 6.0/8.0L (because torque ranges from 443 to 664 lb-ft with no electric motors); treat non-S models like 6.0L models and S models like 8.0L models
AMG 55 — 4.0L x 2.0 = 8.0L (for new SL55 AMG model since it produces 516 lb-ft at a very low rpm, it doesn't qualify for "just" 6.0L)
AMG New 53/New 63 — 2.0L x 3.0 = 6.0L (multiplication by 3 is rare; for instance, Koenigsegg Gemera of similar displacement needs multiplication of "only" [2+⅔])
AMG 45 — 2.0L x 2.25/2.5 = 4.5/5.0L (non-S models are 4.5, S models are Mustang/Lexus-grade 5.0)
AMG New 43 — 2.0L x 2.0 = 4.0L
AMG 35 — 2.0L x 2.0 = 4.0L (at 295 lb-ft of torque it's pretty much maxed out for a n.a. 3.5L engine)
AMG ONE — 1.6L x 3.0 = 4.8L (at 443 lb-ft without electric motors this hypercar would have THE most torque per liter of ANY production engine!)
Old 43/53 is 3.0L x 1.5 = 4.5L equivalent because they make less than 400 lb-ft of torque.