The conventional L6 (1-5-3-6-2-4 firing order) engine, and its performance use in side-valve, pushrod OHV, and OHC variants, is about 100 years old. The whole "what should the exhaust look like?" had already be resolved before WW2:
1. front 3 and rear 3 grouped together
2. keep the 2 pipes separate as far back as possible
This describes 1920 Grand Prix engines, Jaguar XK-E, and Toyota Supra.
