Originally Posted By: expat
I don't believe that the number of cyls is really a factor in how long or how many miles an engine will last.
Design and the way the engine is treated/maintained are the main factors.
81 Toyota PU 22R 520k
For the most part, I agree. But inline 4-cylinders do have inherently [censored] 2nd-order imbalance that has to be compensated for in the design, usually with balance shafts or oversized engine mounts. The vibrations don't usually bother the engine itself, but they can be pretty hard on accessories and the complexity of balance shafts adds a failure mechanism. Not usually a factor in the real world, but on paper its a disadvantage to inline 4-bangers.
V6s are a mixed bag depending on whether they're 60-degree, 90-degree, fully even-firing split-pin cranks, odd-firing split-pin cranks, or odd-firing conventional cranks.
V8s are better, inline 6 and V12 engines have inherently perfect balance and in theory should last the longest. Most v12s are performance engines and die young for other reasons, but there are plenty of forever-living inline sixes out there- Jeep 4.0 and 258, Ford 300, Chrysler slant-6, etc.