The inline four-cylinder engine is the most popular layout for bikes because it combines high performance with relatively low cost.
First made popular by Honda in 1970s, after they’d gained experience making ridiculously high-revving 125s for grand prix racing, the other Japanese manufacturers soon followed suit, and nowadays BMW and MV Augusta also offer this layout.
The inline-four is much smoother than singles, twins or triples because the pistons move in pairs giving it perfect primary balance.
Having several smaller pistons also allows it to pick up revs quickly compared to a single or twin, getting up the rev range to where the power is concentrated and making this engine feel free-revving and smooth, even in a high gear at relatively low speed. Testers talk of “creamy” power delivery and “super-smooth” response.
The downsides to this engine design are that it is complex, with four of lots of components, from fuel injectors to bearings, and relatively heavy for that reason. Servicing can be expensive too, with up to 16 valves to check
Poor torque at low revs
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