I've always heard that Volvo developed the B36 V8 first, both as a commercial truck engine, and as a potential (but never used) larger, fancier engine for some more upscale replacements for the PV they had in mind.

Then the B18 was developed as half of the B36 - one bank of the V8 engine. Sort of like the GM 'Iron Duke' 4 cylinder - which is 1/2 of a small block V8.



In reality, there's not that much in common between a B36 and a B18 - cylinder bore and stroke and spacing, mostly. But enough to certainly lead credence to the basic theory. And it also helps explain why the bottom end was so overbuilt on the B18 - it was meant to handle twice as many cylinders.

But the B18 was developed first and foremost as an automobile engine. But Volvo is a big company and makes lots of products, and as always, the automobile engine designs were leveraged by using them in their boat and tractor lines, with suitable modifications to cams and carbs to suit the different needs.

So there *are* B18 engined tractors out there (never really sold in the US, but some have been imported over the decades), and B18 engined boats, and even some B18 engined military Jeep like things.



Mostly it's called a tractor motor because unlike most Euro engines of the day, they weren't made to rev high and make more HP than torque. That was part of Volvo's recipe for longevity, make an engine that doesn't try too hard, and thus just keeps on going.
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)