Daniel Becker, Certificate Automotive Engineering, CU-ICAR (2018)
It depends. What are you trying to do with it? What are your space, weight, and cost constraints? How do the engines compare in bore, stroke, rev limit, and airflow limitations? Given the same bore and stroke, a 4L v8 would reduce to a 3L v6. A 4L v6 would scale to a 5.5L v8.
So, I suppose the question is whether a road car engine with fewer cylinders but equivalent displacement is better or worse, and whether there aspects of v6 and v8 comparison that overcome that assessment. If that is all accurate, then:
because production cars are increasingly biased toward fuel efficiency and engine manufacture is more easily varied in stroke than in bore, it is reasonable to presume that the engines are likely to have a roughly similar bore. To whatever degree this is right, then the v6 is more likely to have a longer stroke, and therefore a greater average piston speed. This means a lower rev limit. That will likely mean lower peak power output than the v8. Greater displacement per cylinder, with the same air:fuel ratio means that each combustion event in the v6 will therefore be larger, so it may be reasonable to presume that the torque peak at a lower engine speed. Because power is a function of speed and torque, having the same torque at a lower speed means a more progressive build of power in the range of speeds around the speed of peak torque. Therefore, this may mean better “driveability,” which makes it easier to realize fuel economy that is closer to stated ratings (this is why diesels are surprisingly successful in demonstrating high mileage, compared to hybrids of similar rating. Because the v6 is shorter, and has fewer components, should have lower reciprocating mass, and should be able to rev (change speed) more quickly (though flywheel mass can overcome this comparison/ make it neglibly different), but, again, won’t necessarily feel wildly sportier than the v8, because the rev limit may be lower and peak power may be lower.
The v6, for being shorter, is probably easier to package into a same-size vehicle. The last v8 powered transverse drivetrain installation, to my knowledge, was the 4.0L v8 in the previous generation Volvo S80 and xc90. Transverse v6s have been the standard arrangement for transverse drivetrains, for decades. In the case of Audis and Porsches, v6s have greatly decreased the moment of inertia of their cars, and made them much “friskier.” In particular, the 4.2L v8 Audis sounded magnificent and had a wonderful and thrilling powerband, but the supercharged 3.0 v6 enginee made every Model with the later engine much sportier and better able to resist understeer.
V6s of the same displacement are also narrower, because having a 60 degree vee angle is narrower than a 90 degree vee. V6s are taller, though (assuming both the v6 and v8 have similar valvetrain).
In all: if you are a car manufacturer, being able to achieve similar outputs and economy, with fewer parts and requiring a smaller space allocation are probably compelling. Using a v6 instead of a v8 is probably the better choice.
BUT: if you are an enthusiast planning an engine swap, or a supercar manufacturer (particularly for a longitudinal-drivetrain vehicle, where cost is no object, and the tingly qualities of the experience are paramount (sound, rev band, drama of smoothly building power), where space limitations are acceptable compromises… I think the v8s are a worthwhile preference.