Ask The Editor: What Happened to Small-Displacement V6s?

Reader Brian Tai writes:

I’ve been an enthusiast and part-time DIYer for years now. I love to learn about everything automotive.

My question for you: why are cars with small engines always inline-fours? Why do manufacturers not put 2.0-liter V6s into cars? I know they don’t usually use big displacement inline-fours because of NVH issues, but what about the other way around?

Thank you for your question, Brian. I’ve been wondering about this very aspect of engineering for a while and you just gave me enough push to go sniffing for answers.

The current landscape of modern, non-pushrod, direct-injection engines is one of patterns. Chances are that the four-cylinder engine humming away under the hood of your Generic Corp. Compact displaces the same volume as a large bottle of Coke and has all the aural bliss of a vuvuzela filled with stadium hot dogs. Add two cylinders and arrange them all in two rows of three, and that engine is more likely to displace 3.0 liters today than it ever has in its history. Add two more cylinders and you see the growing crop of 4.0-liter V8s. The pattern: 500 cubic centimeter cylinders.

C/D tackled this topic not too long ago. 500 ccs per cylinder provides for a nice blend between efficiency and power, especially when paired with turbocharging and a number of other whizbang technologies. Additionally, with global taxation schemes aimed at vehicles with certain displacements — such as the additional tax paid on vehicles with engines that displace over 1.5 liters in China — it makes more sense to build engines with half-liter cylinders.

However, you might remember the small K-series motors from Mazda. The K8 V6 displaced a whopping 1.8 liters in the MX-3. Even the bigger V6s from Mazda of that generation, which many tuners wedge into the MX-3 in order to boost performance, measure in at a paltry 2.5 liters.

So, we asked Mazda: Why aren’t small displacement engines still a thing? The answer was simple.

“Making a piston costs about the same amount, whether it’s a big piston or a little one,” explained Mazda engineer Dave Coleman. “Boring a cylinder costs about the same whether it’s big or small. Casting and machining two three-cylinder heads costs almost twice as much as one four-cylinder head. Making four variable cam timing actuators costs exactly twice as much as making two. Six injectors costs 50 percent more than four.”

When you’re developing a car to a price, that overall price and profit is directly related to the amount it costs to build said car. Why build a V6 if you can engineer a four-cylinder engine with the same displacement and similar output at a lesser overall cost?

And to drive the point home even further, there’s not much practical point in building a V6, as they “take up more space than four-cylinders, making a bigger, heavier car,” continued Coleman.

So, if you are one of those owners of a Mazda MX-3 with its diminutive 1.8-liter V6, you could be sitting on a historical artifact for years to come.

[Image: Mazda]