For a long time, turbocharging has been referred to colloquially as the “replacement for displacement.” The latest trend embraced by both foreign and domestic automakers seems to support that title, as larger engines are being replaced by smaller — both in displacement and cylinder count — turbocharged engines.
While most of the videos Jason Fenske of Engineering Explained makes are wholeheartedly in support of new technology, this one is a little different. This time, he takes a look at the downsides of turbocharged engines, specifically smaller-displacement turbocharged engines which have replaced larger naturally aspirated ones.
“Not that long ago, naturally aspirated cars were the norm, and turbocharged cars were the rare, crazy-fast cool ones,” Fenske says. “Today that’s kind of flipped. We’re moving towards downsized engines with turbochargers, and the big, naturally aspirated engines are becoming the more rare ones.
In that vein, Fenske has listed five disadvantages experienced by smaller-displacement turbocharged engines when compared to the larger-displacement engines they replace.
A naturally aspirated engine is going to have a more linear throttle response than a turbocharged engine, due to the way a turbocharger works and turbo lag, although throttle response is different than turbo lag. “You want a linear relationship between throttle position and the power that’s delivered,” Fenske explains. A linear throttle is much easier to modulate.”
“With a turbocharged engine, you’re waiting for the turbo to spool up before you have that control. There’s a gap in the throttle pedal where you don’t have much modulation authority. I’ve driven cars where at 50-percent throttle, it’s capable of full boost. At that point, why is the throttle pedal even there? It’s just an on-off switch.”
Like the throttle response section above, for an all-around performer, a smooth torque curve is desirable. While a turbocharger can provide a flat torque curve at peak boost, that plateau is just that, a plateau sitting on top of often-steep ramps.
“This is something that modern turbos are actually very good at,” says Fenske. “Looking at the torque curve of a turbo engine might cause you to say, ‘Look at that nice flat torque curve!’ but the areas before and after that flat section, things kind of fall apart. You have to figure out where you want to put that flat section of torque. Early in the RPM range will give you throttle response, and later in the RPM range to make big power.”
Reliability Vs. Cost
This subject is one of great contention, since to some, cost shouldn’t be factored into performance. For those who don’t have a money tree in their backyard, cost is a significant factor when it comes to horsepower.
“I’m not going to claim that a turbocharged engine is less reliable than a naturally aspirated one. But it takes more money to make a forced-induction engine as reliable as a naturally aspirated one,” Fenske says.
In addition to the more robust internal components required by an engine with forced-induction, the additional hardware is also a factor in the increased cost over a naturally aspirated engine. “Besides the increased pressures in the engine, there are increased temperatures that a turbocharger exposes the oil, and the engine itself to, which have to be accounted for,” lists Fenske.
While there are benefits of a physically smaller engine such as improved packaging leading to better aerodynamics and crash safety, one of the lead benefits touted has always been increased fuel economy. So how is Fenske justifying it as a downside of a turbo?
“While yes, a smaller engine is more fuel-efficient than a larger one, slapping a turbo on that isn’t always the most efficient thing,” Fenske says. “Generally, when putting a turbocharger on an engine, the static compression ratio is lowered to account for the boost. There is a correlation between compression ratio and thermal efficiency, where lower compression ratios are less thermally efficient.”
In addition to thermal efficiency, Fenske cites the fact that to get the same power as a larger N/A engine under normal driving conditions, you need to put your foot to the floor on the smaller, turbocharged engine.
“Running the engine hard increases the temperatures. Increased temperatures increase the likelihood of knock. To combat that, you add fuel, and your fuel economy goes right out the window. Turbocharged engines are going to have to dip into richer air-fuel ratios, earlier, and at wide-open throttle, they will probably be richer than a naturally aspirated engine, as well,” he concludes.
The final category is one of pure subjective opinion, and Fenske knows it. Presented almost tongue-in-cheek, the sound of a turbo car being worse than that of a naturally aspirated V8 will probably generate a lot of debate.
“One of the reasons a turbo engine doesn’t sound as good, is that the turbo acts as a muffler. Sure there are the turbo noises like spooling and the blow-off valves, which is an added value, but you’re pulling noise from the engine with a turbocharger,” Fenske says, knowing exactly the buttons he’s pushing with that statement.
“Since production turbo engines tend to be smaller than naturally aspirated production engines, they simply don’t have the same bark. With a V8 vs. a four-cylinder, your engine is firing twice as often at the same RPM to give you that nice, visceral sound.”
However, what Fenske isn’t taking into account with the final section, is a turbocharged V8 (or V10, or V12). That’s not really the point of his video, but still, we felt the need to include the option of the larger engine AND a turbocharger giving you the best of both worlds. We’re sure the OEMs will disagree with us, though.
This list is only a condensed version of Fenske’s comments in the video, so be sure to watch it in its entirety for all of Fenske’s thoughts on the matter, both serious and lighthearted.