A Tale of Two EcoBoosts

With the introduction of the 2014 Ford Fiesta ST, there's a new sweet spot for hot hatches. Let's see.... The Fiesta ST undercuts the Mazdaspeed 3, Mini Cooper S and Volkswagen GTI by a couple grand and delivers good fuel economy, yet doesn't skimp on performance or agility. It's the hot hatch for the masses.

Part of what injects fun into the Fiesta ST's bloodstream is its boosted, direct-injected EcoBoost 1.6-liter four-cylinder engine. Ford rates the mill at 197 horsepower and 202 pound-feet of torque. Sounds good, right?

Here's the thing. We've found that turbocharged engines can be something of a wild card when subjected to the truth-telling of a dyno test. Sometimes, the engine overachieves (looking at you, Volkswagen). Other times we find they're sensitive to heat and deliver inconsistent results.

Curious to see how the 2014 Ford Fiesta ST would fare, we headed over to MD Automotive and strapped it down to the shop's Dynojet chassis dyno.

Here's what we found:

Well, that's rather impressive. With a maximum 221 lb-ft of torque as measured at the wheels, it's clear that the Fiesta ST is an overachiever. Boost comes on very early in the rev range, allowing it to generate at least 200 lb-ft from 2,000-4,300 rpm. From there the torque rolls away in a more or less linear fashion all the way to the 6,300-rpm fuel cut.

The Fiesta ST's power peak of 184 hp at 5,200 rpm more closely aligns with its rated number at the flywheel. Beyond the peak numbers, the Fiesta ST laid down its power with excellent consistency. There was very little variation from pull to pull, indicating that the EcoBoost 1.6 isn't some highly strung, sensitive power plant. Furthermore, keep in mind that our results were obtained with 91 octane, which is the internal combustion equivalent of trying to sharpshoot while wearing Vaseline-smeared goggles.

The same consistency wasn't found in the Fiesta's stablemate, the Ford Focus ST. We happened to have our long-term 2013 Focus ST the day we went to the dyno. You can guess what happened next.

The result:

The Focus ST was far grumpier on the dyno than the Fiesta ST, exhibiting wild variation from run to run. A few pulls in, several worryingly loud crackles of detonation prompted me to call it a day for this car's dyno appointment. With that said, the Focus ST is clearly generating a whole lot of grunt, belting out more than 250 lb-ft to the wheels between 2,200 and 4,800 rpm.

Torque falls precipitously from this point, more so than in the Fiesta ST. The Focus ST's 2.0-liter EcoBoost four is being asked to work harder than the Fiesta ST's 1.6-liter mill, and the limitations of 91 octane aren't helping here. We'd anticipate that higher octane would help the Focus ST better maintain its post-5,000 rpm output. Furthermore, the Focus ST's intercooler could be undersized or otherwise compromised when it comes to shedding heat. Whatever the cause, it was surprising to observe such diametrically opposed run-to-run characters in two fundamentally similar cars by the same manufacturer.

As for why the Ford Fiesta ST outstripped its factory rating, the intercooler, again, might be the reason. Intercooled turbocharged engines present a unique situation for automakers when devising their rated power and torque. This process is conducted on an engine dyno, of course, where the engine travels exactly zero mph. Yet intercooler performance relies in part on vehicle speed (cooling airflow). Thus, the effect of the intercooler must be simulated during the engine rating process. And it turns out there are no hard-and-fast rules when it comes to simulating intercooler performance. On the dyno we use a massive, high-powered electric fan to create cooling airflow that would normally come from vehicle speed.

To be on the safe side during engine rating, some manufacturers treat the intercooler very conservatively, presenting a worst-case scenario of heat. This tends to make the rated power more conservative than it would be otherwise.

And that appears to be the case with the 2014 Fiesta ST.

The manufacturer provided Edmunds this vehicle for the purposes of evaluation.