It's almost a perfect correlation that the higher Porcello's ground ball rate, the lower his ERA. In fact, take out his rookie season and his current season and it is a perfect correlation. The chart below shows Porcello's first 5 seasons in MLB and the trend line:
So when Porcello is showing his lowest ERA along with his lowest ground ball rate in 2014, it raises a red flag, even if it is only in 6 starts. Using the formula of y = -0.0451x + 0.7321, with a GB% of 43.9%, we would expect an ERA of 6.50. With an ERA of 3.49, we would expect a GB% of 57.5%. Did he change his approach? Is he just getting lucky? Is it just a 6 game anomaly?
Rick Porcello has an 18.2% strikeout rate, a 4.1% walk rate and a 2.0% home run rate which results in a 3.16 FIP, the lowest mark in his career. FIP uses strikeouts, walks and home runs and shows what a pitcher's ERA should look like.
However, he has a 29.8% line drive rate and a 26.3% fly ball rate to go along with his 43.9% ground ball rate, which results in a 4.83 tERA. tERA is another ERA predictor, but uses batted ball information. SIERA kinda uses a combination of both FIP and tERA and says Porcello's ERA should be 3.50 (which is the closest to his actual 3.49 ERA). His ERA estimators are giving off mixed signals.
The 29.8% line drive rate is the highest of his career:
Conveniently, Baseball Reference gives the exact same batted ball figures (FanGraphs and Baseball Reference use different sources for their batted ball data and usually they disagree on few batted ball types). I have to switch to Baseball Reference here since FanGraphs doesn't go as deep into batted ball data for pitchers as they do for hitters. According to B-Ref, Porcello has a .559 BAA on line drives in 2014. For his career, his BAA on line dives is at .696 and typically it's around .700. Simply put, Porcello is very fortunate that more of his line drives haven't fallen in for base hits and it's keeping his ERA low.
Going to Brooks Baseball, we can see exactly what pitch is causing Porcello's lack of ground balls:
Porcello's ground balls on 4-seam fastballs are usually in the mid-40% range, but it has dipped all the way down to only 5.9% so far in 2014. Conversely, his fly ball rate on 4-seam fastballs has gone up:
Porcello's lack of ground balls, high line drive rate and crazy 4-seam fastball trend hasn't hurt him yet, but it's something to keep an eye on in his next start and going forward. I don't believe Porcello can continue to have success with this combination.