If Coors Field is hell for pitchers, then Great American Ballpark in Cincinnati might just be Death Valley during a heat wave. Cincinnati Reds pitchers have long known that when at home, the answer to “Would It Dong?” is almost always “Yes” anytime a ball goes in the air. ESPN’s Park Factors has Great American in the top-10 most home run-friendly parks in every season for the last decade. This includes an average ranking at 4.1 and the top spot in three of the past five seasons.
Being an absolute launching pad means that the friendly confines of home wreak havoc on Reds pitchers’ ERAs. No one knows that better than Tyler Mahle, the Minnesota Twins’ newest starting pitcher. Historically, Mahle gets teed off on at home, surrendering a whopping 1.92 home runs every nine innings over his career. Given his average start hovers around the five-and-a-half inning mark, you could pencil him in for a home run every time he took the mound at Great American.
When you’re as homer-prone at home as Emilio Pagán is, well, everywhere, that’s generally not going to help your numbers. And it sure didn’t in Mahle’s case, as his career home ERA is 5.02.
But the Twins are obviously making a bet that Mahle’s much, much better than that home ERA. And because roughly half his innings were at home, they’re also betting that Mahle’s 4.35 career ERA is deceiving, as well. Change of scenery is a well-worn cliche in sports that crops up during virtually every trade. Getting out of Great American Ballpark, a literal change of scenery, is the best reason to bet on Mahle.
Mahe’s home run problems go away once he is pitching in literally any other park. For his career, his homer-per-nine drops down to 0.82 when he’s on the road. That’s not only cut in more than half, but that home run rate would rank him 15th in the MLB among 62 qualified starters, placing comfortably between Shane Bieber and Joe Musgrove. As it stands now, his dinger rate (including nine dongs surrendered at home and three jacks on the road) puts him at a respectable but not great 36th, with a career-low 1.04 home run-per-nine rate.
The hope is moving Mahle’s home park from the Great American Bandbox to Target Field will be a boon to him and the Twins. Target Field historically has a neutral-to-depressing effect on the long ball. The ballpark has ranked 20th or lower in home run-friendliness for the past decade, according to ESPN’s Park Factors. That includes the juiced-ball-fueled Bomba Squad year of 2019 when Target Field ranked 24th. It’s a simple idea, but one with massive potential for Mahle.
In the past three seasons, when Mahle started entering his peak at age-25, he’s pitched 156.2 innings on the road. He owns a 2.93 ERA over that time, 12th among 84 qualifying pitchers. If getting away from Cincinnati means he can keep that home run rate down and do this at home and on the road, that’s a massive coup for the Twins. We’d be talking about an ERA lower than recent Twins free agent/trade target pipe dreams like Gerrit Cole (3.27), Marcus Stroman (3.28), Yu Darvish (3.45), Luis Castillo (3.56), and “Where’s?” Frankie Montas (3.66).
Improving this could have ripple effects on the Twins beyond Mahle’s innings or even the games he starts. Home runs turning into fly balls means Mahle can pitch deeper into games. Mahle averaged 5.38 innings per start at home over the past three years. Even adding another third of an inning per game to that total will reduce stress on an improved but unproven bullpen.
But the question becomes: How much will changing parks actually help? Can we count on Mahle’s home run rate to be cut down significantly? Great American Ballpark punishes fly ball pitchers, but so does an MLB that is more focused than ever on clobbering fly balls. Besides, home run rates can be subject to small sample size flukes. Is it possible that Mahle got unusually unlucky at home and atypically lucky on the road?
So let’s look at Reds starting pitchers of a similar caliber to Mahle and observe how their home run rates at home changed after they left (and, if applicable, before they arrived at) Great American Ballpark. We will use Johnny Cueto, Mat Latos, and Sonny Gray as examples. 
The good news is that each of these three pitchers controlled home runs better when they got out of Cincinnati. The bad news? The improvements are relatively modest, at least held against Mahle’s career rate at home. The improvement in home run rates at home was only 7% for Cueto, 11% for Latos, and 17% for Gray. Even if we apply that 17% improvement to Mahle, that would lower our expectations to about 1.59 home runs per nine at home, still a ghastly number.
Each of those pitchers also had extreme home/road splits when it came to home run rate. In their times in Cincinnati, Cueto gave up 0.81 HR/9, Latos surrendered 0.55, and Gray gave up 0.70. Only Gray improved his low home run road splits outside Cincinnati (0.64).
So does that mean that Minnesota’s gamble will fail? Remember, we said earlier that Mahle has definitely improved his home run rate over the past three seasons. That also applies to stopping the bleeding on giving up dingers at Great American.
The worst of Mahle’s home run issues came in his first three seasons. He served up a ridiculous 28 home runs in only 110 innings at home. The result was an unfathomable 2.29 HR/9 at his home park. Aside from a broken-down Matt Harvey and Edwin Jackson, no other starting pitcher has had a three-consecutive season run where they’ve been more homer-prone at home. That’s gone down to 1.69 over the past three seasons. Still pretty bad, but at least enough to keep his ERA under 5.00. Apply that 17% improvement, and you get a palatable-enough 1.40 mark.
However, you have to ask: How real is his low home run rate on the road? Mahle’s been less homer-prone on the road in the last three years, allowing only 0.52 HR/9 in away games. That’s almost definitely not sustainable. A pitcher probably isn’t going to be allowing under 6% of his fly balls to leave the park in every stadium except one, where the balls go out ~15% of the time. At least, not over the long term.
So the smart money is that the homer rate at home will go down with the road going up. Cancel those out, and you probably get something like the 1.12 HR/9 rate he’s posted over the past three seasons. Is it the elite of the elite? No, but it’s definitely in the realm of Montas (1.10) and Darvish (1.12). It’s also a cut above what potential postseason starter Joe Ryan (1.38) has provided in his Twins career. It’s also at or above the level of recent postseason starters in José Berríos (1.27) and Kenta Maeda (1.30) over that time.
It also helps that the rest of Mahle’s game is pretty spectacular. According to Baseball Prospectus, Mahle generates a whiff on 27.8% of swings against him. Again, that’s right in the Montas (28.0) and Castillo (26.7) range. He’s struck out 27.5% of his batters over the last three seasons, a borderline elite rate putting him on par with Maeda over that time. Mahle is also near-elite at giving up soft contact, doing so 18.9% of the time. That ranks 20th among starting pitchers with 150-plus innings.
All Mahle has to do is keep the ball in the park. If his home and road splits just even out, the Twins have a damn good pitcher on their hands. If he can show that the Mahle outside of Great American Ballpark is closer to the real one than the one inside it, it’ll be a steal.
