This will be Jake Peavy's first full season with the Chicago White Sox, and it's well documented that his new home ballpark, U.S. Cellular, inflates home run totals while his former park, Petco Park, suppresses them quite a bit. So we should expect Peavy to give up more home runs -- but how many more?
One way to answer the question is to look at the home run per fly ball rates, or HR/FB, of each park relative to Peavy's fly-ball numbers. We know fly-ball rates normalize more quickly than simple home run rates, and pitchers generally allow homers on about 11 percent of their fly balls. So this approach allows us to take a pretty nuanced look at the problem.
A study by Dan Turkenkopf of The Hardball Times shows us that "The Cell" and Petco have HR/FB indices of 118 and 75, respectively. What that means is that if you multiply those numbers by .11 (percentage of all fly balls that become homers), you discover that about 13 percent of fly balls become homers in Chicago, versus only about 8.25 percent in San Diego.
Before we see how that affects Peavy, let's look at his record with the Padres.
Peavy away from Petco Park
He’s headed to U.S. Cellular, which inflates home run totals.
Now, here are Peavy's numbers at Petco:
Peavy at Petco Park
His old home field suppresses homer totals.
We see a couple of things here. First, Peavy benefited from Petco's home-run suppression, as seen by his lower HR/FB rate. Also, he gave up a greater number of fly balls at Petco. The latter is due to a significant imbalance in his innings-pitched home/away splits between 2004 and 2009: 591 innings at Petco versus only 458 innings away.
Therefore, we need to figure out how many fly balls Peavy allows on average. Since 2004, he's allowed 477 flies in 458 away innings (1.04 FB/IP) and 590 flies in 591 home innings (1.00 FB/IP). Basically, he allows almost exactly one fly ball per inning.
If we project Peavy to throw 200 innings, split equally between The Cell and opposing parks, we get something like this:
Home: 100 IP = 100 FB at 13% HR/FB = 13 HR (1.17 HR/9)
Away: 100 IP = 100 FB at 11% HR/FB = 11 HR (0.99 HR/9)
Total: 200 IP = 200 FB = 24 HR (1.08 HR/9)
Of course, this isn't a stone-cold lock. There are other variables to consider, such as the fact that he is moving to a league with a designated hitter, but 24 is a legitimate estimate.
How does that compare to his usual numbers? Well, since 2004, when Petco opened, Peavy has posted a 0.77 HR/9 rate, meaning that, in any given 200 innings, he'd allow about 17 home runs.
So over 200 innings, Peavy should give up around seven more homers than he did with the Padres. That's enough to give his ERA a bit of a bump but not enough to prevent him from remaining an elite pitcher.
Carson Cistulli is an author of FanGraphs.