The cowling is off of the Flying Heritage Collection’s Hellcat at the moment.
The stripped-down nose of the fighter gives you the opportunity to trace the path of the plane’s 10 exhaust stacks (five per side). Since the stacks serve the 18 cylinders of the plane’s Pratt &Whitney R-2800 engine, the math doesn’t quite add up.
Looking closely, four out of the five pipes (per side) carry away the exhaust gasses of two cylinders each — that takes care of 16. Two of the pipes (one per side) only serve one cylinder each. To be specific, one of the “slackers” is the second one from the top in this photo.
The 16, plus two more, give a grand total of all 18 cylinders.