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Engine design question

700 Views 3 Replies 3 Participants Last post by  frankamartin
  • Bore-Stroke
  • 94mm - 90mm; 625cc per cylinder; 2,498 cc total displacement; 182 hp -- FB25D
  • 94mm - 86mm; 597cc per cylinder; 2,387 cc total displacement; turbocharged 260 or 271 hp -- FA24F
Looking at the engine design of the two most popular engines in Subaru's lineup where the 182 hp engine will be used in the RS trim of the 2024 Impreza, one question seems obvious. Why not make the cylinders of the two engines identical? You would think the very slight difference in stroke length of 4 mm wouldn't make that much difference in fuel economy, so why have the logistic issues of making two engines when you could have one engine either with or without a turbocharger..
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An engine is more than just it's measurements. The FA24F is built to withstand the added stress of forced induction, so it's going to be heavier and more expensive than it needs to be for a small car application.
I just assumed that the Subaru FB25D and the FA24F were about 80% the same. After all you can purchase a Legacy or an Outback with either engine. Only the WRX and the Ascent can only be purchased with the FA24F. But, I suppose the question was naive.

The Hyundai Elantra and the Hyundai Elantra N have different bore and stroke. One is a long stroke design and the other engine is square.

2023 Hyundai Elantra
2.0L MPI Atkinson DOHC 16-valve Inline 4-cylinder
Displacement: 1,999 cc
Horsepower (HP): 147 @ 6200
Torque (lb.-ft.): 132 @ 4500
Bore & Stroke (mm): 81 x 97
2023 Hyundai Elantra N

2.0L turbocharged direct injected “Flat Power” 4-cylinder
Displacement: 1,998 cc
Horsepower (HP): 276 @ 5500 ~ 6000
Torque (lb.-ft.): 289 @ 2100~4700
Bore & Stroke (mm): 86 x 86
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