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Old 09-01-2019, 09:19 PM   #23
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Join Date: Apr 2015

Originally Posted by WRXnick16 View Post
I haven't seen much info on the FA24 engine design. Prime Motoring has only mentioned that Subaru improved upon the FA20's weak areas like the rods.

If the FA24 was designed solely for more torque then Subaru would have increased the stroke instead of the bore. The FA20 is a square engine (86mm bore & 86mm stroke). The FA24 is oversquare engine (94mm bore & 86mm stroke). This is more typical with high performance, higher revving engines. The EJ257 is an oversquare engine as well (99.5mm bore & 79mm stroke). The EJ257 is limited by the UEL header & head design (compared to the EJ207 - 92mm bore & 79mm stroke). The smaller stroke allows for lower piston speeds at high RPMs. Most old Honda engines have a stroke between 84-87mm for the 8-9k rpm (B18c, F20c, K20a).

The Ascent's FA24 is probably hp limited by the small WRX turbo, stock tune, intake manifold, tmic, etc.
Straight from Subaru, the FA24 was designed for higher torque not horsepower. Main goal was towing capacity and low end torque. The rods are not forged, they are the same J shape design and material as FA20, just larger. The FA20 has a very high compression ratio for a turbo engine. The cylinder pressure can get out of control, that is why good quality gas is key and also why you see some people make 400 whp for thousands of miles and some toss rods on the stock tune.
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