by dr20t » Thu Jul 30, 2015 10:08 pm
EJ255 is not the same as EJ257, even though they may appear to be so
Different cases (webbing is different, although same thickness bore sleeve)
Pistons are different (257 has higher silicon content and are hyperteutetic vs cast)
Crank is different. 2011 onward 257 crank is nitrided (hardened) and from memory has cross drilling like the spec c 207 crank.
Tolerances are better in the 257 and more suitable to "higher" output.
255 was a budget assembly build. 257 was the "performance" equivalent. If there wasnt a difference, they wouldn't have put the 255 in the wrx, forester and liberty and the 257 only in the Sti.
Don't get me wrong I'm not suggesting for a minute that the 257 is a super freak (they still have weaker pistons than 207, thin wall linings and still prone to the lower timing threshold required of the 2.5 compared to the ej207 2.0 litre).
The main and most important benefit of having the 257 block however is in my opinion the fact it is a factory assembled block with relatively decent tolerances up to say 250awkw.
Once you split a Subaru block, it is almost impossible, quite literally, to ever get the truances correct. They distort just by splitting them (ie once you remove the case bolts the two case halves are instantly distorted). This is even before any machining (bore/ hone / decking etc). The machining will exacerbate this distortion in an alloy boxer motor, unless your machining is completed by someone who knows exactly what's required (at a minimum, a torque plate to bolt down to the block surfaces, tunnel line boring with the torque plates in place, and an ability to prevent case flex without the bottom of the case halves (where the sump would go) being braced. This is a MINIMUM in my book. Add the cost to properly balance and blueprint once this machine work is done (to do this properly and ensre you get the exact tolerances which have now changed as a result of the machine work), then you have just turned this into a 10k build.
Let me ask you - how many built subaru engines do we see go pop within 15,000-20,000 kms of being "fully built"? How often is this the case? Ask yourself why. Its not because the pistons, rods and crank chosen are inferior parts. Its because the machine work is not up to scratch, or because the components have not been chosen to correct tolerances. Occasionally with big power subie builds you will also see that its due to the thin cylinder walls (if stock sleeves) on the 2.5's, which measure a measley 7.6mm from memory, will flex under high cylinder pressures (especially so with e85). Or if sleeved, will drop a sleeve or ovalise under certain conditions.
This is why I say unless you're going the whole hog on a 2.5 bottom end, then stick to stock, Subaru factory assembled block with decentish tolerances (I'm only talking about the 257 here as I truly believe the 255 in all iterations was rubbish).
Mick
