Quote:
Originally Posted by Dimman
It's not so much the under/oversquareness, as the total length of the stroke and rpm. That's was affects piston speed, and therefore acceleration. The more acceleration the piston goes through up and down, the more force on all the components. And the stronger and either heavier or more expensive the parts have to be. There's a reason the Mugen redlines a bit less. (Is the Mugen on factory rods?)
And it's making its power partly from a displacement increase (2.16L so it's not huge, but it is bigger). If they increase flow potential from porting, cam lift, whatever, they still need to get more air in, either through more air per revolution (bigger stroke) or more revolutions.
To me it's that the B X S of the FB20 reminds me a bit too much of the 7M (83mm x 91mm), a motor not known to enjoy revs vs the 1JZ with 71.5mm stroke which LOVES to rev. Yes, they're old tech these day, but physics doesn't change...
Edit: Self facepalm... 'factory rods? It's stroked... fail.
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Ahh i get what your saying, yeah a 90mm travel per 2 rpms is pretty worry some, even if it does make power at that travel (which it could) it does make that power strenuous and unstable. But can be easily remedied with engineering even if they retained those same stock FB20 stroke ratio.
But your 7m and 1jz comparison is kind of confusing to me;
7M-GE (83mm x 91mm) 3L I6 engine with a 9.1C/R makes power at 6000rpms
vs
1jz-GE (86mm x 71.5mm) 2.5L I-6 and a 8.5 C/R and makes power at 6000rpms
...where exactly is this high reving?
if we compare the turbo models
7m-GTE has the same stroke but with lower compression to compensate boost making the the power band lower (5600rpms). The 1jz-GTE retains the same compression ratio and later gains compression at 9.0:1 with VVT-i raising the Powerband RPM slightly higher (6300rpms).
Your right about physics but im having a hard time finding the correlation and differences in your reasoning.
Key items that make the difference regardless of stroke in a small total displacement engine is, high compression+low friction.
If the 7m-GE using the same stroke and bore, had 11.0:1- 12.0:1 compression, low friction pistons, stronger rods, a lighter forged crank, and a suitable intake cam lift. It wouldn't surprise me to see the 7m-GE powerband in the 8000-9000rpm range.