Quote:
Originally Posted by Dimman
I'm beginning to consider this motor as an extension of the auto-tuned BEAMS 3SGE, which produced 197 bhp @ 7000 rpm and 159 lb-ft @ 4800 rpm. Same 86mm X 86mm BxS, same developer Yamaha.
The BEAMS did those numbers on 11.1:1 compression, port fuel injection, and bucket/shim lifters, without some of the inherent better balance/vibration elements of the H4.
We're looking at 12.5:1 CR with significantly better combustion from the D4-S (which, coincidentally, Yamaha, along with Denso and Toyota was a primary developer on) reduced valvetrain friction with finger followers on needle bearings, and the better balance and vibration properties of the H4.
168 lb-ft is realistic.
Also given the 86mm stroke, 7500 rpm (redline has been listed at 7400) put the mean piston speed at around 4200 ft/min which is often the upper range of production cars, especially those that involve a cost factor.
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Now that you mention it, I'm starting to think you may be onto something. The 3S didn't have variable lift afaik...
HOWEVER that makes 197hp peak even more fishy. Something isn't right.
As for the rpm, I computed a rod:stroke ratio of 1.55, which may or may not be an issue. Using simple geometry, increasing it to 1.60 which a lot of 8000rpm engines use decreases bearing load by about 3% which isn't much, but there's a reason they set the limit there.