Rod to Stroke ratio changes

[rigidthumper]

 Gentleman:
Any thoughts on whether the decreased the Rod/Stroke ratio is a partial cause of the decreased power output of the newer bikes? On the 4" stroke motors (TC88,1.91:1 R/S) were capable of 1.3 or more HP/cubic inch,  or better numbers, whereas the 4 3/8s stroke motors (TC96, 1.75:1 R/S ) seem to be having a harder time hitting 1.2HP/Ci.

Robin

[uglyDougly]

  Specific power output (HP/CI) isn't a criteria that is very useful. Specific torque is something you can use to compare the efficiency of any two engines.

  When you stroke an engine the peak RPM goes down (as long as you are using the same peak piston speed as a target.) If you've maintained the same specific torque and the peak RPM goes down proportionally, the power will be exactly the same.
  When the H-Ds were originally attempting to run in NHRA Pro-stock with the big-twin based engines, they tried to get an increase of power by stroking them but without increasing the peak pistons speed (and H-Ds run jinormous pistons speeds in drag-race form) they also had to numerically lower the gearing which nullified any torque increase they got from the displacement increase. With the same peak power the larger engine would be exactly as fast as the smaller one.

   About rod-to-stroke ratio. The American V-8 take on that subject is about matching the rod-to-stroke ratio to the heads/cams. If you have control, or decide to take control, over your ports and cams, the rod-to-stroke ratio won't control anything about your engine characteristics.

   The only place where rod-to-stroke ratio is beneficial is in automobile Pro-Stock/Pro-Comp where the CRs are so high that the pistons rapid drop from TDC with a short R-to-S drops the peak pressures quickly enough to avoid the death rattle. I think a longer rod does hold the huge peak pressures longer.
   A friend of mine had to replace his Chevy block and I encouraged him to use 0.100" longer rods with his new Bowtie block, which was that much taller at the deck than his previous block. David Nickens had originally built his engine and he asked Nickens about the rod issue. Nickens assured him that the shorter rods would make more power, and he, of all people, had done the testing. But he didn't say why!

  I think it's about the cylinder pressure, because every other high RPM engine (F1, MotoGP, Superbike) use long R-to-S ratios to keep the max accelerations down.
  None of these other racing classes have such high CRs, and they run for extended periods at those elevated RPMs.

   Doug