|
The Wallace Racing site has a simple online calculator that let's you submit the engine stroke and RPM and it will spit out the mean piston speed here: ww.wallaceracing.com/calc-pistonspeed.php
-I can't post links yet, so add one more W to the beginning of that and copy/ paste it.
These simple calculations only look at the MEAN /AVERAGE piston speed, though the piston velocity at each crank angle changes, though and depends upon the rod length / rod ratio, too.
A lot of the old rules-of-thumb of safe max piston speed are just that and based upon old assumptions on materials and things like piston weight, and modern materials are moving the actual limits faster and faster.
When it comes to fuel economy and balancing fuel economy and performance things get murkier; you can go with a larger cylinder bore and keep piston speed the same and you'll make more power from having more cubic inches, but the larger bore makes for a wider combustion chamber that requires more ignition advance / a burn that takes longer to accomplish and you'll have more negative work and reduced efficiency from that wider chamber. You could instead choose to go with a longer stroke and the same initial engine bore and you'd have more efficient combustion because of the more compact chamber -this would increase the piston speed / FPS at the same RPM as the original unstroked engine.
Increased stroke AND increased bore size increase the friction between the rings and the cylinder walls and oil ring friction is the largest source of friction in an internal combustion engine.
IMO, Increases in stroke or bore should be paired with thinner piston rings to reduce friction.
With wedge-type heads that have a decent quench area and quench distance, the increase in piston speeds increases the max quench velocity which can increase the detonation resistance of an engine and allow higher "dynamic" compression / cylinder pressures and reduced friction + the ability to support higher max cylinder pressures without detonation can together help increase fuel economy. -Low RPM eco-focused SBCs have particularly poor quench velocity at their low RPM torque peaks where knock / detonation risk is highest and rebuilding with a tight quench and more piston speed can help them to run with more cylinder pressure / compression on a given octane fuel.
Adam
|