grumpyvette
Well-known member
look here
http://www.iskycams.com/ART/techinfo/ncrank1.pdf
now look at the cylinders pressure curve,
http://www.auto-ware.com/combust_bytes/pv.htm
notice that almost all useable pressure is in the 25 degrees after TDC (right were the longer rods longer dwell time can use it)
what youll find if you carefully look into it is that the longer 6" rod has a SLIGHT ADVANTAGE over the 5.7" rod for several reasons , one is that the 6" rod has a longer DWELL time , what this means is that it takes slightly longer in rotational degrees for the 6" rods piston to move away from TDC thus allowing a slightly more effective use of the cylinders presure peak, that together with the better rod angle produceing less cylinder to piston drag allows the 6" rod spinning the same rpms as a 5.7" rod to produce slightly more power in the over 5000rpm to 8000rpm range IF the cam timeing and compression ratio are the same and about 1%-2% more if the cam timeing and exhaust scavageing are set up to take advantage of the longer dwell time. now if your not sure what Im talking about look at it this way, at 7000rpm the piston changes direction 233 times a second and has a power stroke 116 times a second now that means youve got a power stroke every 116th of a second so the longer rods slower acceleration away from the tdc possition allows the longer rods piston to more effiecently use of the cylinder pressure peak. to put it bluntly the 6" rod has a slight but provable advantage, or to paraphraze SMOKEY YUNICK " use the longest rod you can economically fit in the engine block for the best power curve"
http://www.iskycams.com/ART/techinfo/ncrank1.pdf
now look at the cylinders pressure curve,
http://www.auto-ware.com/combust_bytes/pv.htm
notice that almost all useable pressure is in the 25 degrees after TDC (right were the longer rods longer dwell time can use it)
what youll find if you carefully look into it is that the longer 6" rod has a SLIGHT ADVANTAGE over the 5.7" rod for several reasons , one is that the 6" rod has a longer DWELL time , what this means is that it takes slightly longer in rotational degrees for the 6" rods piston to move away from TDC thus allowing a slightly more effective use of the cylinders presure peak, that together with the better rod angle produceing less cylinder to piston drag allows the 6" rod spinning the same rpms as a 5.7" rod to produce slightly more power in the over 5000rpm to 8000rpm range IF the cam timeing and compression ratio are the same and about 1%-2% more if the cam timeing and exhaust scavageing are set up to take advantage of the longer dwell time. now if your not sure what Im talking about look at it this way, at 7000rpm the piston changes direction 233 times a second and has a power stroke 116 times a second now that means youve got a power stroke every 116th of a second so the longer rods slower acceleration away from the tdc possition allows the longer rods piston to more effiecently use of the cylinder pressure peak. to put it bluntly the 6" rod has a slight but provable advantage, or to paraphraze SMOKEY YUNICK " use the longest rod you can economically fit in the engine block for the best power curve"
