times. >Connecting Rod Length Comparison > >By Rick Draganowski >dragan@harborside.com > > >Piston movement was computed by simulating the crankshaft /connecting rod >/piston assembly in several precise engineering drawings (DesignCad) and >then determining the exact amount of piston movement for each of 256 >divisions of one rotation. > >The piston movement data was then used as an input vector in a MathCad >program to calculate velocity, acceleration, and dynamic forces.. > >The simulation of an infinitely long connecting rod which imparts true >harmonic motion to the piston is the starting point for comparision >purposes. > >The motion generated by a finite length connecting rod is quite distorted >by comparison. It has much more velocity and acceleration at the top of the >stroke compared to the bottom. A graph of the movement is peaked at the >top of each cycle and rounded and flattened at the bottom. This is caused >by the rod angle increasing and pulling the piston down and adding to the >motion caused by the crankshaft rotating down from top dead center. At the >bottom as the rod journal slows the angle decreases. This retards the >movement of the piston by subtracting the rod angle component which was >added at the top of the stroke from the crankshaft movement component at >the bottom of the stroke. > >Compression and combustion pressures are in opposition to the inertial >forces so the top of exhaust and intake strokes generate the largest forces >on the rod. > > >1) Maximum Piston Acceleration > >This table is for a 3.75" stroke used in a 400 0r 383 small block Chevy >engine. >------infinite rod--6.0" rod---5.7" rod---5.565" rod >5000rpm 1332G 1749G 1776G 1790G >6000rpm 1933G 2525G 2558G 2578G >7000rpm 2631G 3437G 3482G 3509G > >Percent difference due to rod length in above table. >Difference between 6" rod and 5.565" rod 2.34% >Difference between 6" rod and 5.7" rod 1.54% >Difference between 5.7" rod and 5.565" rod 0.79% > > >This table is for a 3.48" stroke used in a 350 or 305 small block Chevy >engine. >------infinite rod--6.0" rod---5.7" rod >5000rpm 1240G 1600G 1623G >6000rpm 1786G 2305G 2338G >7000rpm 2432G 3138G 3182G > > > >2) Maximum Connecting Rod Dynamic Load (Tension) > >This table is for a 3.75" stroke used in a 400 0r 383 small block Chevy >engine. The forces are based on the weight of the piston and pin assembly >and does not include the percentage of force generated by the acceleration >of the end of the connecting rod. The reference piston is the stock >replacement Silv-O-Lite piston for a 400 engine. > >------infinite rod--6.0" rod---5.7" rod---5.565" rod >5000rpm 2249LBS 2938LBS 2976LBS 3000LBS >6000rpm 3239LBS 4232LBS 4287LBS 4320LBS >7000rpm 4409LBS 5769LBS 5834LBS 5849LBS > >Percent difference due to rod length in above table. > >Difference between 6" rod and 5.565" rod 2.34% >Difference between 6" rod and 5.7" rod 1.54% >Difference between 5.7" rod and 5.565" rod 0.79% > > >3) Maximum Rod Angularity > >This is the angle the conecting rod makes with the axis of the cylinder >bore at 90 degrees after top dead center (maximum excursion from bore axis. >This measurement is for the 3.75" stroke of the 400 and 383 only. > >6.0" rod---18.21 degrees >5.7" rod---19.20 degrees >5.565" rod-19.69 degrees > > >4) Cylinder Wall Load > >Percentage of compression and combustion force against the top of piston >transmitted to the major thrust face of the piston and then to the cylinder >wall. > >This table is for the 3.75" stroke. >6.0" rod---32.89% >5.7" rod---34.83% >5.565" rod-35.64% > >This table is for the 3.48" stroke. >6.0" rod---30.31% >5.7" rod---32.05% > >5) Piston Speed > >Maximum piston speed for the 3.75" stroke at 5000 rpm. > >infinite rod--81.68 feet per second, 55.69 MPH >6.0" rod------85.64 feet per second, 58.4 MPH >5.7" rod------86.01 feet per second, 58.6 MPH >5.565" rod----86.20 feet per second, 58.8 MPH > > >6) Effective Stroke > >Because of the mechanical advantage provide by the toggling effect of the >rod the shorter rods act as if they were in a longer stroke engine at the >top of the stroke. This effect would make the short rod engine rev faster >from 2000 to 4000 rpm and the circle track people claim that acceleration >out of the turns is signifigantly improved with the shorter rod. In all >other factors the longer rod comes out superior... > >Effective stroke as compared to the infinite rod model for the 3.75" >stroke. > >infinite rod-- 3.75" >6.0" rod------ 4.20" >5.7" rod------ 4.23" >5.565" rod---- 4.25" > >Note that the differences are subtle... > > > >7) Dwell Time > >This measurement is of the number of crankshaft degrees the piston is >within 0.250 inches of top dead center. It is the subject of much >conjecture and controversy in the automotive literature. > >This table is for a 3.75" stroke used in a 400 0r 383 small block Chevy >engine. > >Infinite rod--59.853 degrees >6.0" rod------52.397 degrees >5.7" rod------52.071 degrees >5.565" rod----51.915 degrees > >Percentage difference in dwell time between the 6.0" rod and the 5.7" rod >is 0.626%. > >Percentage difference in dwell time between the 5.7" rod and the 5.565" rod >is 0.3%. > >Percentage difference in dwell time between the 6.0" rod and the 5.565" rod >is 0.928%. (Still less than 1 percent) > > >This table is for a 3.48" stroke used in a 350 or 305 small block Chevy >engine. > >Infinite rod--62.188 degrees >6.0" rod------54.929 degrees >5.7" rod------54.605 degrees > >Percentage difference in dwell time between the 6.0" rod and the 5.7" rod >is 0.593% at the 3.48" stroke. > >8) Authors comments. > >The data in this report seems to indicate that the differences between the >rod lengths are exaggerated in the literature. In many (most) cases claims >are anecdotal and represent the vested interests of the suppliers. I have >seen no objective dyno testing of rod lengths but keep hoping for one. > >There are real gains to be had by going to longer rods but they are small, >usually a lot less than 2 percent. However, the hard core racers are >grasping at every tiny bit of performance and can justify the expense. For >the more average rodder I would suggest staying with the rod length >specified by the factory. Money would be far better spent on improving >the heads, cam, and induction and exhaust systems. (and perhaps a >supercharger..) > >-------- >Chevrolet Nova Internet Source Listserv >http://nova.smu.edu >---------- >From: Gary Sell >Sent: Saturday, April 24, 1999 8:52 PM >To: Rick Draganowski >Cc: 'Nova List' >Subject: Re: [nova] Draganowski's Rod Study > > Rick, > Thanks a ton for the all the work you went through to provide us > with this information. I know most of us, (if not all) appreciate > your abilities and willingness to share them with us. I just > printed your findings, and if it's O.K. with you, I'd like to > share them with the guy I use for my machine work. He builds > 1/4 mile, circle track, and Pike's Peak climber engines and > has been trying to keep me out of the 6"/ 6.125" rod candy > stores. He has run dyno comparisons on available rod lengths > and your data backs his up to the letter. > My last go-round with him was with putting my 406 together. > I was sure I needed 5.7 rods, and felt that I should go > with 6" rods since I had to make the change. He all but > insisted I stay with the 5.565 rod. That's when he shared > with me that the average smallblock bruiser, (400/ 450 > real horsepower) only gains about 2 to 5 horsepower from > this modification. This guys' word is gospel around here, > but I sure am glad to hear your results were the same. > > Thanks Again, > Gary > '66 Coupe
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