short rod 347 rod angle = 18.65 degrees
long rod 347 rod angle = 18.35 degrees
whoop dee doo. Not an issue. Rod angles above may not be exactly correct if the wrist pin is off center on the piston which I believe it is.
Rod length does very little to change piston motion unless the rod is SIGNIFICANTLY longer than the shorter rod. Even 1/2" longer will do very little (literally about .015 or so difference max in piston motion throughout the stroke cycle). Ask me how I know...I purposely designed and built a short rod 331 using 5.155 289 length rods and I wrote a program to compare the piston motion of a 5.4 rod 331 vs a 5.155 rod 331. The difference was .012 max at 90 degree crank angle and even less everywhere else. The rod angle is 0.85 degrees more than a 5.4 rod (yet it's the same as a 5.4 rod 347), but when you do the math to determine the increase in side loading ,it's small (about 5%) and my piston has skirts that are more than 5% bigger so the force is spread over more surface area helping to negate the increase in side loading. Longevity was my concern when I designed my 331. I just went road racing (hi performance driving event) and drove 100 miles each way and I didn't burn any oil. The main thing a longer rod buys you is reduced internal friction because the piston skirts are smaller and the rod angle is smaller, but it costs you in higher potential of piston rocking and ring instability (doesn't seem to be a big factor with well built 347s nowadays).
I built my 331 3 or 4 years ago and your options were less than they are today. They may have indeed improved upon the oil ring wrist pin intersection issue with better piston design.
Tracy Blackford: Corona, Ca
'65 FB Mustang 331, 282S cam, ported 351W heads. T5z, 3.50 9" posi.
346 [email protected]
on a warm spring day (335 RWHP SAE corr.)
<font size=-1>[ This Message was edited by: blkfrd on 5/13/06 1:04am ]</font>