Started a new thread for this, four graphs, piston velocity and acceleration for 4 dif rod lengths same stroke, and 3 dif stroke same rod length. Pretty clear rod length has little effect on velocity or acceleration. If you go from like a 5" rod to a 20" rdo then yes Piston velocity will change from like 86 fps to 93 fps on a 4" stroke but the difference between a 6.135 rod and 6.535 rod is very little

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Very nice data. Is there any way to determine the effect of rod length on torque or rpm at the crankshaft for a given piston velocity?
This is interesting to me because I've been considering how to get a lot of HP out of a large cubic inch, low rpm engine (ie 4600rpm, 540 supercharged motor). In addition to the effect of rod length on piston time near TDC, I was curious about the effects on piston speed and torque.
Thanks for the info.
Gary

Based on own experiences with engines, found that longer rods provide more low-end torque and broader torque curve, but tend to be less rpmish engines and short rods provide lesser torque and being more rpmish.

Found this on R/C engines and the differences are quite noticeable (what you think on a .12cid engine two stroke capable of pulling 1.3Hp @ 36000 (yes, thirty six thousand rpms)? Different rods make different power curves and provede more or less torque, depending your needs.

I think crank arm length (stroke) has a larger affect on torque than rod length. When you change rod length without changing the crank or piston, all you are doing is changing the compression ratio. It may give you a larger "pop" but the mechanical advantage the piston has on the crank comes from stroke. This is why the straight six engine has the reputation as a low rpm high torque motor since most manufactured, by US auto companies, were long stroke engines. Lots of grunt.

You can see the trade offs engine designers make every day. You want to minimize piston velocity. That is the limiting factor for reciprocating engines. Piston acceleration is not as important as the wrist pin, rod and bearings are under higher loads during compression and combustion than when they change directions. At least on engines running less that 8,000 or 9,000 rpm. Piston velocity is important due to sliding friction and lube film issues.

I guess I missed the first thread......what are you trying to accomplish? If you want to grow torque at a lower rpm, lengthen the stroke. If you want more rpm, shorten the stroke. Then choose the rod length that gives you the compression ratio you want to run.

So am I full of it or what

Thanks for the charts! They are very good!!!
There are more issues than just piston speed. The best place for the wrist pin center is in the plane of the piston top. That being impossible, the higher the better. You must leave room for a proper ring package, but I usually add up all the numbers and go for the longest rod I can conveniently get.
Next, the longer the rod, the better the side loading on the piston skirt. If you built two engines just alike except the rod length, and ran them for a long period of time under load, the one with the longer rod would show the lesser cylinder wear. It MAY show more power due to less piston weight due to shorter skirts, less rock in the cylinder(better ring seal) etc.
Keep it coming, Guys, I love this stuff!!

ursus - cool data.
Hard to argue with the curves. I see that rod length has far less impact on piston accel that I had imagined. Looks like it all boils down to piston dwell at stroke extremes, piston and bore wear, side loading, and friction loss from same...

ANdrea,
Am familiar with the rod length issues you mention on RC motors. What you're getting though is more to do with port timing. In a 2 stroke motor, raising the port package (via cylinder spacer OR short rod) raises the exhaust port and tunes the motor for more topend. Lowering the package (less spacer or longer rod) does the opposite.
Adding a cylinder spacer AND installing a long rod makes for a better low-mid package cause it changes the port dwell in relation to crankshaft position...

All of this creates dramatic performance alterations in a 2-cycle engine but in the 4-cycle applications we are discussing, induction and exhaust timing are completely unchanged (although piston dwell at stroke extremes are slightly altered with a rod length variations)

ursus

Thanks for the graphs.. makes it easy to understand the math!!
I built the long rod motor in hopes of finding octane insensitivity.. In other words, run regular gas at 10:1. I have not experimented, but at the onset of this project I was lead to believe that a longer rod motor could be run on less octane with the same compression ratio..
Has anyone found this to be true??
For those that didn see the first thread I built a 565" motor with 1.88 rod ratio, that is an 8" rod on a 4.25 arm. Pin height is 1.537"

Thanks
Dick