May be a silly question but i have been looking into forged connecting rods and see that they generally come in 2 sizes.

I presume that the std size is 5.7 so what advantages / disadvantages does the longer rod give you.

Many thanks

 

Advantages of longer rod:
Slight increase in engine's "thermal efficiency" (~1% or so higher HP possible)
Slight decrease in side loading of piston against bore (rod angle decreased by a degree and a fraction, depending on stroke; near 2 degrees with a 4" stroke)
Allows internal balancing of crankshaft in long-stroke applications, by increasing max possible size of counterweights

Disavantages:
Cost (although if you're already going to buy a high-cost rod, there is often no difference in price)
Piston cost (similar to above.... the long rod automatically puts you into "custom" piston territory)
Rings are narrower and/or closer together and/or located above the wrist pin, reducing their long-term survival and effectiveness
Weight (mots of the time, rod material is heavier than piston material; not a big factor, but you know how every little bit counts)


The cost difference is mainly because of the step up from "stock" configuration parts, to "custom" parts. If your build already involves forged aftermarket rods and forged racing pistons regardless of rod length, you're already there; so there is little or no cost increase.

People talk about clearance issues; with properly designed and chosen rods, this is not an issue, except in stroker applications.

 

Thank you very much for the reply

 

A longer stroke will also cause the average piston velocity to be higher than a shorter stroke, putting more stress on the internal components.

 

He isn't asking about stroke length.

-Tom

 

Thanks for correcting me. I'm running on about an hour of sleep today and I can't think straight .

fin170703, longer rod length will actually reduce the mechanical stress because the instantaneous velocity is slightly less at TDC (its acceleration and deceleration at the top of the bore isn't as much as with a shorter rod). I doubt that makes much of a difference on a street motor, though.

 

TDC and BDC isn't where the stress is greatest anyway. That's a total non-issue.

Of course, instantaneous piston velocity at both BDC and TDC is ZERO; it stops, and reverses direction.

A longer rod causes it to "dwell" around the top and the bottom for longer, and to move more rapidly at the halfway points (the point of max velocity). The longer "dwell" at TDC is what creates the slight increase in "thermal efficiency"; because the compressed gas has a little bit longer to turn its heat energy into mechanical energy.

It makes very very very little difference on ANY kind of motor. All of the effects on HP output and that sort of thing, are just little tweaks. About all you can say is, that if you build 20 otherwise identical motors and use longer rods (and therefore shorter pistons) in the half of them than in the other half, the half with the longer rods will make, on average, slightly more power. Not necesarily each one in each group will make more or less than the other group; but on average, they're just measurably different.

The main reason for running a longer rod is to allow internal balancing with longer stroke, which requires larger counterweights, which need the longer rod in order to "fit" under the piston at BDC. And that is a major ease-of-use upgrade, regardless of the power output or anything else.

This is one of the most poorly understood design issues in all of this hobby. People get this stuff out of magazines or whatever, and if they have no scientific or engineering background (or sometimes even if they do) it gets all twisted and distorted in the re-telling. It has achieved the status of black magic and voodoo among the hobbyist crowd, most of whom have no clue what's really going on.