Actually rod length to stroke ratio. 1.9 to 1 is the number I remember as optimum. The rod length to stroke ratio effects piston speed. Does this relate directly to Volumetric efficiency? I have the answer buried in a pile of magazines but this way someone else might actually learn something.
I've been told that the secret to this increased efficiency is the longer "dwell time" at top dead center with a rod closer to "optimum" ratio. It's supposed to allow a better burn time for the charge, thus more complete combustion and better efficiency. I run the longest rods I can without putting the wrist pin on top of the piston.
good start, but it depends solely upon the motor you have chosen...for example, alot of big blocks are already "oversquare", and therefore don't gain anything by adding length to the rod. this is why a Pontiac 462 stroker will make more power than a pontiac 455 bored 30 over (463?) the change in ratio is alot different due to the difference in stroke. That said, a 6 inch rod in just about any small block chevy is worth power, so long as the piston on top of it is high quality.
To directly answer the question, rod length DOES have an effect on VE, but there are a LOT of other factors involved- port efficiency, port entry angle, chamber design, piston top profile, intake valve back design & seat angles, intake valve opening & closing points, intake lobe acceleration, rate of change of that acceleration....oh yeah, and intake manifold design & carburetor venturi size/design etc. Whew! The major affecting factor to VE is the relationship of the stroke length to the efficiency of the complete intake tract. Rod length has more of an effect on piston speed at certain places in the piston travel. The piston still goes the same distance, in other words; but changing the length of the rod speeds it up/slows it down in different places, something like a stock car going around turns/going down the straight. For example, if you had a situation where you felt an engine might be susceptible to detonation (and wasn't computer-controlled), you would likely use a shorter rod, because it gets the piston away from TDC more quickly. As Retrorod said, a longer (comparatively) rod tends to keep the piston at or around TDC for a greater period of time. If you have a chamber that's not very efficient, this might be helpful. My opinion on all of this is, for what it's worth, that there has been WAY too much emphasis on rod length over the past few years. There are enough good cylinder heads with modern port design & efficient chambers; the newest cam profiles; piston top designs; & piston/crank stroke/rod length options that you can get almost any powerband you want without sacrificing other factors just to get a hypothetical optimum rod length. Warren Johnson said, in a conversation with Jon Kaase, "At the end of the day, the rod is there to connect the piston to the crank; nothing more, nothing less." Kaase added that people are getting too obsessive about it (optimum rod length). Given all else being equal, with no limits on engine size, and assuming I can get the intake port flow & velocity to feed it, I'd prefer to increase the stroke & sacrifice a little rod length to get it if I have to, with a reasonable pin location. Rod length is usually (NOT always) a lower priority. It all boils down to the intended use of the engine; there's no "universal" perfect answer. A typical short track 355 SBC may want longer rods than one built for, say, a one mile track with no restrictor plates.
Volumetric efficiency has much more to do with cam timing, port/valve sizes than rod length. The whole long rod discussion is more to do with getting the most power (or more power than the other guy) when you are stuck with a stock or spec cam. The stroke stays the same but what changes is the time period spent at TDC and BDC - the piston pauses a bit longer at the ends of the stroke giving a limited overlap cam more time to take advantage of any extraction effect when both valves are open.
There is the issue of side loading the piston w/short rods also. Thats why small block strokers don't live to long on the street. The side loading not only causes some additional friction on the piston skirts, they tend to rock a little, [short piston] and that has an effect on ring life. Could have an effect on ring seal as well. Thats probably another good reason to run a longer rod. Or so I've been told. -Mike
Smokey Yunick was a long rod advocate. Read his power secrets book. SBC 327 was about the perfect all 'round combo. Quick reving and high RPM's, but still had some bottom end. A Z-28 had same length rods and shorter stroke. Again, quick revin' and ultra high rpms. Not much on bottom end. GM stroked the 327 to 350, used the same length rods and screwed it up. Racers fixed GM's screwup by installing 6" rods in a 350. Same thing for a 427 increased to a 454. GM used the same length rods in both engines. A SBC 400 has real short rods and was a good torque engine. Used in heavy Impalas and pickups. Will rev better with longer rods, but ya' loose bottom end. All depends upon whatcha' want to do.
Well, I wanted to pose a question that people might be interested in. This thread got some good input, and had quite a few people check it out....all while a ceertain O/T post was allowed to quickly disappear to page ten where it belonged. A few of us learned something about building engines and a few of us learned something about letting go and letting O/T posts die. "Thank you for your support" BTW....did you get that small journal crank...or is there a chance that I can get to the next meet before you and snap it up?
The Jalopy Journal
