ok, i'm no expert here, but i'll just make a quick couple of quotes:
warren "the professor of pro stock" johnson - a connecting rod is there simply for nothing more than to link the piston to the crankshaft.
reher-morrison - We also wanted to point out some of the common myths and misconceptions about high-performance motors. For example, I've seen dozens of magazine articles on supposedly "magic" connecting rod ratios. If you believe these stories, you would think that the ratio of the connecting rod length to the crankshaft stroke is vitally important to performance. Well, in my view, the most important thing about a connecting rod is whether or not the bolts are torqued! If I had to make a list of the ten most important specifications in a racing engine, connecting rod length would rank about fiftieth. Back in the days when Buddy Morrison and I built dozens of small-block Modified motors, we earnestly believed that an engine needed a 1.9:1 rod/stroke ratio. Today every Pro Stock team uses blocks with super-short deck heights, and we couldn't care less about the rod ratio. A short deck height improves the alignment between the intake manifold runners and the cylinder head intake ports, and helps to stabilize the valvetrain. These are much more important considerations than the rod-to-stroke ratio. There's no magic - a rod's function is to connect the piston to the crankshaft. Period.
ed iskenderian - Rod Lengths/Ratios: Much ado about almost nothing.
Why do people change connecting rod lengths or alter their rod length to stroke ratios? I know why, they think they are changing them. They expect to gain (usually based upon the hype of some magazine article or the sales pitch of someone in the parts business) Torque or Horsepower here or there in rather significant "chunks". Well, they will experience some gains and losses here or there in torque and or H.P., but unfortunately these "chunks" everyone talks about are more like "chips".
To hear the hype about running a longer Rod and making more Torque @ low to mid RPM or mid to high RPM (yes, it is, believe it or not actually pitched both ways) you'd think that there must be a tremendous potential for gain, otherwise, why would anyone even bother? Good question. Let's begin with the basics. The manufacture's (Chevy, Ford, Chrysler etc.) employ automotive engineers and designers to do their best (especially today) in creating engine packages that are both powerful and efficient. They of course, must also consider longevity, for what good would come form designing an engine with say 5% more power at a price of one half the life factor? Obviously none. You usually don't get something for nothing - everything usually has its price. For example: I can design a cam with tremendous high RPM/H.P. potential, but it would be silly of me (not to mention the height of arrogance) to criticize the engineer who designed the stock camshaft. For this engine when I know how poorly this cam would perform at the lower operating RPM range in which this engineer was concerned with as his design objective!
Yet, I read of and hear about people who do this all the time with Rod lengths. They actually speak of the automotive engine designer responsible for running "such a short Rod" as a "stupid SOB." Well, folks I am here to tell you that those who spew such garbage should be ashamed of themselves - and not just because the original designer had different design criteria and objectives. I may shock some of you, but in your wildest dreams you are never going to achieve the level of power increase by changing your connecting rod lengths that you would, say in increasing compression ratio, cam duration or cylinder head flow capacity. To illustrate my point, take a look at the chart below. I have illustrated the crank angles and relative piston positions of today's most popular racing engine, the 3.48" stroke small block 350 V8 Chevy in standard 5.7", 6.00", 6.125" and 6.250" long rod lengths in 5 degree increments. Notice the infinitesimal (look it up in the dictionary) change in piston position for a given crank angle with the 4 different length rods. Not much here folks, but "oh, there must be a big difference in piston velocity, right?" Wrong! Again it's a marginal difference (check the source yourself - its performance calculator).
To hear all this hype about rod lengths I'm sure you were prepared for a nice 30, 40, or 50 HP increase, weren't you? Well its more like a 5-7 HP increase at best, and guess what? It comes at a price. The longer the rod, the closer your wrist pin boss will be to your ring lands. In extreme situations, 6.125" & 6.250" lengths for example, both ring and piston life are affected. The rings get a double whammy affect. First, with the pin boss crowding the rings, the normally designed space between the lands must be reduced to accommodate the higher wrist pin boss. Second, the rings wobble more and lose the seal of their fine edge as the piston rocks. A longer Rod influences the piston to dwell a bit longer at TDC than a shorter rod would and conversely, to dwell somewhat less at BDC. This is another area where people often get the information backwards.
In fact, this may surprise you, but I know of a gentleman who runs a 5.5" Rod in a 350 Small Block Chevy who makes more horsepower (we're talking top end here) than he would with a longer rod. Why? Because with a longer dwell time at BDC the short rod will actually allow you a slightly later intake closing point (about 1 or 2 degrees) in terms of crank angle, with the same piston rise in the cylinder. So in terms of the engines sensitivity to "reversion" with the shorter rod lengths you can run about 2-4 degrees more duration (1-2 degrees on both the opening & closing sides) without suffering this adverse affect! So much for the belief that longer rod's always enhance top end power!
Now to the subject of rod to stroke ratios. People are always looking for the "magic number" here - as if like Pythagoras they could possibly discover a mathematical relationship which would secure them a place in history. Rod to stroke ratios are for the most part the naturally occurring result of other engine design criteria. In other-words, much like with ignition timing (spark advance) they are what they are. In regards to the later, the actual number is not as important as finding the right point for a given engine. Why worry for example that a Chrysler "hemi" needs less spark advance that a Chevrolet "wedge" combustion chamber? The number in and of itself is not important and it is much the same with rod to stroke ratios. Unless you want to completely redesign the engine (including your block deck height etc.) leave your rod lengths alone. Let's not forget after all, most of us are not racing at the Indy 500 but rather are hot rodding stock blocks.
Only professional engine builders who have exhausted every other possible avenue of performance should ever consider a rod length change and even they should exercise care so as not to get caught up in the hype.
sorry for such a long post, but i feel it's worth posting. if you get caught up in all this stuff about rod length in your engine, i honestly believe that you're wasting money and time with something that is pretty much futile. spend your money on high quality parts, and match the parts that count, intake system, camshaft, compression, exhaust, etc...............just my $5.00 (adjusted for inflation, 2 cents just doesn't get what it used to anymore.....)