Longer con rods extend the time the dwell time the piston spends at TDC, in effect it gives the mixture more time to burn (under compression) and allows for the flame front to expand completely before the piston begins to travel down the cylinder bore.
The reason why this is good is it will increase the time available to extract the energy from combustion which allows the engine to achieve a higher volumetric efficiency, the only downside to longer rods are the added weight to the reciprocating assembly and the packaging restraints of having long con rods.
There are ideal rod stroke ratios for different fuels but in the automobile 4 stroke they are nowhere near the ideal length and it is unlikely that you could ever get a long enough rod into any conventional automotive engine. You can see this when you compare automotive engines to stationary engines where there are no packaging problems, of course rpm limit adds another dimension to the problem as a longer rod weighs more and is weaker as a result at high rpm.
I seem to remember reading that an ideal rod/stroke ratio for gasoline is in the range of 6:1, this would mean a rod length of 21 inches in a small block Chevy or Ford of 350 ci displacement!! Just a little impractical in the real automotive world, but very possible in a generator set operating at low rpm.
Check out this link to see how bore/stroke/rod lengths compare in aircraft and stationary engines.
<a href="http://www.prime-mover.org/Engines/index.html" target="_blank">http://www.prime-mover.org/Engines/index.html</a>
Don't miss the 100 000 HP Sultzer diesel engine.
A long rod ratio is a good thing for piston dwell and a little bit of more power, but there is another bad thing to it besides being heavier. In the case of a 347 (302)stoker kit, they usually come with a 5.4 inch rod, as oppsed to a 5.09 inch stock rod. With this rod in the stroker kit, the piston pin is pushed up into the oil control ring. So what does this mean? Well in some cases ive read that this creates unstable (wobbly) pistons at higher RPM ranges and burns oil. They make connecting rods for a 347 with a 5.31" rod which has the piston pin below the oil ring, where god and the rest of us determined it should be. Sure, the 5.4 inch rod has a better ratio, but i'll give you a million bucks if any dyno in the country could tell the difference between the two. So, longer is not always better.