the rod ratio in a gasoline engine determines the piston dwell at TDC. the longer the rod, the less downwards piston motion will be the result of the rod angle changing.
if the crank rotates 10 dergees, then a very short rod will be at a greater angle than fi the rod is very long. since the rod doesnt change length then some of the pistons downwards motion will be a result of this phenomenon, so the longer the rod, the longer the "dwell" and the longer the dwell the more complete combustion and higher cylinder pressure. The rod length is limited by other engineering conserns, like bloch deck height, piston height, and thus inherrent stability, and resultant wear. and so on.
In most large cubic inch engines it is hard to get a very high ratio. A Stock 350 Chevy has a 1.64:1 RL/S ratio, which is not very good. By increasing the rod length to 6" the ratio increases to 1.72:1, which is much better. You can squeeze a 6.1" rod in a 350 with little trouble, but longer than that requires plugging the piston pin bores after assembly to support the oil rings. It is not worth the extra expense for the little gains, so a 6" rod in a small-block Chevy has become common because everything fits right in. With endurance engines, longer rods are always better. Most endurance engines are using a RL/S ratio of at least 1.9:1 and some as high as 2.2:1. Before you go out and buy longer rods, let me just say that the gains are very small. This debate has been argued for years and will not end anytime soon. In my opinion, if are building an engine and need new rods and pistons, a longer rod will cost about the same. That makes the small benefits worth it. I would not waste my money buying longer rods if you have a good set of rods that you can use. Use that money to make more power elsewhere in the engine.
Rod Angularity
A longer rod reduces the maximum rod angle to the cylinder bore centerline. Less rod angle will reduce piston side loading; there will be less friction and less bore wear. Less rod angle also gives better average leverage on the crank for a longer period of time. A 5.7" rod with a 3.48" stroke (stock Chevy 350), will have a maximum of 17.774° rod angle. Switching to a 6" rod will reduce that to 16.858°, assuming that the wrist pin has no offset. (on a ford small block, installing the pistons backwards will actually gain almost 20 Hp. since the piston acts like it was mounted on a longer rod, as the ofset is reversed, at the price of increased noise and wear)
Piston Pin Height
A higher pin height will reduce piston rock and aid ring seal. please no screaming and name calling about anything about a tighter ring pack, we are talking about pin height and pin height only. Moving the pin closer the the center of gravity of the pistons makes the piston more stable.
Rings
As the compression height is reduced, the space for the ring pack also get reduced. This can be a problem on some engines. It is good for power to have the top ring as close to the top as possible, but this is limited to the strength of the top ring land. As it becomes thin, it becomes weak. High output engines (especially nitrous engines) need a thicker ring land to keep the cylinder pressure from pinching the top ring. In my opinion, if you have to compromise ring location, it would be better to run a slightly shorter rod.
Skirts
Shorter skirts are usually combined with a shorter piston for a longer rod, but they are not really related. There is no reason to reduce the size of the skirts just because the pin location changed. A shorter skirts are used to reduce friction and lighten the piston. The cost is a little less stability, but it is arguable that a lighter piston with a higher pin height does not need the extra stability. For a street
, I would increase the rod length if it meant a reduction is skirt size. Most of the things listed here are for competition motors to gain a few hp, not worth a lot of effort for most street engines.
Piston-to-Valve Clearance
A longer rod decelerates toward TDC and accelerates away from TDC slower than a shorter rod, so piston-to-valve clearances are tighter with a longer rod. This may require deeper valve relief's in the piston (but probably not). A short rod is just the opposite, there is more clearance because the piston decelerates toward and accelerated away for TDC faster.
Piston Velocity
A longer rod reduces peak piston speeds slightly and delays peak piston velocity until the piston is further down the bore, which gives the intake valve more time to open more. Peak piston velocity is usually somewhere around 75° ATDC and since most cams cannot fully open the cam until at least 106° ATDC, it leaves the valve as a major obstacle when airflow demand is at its greatest. By delaying peak piston velocity, even if it's only 1 or 2 crankshaft degrees, it can allow the valve to open another 0.010-0.015", before peak airflow demand is reached. Not a huge help, but a step in the right direction. With a 350 Chevy, switching to 6" rods from 5.7" ones will delay peak piston velocity from 74.5° to 75.5°.
Piston Acceleration / Deceleration
Reducing piston acceleration / deceleration from and toward TCD will reduce tensile loading of the rod, the number 1 cause of rod failure. A Chevy 350 with 5.7" rods will have a peak piston acceleration rate of 101699.636 ft/sec/sec at 7000 rpm. Swapping in 6" rods will reduce that to 100510.406 ft/sec/sec at that same rpm. That is a reduction of 1189.23 ft/sec/sec.
Intake Runner Volume
Since it is easier for an engine to breath with a longer rod, less runner volume is needed. This allows more room for an intake system (this is a very small gain, but is real).
Exhaust Gas Scavenging
A longer rod is moving slower at TDC, which reduces the speed of the exhaust gasses during the overlap period. This reduces the scavenging effect at low rpm and reduces low rpm power slightly (makes the engine run more cammie). A short rod on the other hand moves faster past TDC and increases the scavenging effect and help low rpm power.
Ignition Timing Requirements
Due to the fact that the longer rod moves past TDC slower, it gives the charge a longer time to burn. So you need less timing for peak power. Using less timing also reduces the chance of detonation; so higher compression ratios can be used. Switching from 5.7" to 6" rods on a 350 Chevy can allow as much as 1 full point increase in compression. In other words, if you could only run 9.5:1 with 5.7" rods, you could run 9.6:1 with 6" rods.
Longer Rod Pros
Less rod angularity
Higher wrist pin location
Helps resist detonation
A lighter reciprocating assembly
Reduced piston rock
Better leverage on the crank for a longer time
Less ignition timing is required
Allow slightly more compression to be used before detonation is a problem
Less average and peak piston velocity
Peak piston velocity is later in the down stroke
Less intake runner volume is needed
Longer Rod Cons
Closer Piston-to-valve clearances
Makes the engine run a little more cammie at low rpm
Reduces scavenging at low rpm
Shorter Rod Pros
Increased scavenging effect at low rpm
Helps flow at low valve lifts (a benefit if the heads are ported with this in mind)
Slower piston speeds near BDC
Allows the intake valve to be open longer with less reversion
More piston-to-valve clearance
Can allow for a shorter deck height
Shorter Rod Cons
More rod angularity
Lower piston pin height (if the deck is not shorter)
Taller and heavier pistons are required (again, if the deck height is not reduced)
More ignition timing is required for peak power
some of this must be attributed to grape ape racing...
