Rod Ratio

The rod ratio in an 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 say 10 degrees, then a very short rod will be at a greater angle than if the rod is very long. since the rod doesn't 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 because of the longer dwell, the more complete the combustion and the higher the cylinder pressure. The rod length is limited by other engineering concerns, like block deck height, piston height and thus inherent stability, and resultant wear and so on. The shorter blocks like the KR and 9A, have a shorter rod than the later tall 2E (8v) and ABF, and it is known that the 9A in particular suffers with piston slap due to its longer stroke.

Rod Angularity

A longer rod reduces the maximum rod angle to the cylinder bore centreline. Less rod angle will reduce piston side loading (as found in the 9A); 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. Take the 9A engine. The 144mm rod with a 92.8mm stroke, will have a maximum of 18.797° rod angle. The ABF in comparison has a 159mm rod that yields a better maximum rod angle of 16.967°.

Piston Pin Height

A higher pin height will reduce piston rock and aid ring seal. Moving the pin closer to the the centre of gravity of the pistons makes the piston more stable.

Piston 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.

Piston Skirts

Shorter piston 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. 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 car, it is suggested that an increase of the rod length would allow a reduction is skirt size.

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.

Piston Acceleration / Deceleration

Reducing piston acceleration / deceleration from and toward TDC will reduce tensile loading of the rod, the number 1 cause of rod failure. Again this can be done by using longer rods. E.g. [TODO]

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 lumpy). 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.

Longer Rod Advantages

Shorter Rod Advantages

Most of the things listed above are suggested to gain a few BHP for competition motors and not really worth the effort for most street engines, but the principles can help with the choice of components for both your N/A 16v and indeed your 16v turbo