You're very welcome, and thank you for noticing. I'm glad I can help.
short rod helps torque.. but long rod helps efficiency
Good point, and one that I should've addressed earlier. Actually, we should think of efficiency in two ways here: pumping efficiency and combustion efficiency.
The shorter rod benefits pumping efficiency due to its "snappier" reciprocating action (speed at which it changes direction) at TDC and BDC. That's why I proposed that a shorter rod compensates for poor intake/exhaust port design (too big and wrong shape, usually).
The tradeoff to that increased pumping efficiency, though, is that the pump (the engine itself) will eventually overcome the ports' ability to flow more air/fuel. After that point the engine will not make good power (it cannot breathe). The pumping efficiency basically becomes too good for the engine's own good.
The net result of a short-rod engine is that it will make decent torque, but only within a very narrow speed range. Take the stock Type IV engine for example. It's great at hauling a Westfalia, two adults, a kid, two dogs, and a weekend's worth of camping gear; however, it doesn't really reveal itself as a great racing candidate until it gets longer rods (among other things like cam timing, port work, induction/exhaust, etc...). That's not to say it cannot be a good powerhouse (which it really is); it's just to say that it's not going to produce intense amount of power over a very diverse speed range.
While the longer rod reduces the engine's pumping efficiency to a degree, it's not necessarily a bad thing. With properly designed ports and combustion chambers that are matched to the overall engine design, the long-rod engine will make torque over a greater speed range.
The longer rod benefits combustion efficiency (longer usually equals better) since the piston stays at TDC longer during the combustion incident. However, there is also a tradeoff to too great of a rod ratio. I'm speaking out of theory rather than expertise here, but I would say that you need only enough dwell time at TDC for the flame front to travel from the spark plug, across the piston head, and to the cylinder walls.
I would also imagine that too long of a rod will decrease the pumping efficiency too greatly.
The key, of course, is matching the rod ratio to the engine's operating speed range. I wish I could tell you the formula for this equation, but it's beyond my comprehension. That's why I defer to the professional engine builders who say, for the most part, that most engines work well at or just below 2:1. Also consider that the Type I engine (1500 and 1600) has about a 1.94:1 ratio.
Now here's the summary: a short-rod engine will build good PEAK torque, but will develop that torque over a more limited speed range. A long-rod engine may not develop just as much peak torque as a short-rod engine, but it will develop a greater amount of torque over a greater speed range.
Now to influence you as to what you want when it comes to a power band, I'm going to reference a prevailing objective among engine builders nowadays. It's the prospect of creating power over a greater speed range. Builders usually refer to it as "power under the curve." This is the idea: you want an engine that makes respectable power over a greater speed range rather than an engine that makes enormous power within a very limited range.
For example, an engine may make 650 lbs-ft torque at 7,500 rpm but will be a total dog if it makes absolutely NO power above or below those speeds. It would have a great peak number but no power elsewhere. Its power curve would look like the Matterhorn.
If, on the other hand, you built an engine whose power curve started making 400 lbs-ft torque at 2,500 rpm, peaked at 450lbs-ft at 5,500 rpm, and made at least 300 lbs-ft until 6,000rpm, you'd SHRED the engine that made 200 lbs-ft more torque. It'd have a torque curve as broad and flat as Kansas.
The 650lb-ft engine would win the bench-racing session, but the engine with the lesser peak torque figure would win on the track. Plus it'd be way more fun to drive (torque=fun). Incidentally, we should all get away from quoting horsepower figures. Horsepower is a mathematical figure derived from torque and speed. It looks good in magazines, however, so it will probably stick around forever.
In short, rod ratio is but one figure in the equation to make power.
Okay, as complicated as all of this may seem, the limited nature of the VW world makes these decisions easy.
A stock VW rod is good to go in a 69mm stroked engine. Here's where the limitation comes in. If you stroke an engine to 74mm (which is very little), you'd need a 5.7-inch (Chevy length) rod to maintain that ~1.9:1 rod ratio. I'm sure you could adapt Chevy-journal rods as long as 6.2 inches (which are common nowadays), but you'd run in to a cylinder-length/pin height/engine width problem at one point. Probably the longest practical rod you could run in a VW would be about 5.7 or 5.9. If you're building an engine with more than a 74mm stroke, I'd say the 5.7 rod would be the best balance of practicality and power. Verify this with an engine builder, of course.
pin height would have to be bigger on a short ron and smaller on a long rod for the same combo
The pin height would have to be HIGHER within the piston on an engine with long rods; it would have to be LOWER on an engine with short rods. Remember, though, that pin height isn't as critical to VWs as it is to V8s as we can shim our barrels to tune compression ratios.
smaller would mean a shorter lighter piston too
A higher piston pin height would technically mean a lighter piston, as the pin would be closer to the crown of the piston. Remember, though, that the piston still has to have skirts on it. After one point, it won't get any lighter from pushing the pin higher.
Also, while I didn't really touch upon it, a longer connecting rod has the net effect of reducing the side loads on the piston skirts, cylinder walls, and so on.
Gotta go for now. Late, late, late!
