ramrod;c-138444 wroteWhilst it's entirely up to you (this is your engine and your money), I'd like to offer my 10 cents. Changing the conrods in an engine is not exactly as simple an excercise as you make it seem. The later rods, whilst being stronger due to being thicker, are also heavier. How much heavier? 9mm to 13mm, an increase of 50%. This is quite a significant increase. You have to factor in that the entire assembly will need to be dynamically balanced, for the excess weight to be taken into consideration if you intend on having a smooth running and revving engine. Add this to the cost. At the power, boost and rev limit you're talking about having, the larger rods just aren't necessary. Look back to the 240 group a race car. It had a b21et, k-jet injection, made 340-370hp, and revved up to 7500rpm, all on 9mm rods (which was all that was available at the time, although they were hardened.) I am pretty sure, the drivers of the race car drove the car harder and faster than you ever will, no offence. I believe the pictures of the holed block above scared you in a way, but, there are always other things involved which we cannot know. I think you will be safe with the standard rods. Red blocks are way strong engines, if you break one, then build it stronger next time.
I understand the importance of a well balanced engine. I recently built a saab 9-5 aero B235R engine and had the entire rotating assembly balance, but this was largely because I removed the balance shafts from the engine. It was my understanding that they can balance a crank, flywheel and clutch plate together as a rotating mass, but the connecting rods and pistons are just weighed to ensure they are perfectly (or within very very fine tolerances) equal to one another. If you know more about this process, i would definitely like to hear about it. I was also under the impression that factory standards for balancing an engine are actually pretty low (tolerances) compared to blueprinting a high revving high power race engine. I thought that largely, it would be application specific as to whether you would balance the rotating assembly after replacing the rods. I'm certain that there can be no harm in balancing the rotating assembly, but is it actually required if the connecting rods are equal weight and mass? they may be heavier, but they will still be equal and because of this, I would assume there wouldn't be a pressing need to have the assembly balanced?