I've started wondering this while trying to make the hypothetical decision between an RSX-S and an '06 Si. I'm thinking the K20Z3 has balance shafts because the rotating assembly (pistons, rods, crankshaft) are not as balanced as the parts in a K20Z1.

I doubt Honda would suddenly realize the engine has vibrations and needs balance shafts after including it in the RSX for a year. The K-series has been in development and production for a long time... the basic R&D should be done. If the Z1 is smooth enough for an Acura, why isn't the Z3 smooth enough for a Civic without balance shafts? I'm betting the internals are different... and that's a bit disappointing.

Does anyone know any more about this? Do the K20Z1 and the K20Z3 have different internals and if so, what is different?

 

First, Balance shafts have NOTHING to do with the balance of the rotating assembly, so it is not that the pistons, rods and crank are not balanced as closely. Balance shafts are there to counteract the secondary vibrations inherent in an inline 4 engine. 2.0L is considered the borderline for needing balance shafts. You will notice that all modern inline 4 cyl. engines larger than 2.0L have balance shafts, and that the Ecotec 2.0SC now has them too.

Engines such as an inline 6, do not have these out-of-order vibrations, and do not need balance shafts.

They put them in the Z3 because they are needed to meet customer demand for lower NVH (noise, vibration, harshness) in modern vehicles. It you go and look at reviews of the RSX, one of the gripes was the engine vibration.

 

Good point. I've read about different types of vibrations in different engine configurations, but I hadn't thought about that here. It still surprises me that Honda thought the supposedly "raw" Civic Si needed balance shafts to eliminate NVH when the RSX doesn't have them. I guess it's the same reason the Civic weighs nearly 3000 pounds... Honda is selling to enthusiasts, but using a decent amount of comfort to do it.

 

So, do you still think that the internals of the z3 might still differ from the z1

 

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Personally I think the changes are probably intake manifold, ecu, and I'm gonna guess the cams. Difference between the K20A2 and K20Z1 was the cams as well. They had the K20Z1 listed at 10hp more because of that.

 

I also heard that the balance shafts reduce the overall power of the z3 and the z3 has tsx heads.

 

Spinning those shafts in a pool of oil sure does add parasitic losses! Honda gave us a way of adding free HP with the downside of NVH that we are all willing to accept as tuners.