I did some research into headers a few months back for the CRX.
A 4-1 header will generally give you more high end power and is better for high RPM applications.
A 4-2-1 will give you more low end and mid range torque.
Exhaust pipe size also matters. Smaller diameter pipe will increase your back pressure which helps on low end torque but suffers at higher RPM's. Vice versa for a larger diameter pipe. Most Honda-Acura (early 90's 4 cylinders) exhaust write ups I have seen recommend either a 2.25" pipe or a 2.5" pipe depending on tuning levels. A 10/10th's engine will be putting out more exhaust flow for more of its operating cycle and can make more use of the larger diameter pipe.
So when changing headers make sure to look into what the effects are going to be on the system. A higher flow rate (4-1 header) could be hindered by using a smaller diameter pipe thus negating its added flow rate.
Note: This is coming from an internet junky who reads forums, tuning articles, and others personal experiences to make informed (seems like a good idea) choices. Take what I said with a grain of salt. Ask some experienced exhaust techs.
If you already knew what I said then well someone else one day may find this useful.
-Sean
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