You can shorten the primary's to gain length. That will raise the RPM of your possible peak power. If that is important to you, then you haven't got too much other choice. As you say, there isn't much room to move things about. If your O2 sensor has a heater, maybe put it after the muffler? Not sure if anyone has done that before. it wouldn't react much different than an emission sniffer. The added distance might affect your tune during transients if you plan on running O2 closed loop. I recall that the Megasquirt can change the delay time for that. However delay only works at some given volume flow rate (RPM) maybe the newer MS firmware has that covered by entering a distance rather that time delay.
Some have had room to run the exhaust pipe between the tire and chassis and place a transverse muffler across the back. Can you do that? It would open up a world of opportunities for locating the O2 sensor, not to mention reduce the exhaust noise to the driver & passenger. If I were building another, that is what I'd try to do.
FYI, here is where the stock 02 sensor is located on my RX7. Certainly not 24 inches downstream. Maybe 12 inches from the engine port? It didn't seem to bother Mazda. Not much laminar flow there either. although the #1 and #2 rotor gasses were certainly mixed. So I think mixing to get an average of all exhaust ports is more important that Laminar flow.
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Chuck.
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