The standard cat on a Skyline is 500 density, sports cat's are available in 300 or 200 density. The lower the density the better the flow, but any lower than 200 and MOT's can be a problem, 200's are also more expensive than 300 density units..go figure as they have less in them! Abbey have a 200 density unit available to order..but you won't get much change out of £400, if any after delivery charges, but that's par for the course on these items if they are quality. No sports cat can possibly flow as much as a decat pipe, but with a bit of tweaking it's a safer bet for MOT's and roadside Vosa tests for later motors.
Straight swap and the sensor is not required, it won't affect the ECU as it's "off" by default. So, if it's not fitted it won't have any input, and if it is fitted the chance of it activating in a decat pipe is zero, so again no input to the ECU. It's there to detect a cat which is blocking and getting hot as a result, a straight through pipe is not going to block or get overheated so no output from the sensor, ergo no input to the ECU.
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