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Old 01-18-2013, 04:45 PM   #10
OEM ///Member
Join Date: Nov 2009
Location: Fairfax, VA USA
Posts: 23,002
My Ride: 330CiC, M5, X5, E93
Just wanted to add since I was typing and had to address some management issues with the kids, I noticed other commented on the SAP purpose and function.

The SAP or air pump system purpose it to add extra air (that includes oxygen) into the cylinder head exhaust ports to add additional oxygen into the exhaust stream to allow the richer cold start fuel mixture to continue burning in the exhaust stream, kind of like an afterburner in a way, but not using fuel, just supplying additional oxygen to allow secondary combustion of any unburnt fuel in the exhaust as it exists the cylinders.

This extra air introduced in the exhaust stream will reduce the initial hydrocarbon emissions on cold start as well as rapidly increase the exhaust temperature to allow both the O2 sensors and catalytic converters to warm up faster and attempt to bring the engine into closed loop operation in typically under 2 minutes.

Another little known fact is stainless steel either reacts and/or heats up faster to allow more efficient hydrocarbon burning to again reduce both cold start and normal hydrocarbon emissions. This is another reason there has been a large amount of stainless steel exhaust components used over the last 10+ years on most cars.
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