On my 2005 Navigator I noticed that the factory exhaust manifolds look the closest to shorty headers than any other factory manifolds I’ve seen. This got me to wondering how well they actually perform when compared to other types of factory manifolds and compared to shorty headers.

My navigator does not have the factory exhaust, I changed to a flowmaster 40 series and eliminated the resonator at the back. I had noticed just before the factory muffler, the exhaust restricted from 2.75 to 2.5 and back to 2.75 at the muffler inlet, so we eliminated that restriction. I’m actually not certain how much that helped me gain and if I went to a 3 inch pipe and muffler, what my gains would be.