This is where all the heat and turbulence problems are. You can bolt up the 2.5" parts to our 3 inch parts, but in order for the downpipe to work you will at least need test pipes that cone back down to 2.5". If you bolt a 2.5" test pipe up to the 3 inch down pipe you will have a 3 inch outlet hitting a 2.5" wall.......This will cause more turbulence than running all 2.5". It takes a little distance for the pipe to cool and you need gentle transitions if you want to change sizes. Coning back at the rear of the test pipe does work well and offers most the 3 inch gains.
21:01:18 07/26/07
