Audiophobe, imagine if you will a straight and narrow creek or stream with running water passing through. Along the way there is a small but wider, deeper spot or pool that the water dumps into before it narrows back together into the little stream and continues on with it’s journey. If you take a close look along the sides of the pool area, you will see some of the water along the bank is curling back around and actually heading upstream towards the original entrance. When it gets there it then dumps itself back into the main flow and shoots along with rest of the current trying once again to squeeze itself through the bottleneck exit so it can continue on down the stream and once again be one with the rest of the water flow.
This is EXACTLY how an exhaust system works! The cat is the pool area that is wider than the rest of the exhaust, thus turbulence is created and exhaust flow is hindered. Not to mention it has internals that slow it down even more. The test pipe gets rid of the “pool” area and creates a smoother flowing straight shot for the exhaust to be blown out. This is true for everything else regarding your exhaust system as well, from the exhaust manifolds and downpipe, to the resonator box and mufflers. More flow is the way to go for an NA, don’t cha know!!!
The test pipe or downpipe will increase your exhaust sound a little, but not enough to make that much of difference. The great thing about the test pipe is it’s CHEAP! If you don’t like the sound (and you will) you can always take it off and sell it to someone here on 3SI and make at least half your money back.
AustinGT, I think about a 5 to 8 hp increase with everything else stock would be more realistic. Then as you free up other sections of your exhaust you will get additional hp outta the test pipe.
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