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The reason for upgrading to a larger exhaust system is to get the hot exhaust away from the engine quicker, in a larger quantity. If you do not enlarge the smallest restriction you essentially gain nothing. The CAT back larger systems fit this description, the 3.5" stock exhaust pipe will still allow the same amount through, regardless of how large the rest of the system is made. Test this theory yourself with a garden hose and water for visual confirmation. Larger systems are quite spendy, especially since the stock system is adequate for stock engines. If you wish to add HP, then you must upgrade the air flow systems first. A Larger exhaust system and an improved air intake system are essential before increasing HP, if the full benefit is to be realized without harming your engine. A diesel engine is a heat engine, 1/3 is exhausted, 1/3 is absorbed by the cooling system and 1/3 is converted into usable power. All three thirds must be balanced when adding HP if reliability is expected, just as the stock unit is balanced for reliability. Cost effectivness enters in too, an $800 SST large exhaust system may gain 2-3 mpg, at $3 @ gal it will take several thousand miles to break even, and that's without also spending money to increase HP, You said your HP is satisfactory now right?
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