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Old 02-08-2018, 12:23 AM   #23
JohnMc
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Join Date: May 2004
Location: St. Louis
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One time I had a cat melt down on my old VBR6 Jetta. The car ran perfectly fine, no codes were set (1995, so OBDI, not OBDII). Idled perfectly, ran 100% fine at part throttle. But put your foot down, and LOTS of missing power. Floor it, not much there at all. Hell, in neutral you could floor it and it would only rev up to about 3500 rpm and just sort of hang there, working hard but not making any power. No restriction in the intake, no fuel issues, no issues with the engine, just a massive restriction in the exhaust causing about 160 hp to vanish. When the cat cam off, there was a hole about 1/3" still flowing.

Turbos don't pump air for free, exhaust system pressure reduces power, a smaller turbo requires more exhaust pressure to make the same PSI.
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