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Originally Posted by fullmann
what happens with the headers and no SC??
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There's not enough back-pressure, which reduces the scavenging effect (getting the exhaust out of the cylinder), which has the net effect of dropping your power band by about 800rpm. That might may you think "Oh WOW... LOW END TORQUE!" but it doesn't work that way - you end up with a boggy engine because of the exhaust not clearing the cylinder before the next intake stroke.
I made this mistake... I drove like that for almost a year. It _sucked_.
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will the ecu still need two cats between its two sensors, or will one of the high flow units off of ebay work good enough? I know the truck came factory with two, but I kinda figured it might be overkill, and toyota covering their a$$ when it comes to EPA guidelines.
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Well, it's not Toyota CYA, it's Toyota having to meet the California _STATE_ emissions guidelines.
If you have two cats, then that truck was originally delivered in (or destined to be delivered in) CA. You need to search around the forum for folks that have gone through this. It's not pretty and it may require replacing the ECU with a non-CA version. There are a couple of threads here from about 9 months ago (I think) where people have documented the experience.
There are O2 simulators out there that have worked for some people, but mostly not. There are people that have made it through this, but there are as many people that are still throwing MIL codes every time they start the truck.
Search on "two cats", "mil cat", "cel cat"... that sort of stuff.