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Old 01-14-2013, 03:26 PM   #16
tmittelstaedt
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Join Date: Jan 2013
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Location: Portland, OR
What I Drive: 2001 Red Focus SE Street Edition

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"...Removing the cat doesn't make you loose power..."

Yes, it does - but to be explicit, it does for 99% of your driving. When you remove the cat the computer goes into open loop all the time, and uses a suboptimal preprogrammed fuel curve - that curve does not matter at WOT of course. Also the loss of backpressure from the CAT reduces low end torque. Granted - you see a gain of high end HP at high RPM because of the lower back pressure and because at WOT the computer is basically dumping fuel in as fast as the engine can take it but the tradeoff is less low end torque.

Basically the only way it helps give you any HP at all by punching out the cat is if you drive around above 4-5K RPM at WOT all day long. Great if your drag racing - pretty terrible for most regular driving.

If you want to do it right, you delete the cat with a custom-welded exhaust, and replace the engine computer with a different one programmed for no cat. THAT is the only way a typical driver will see a HP increase by removing the cat. Incidentally, a cat with an intact honecomb fetches a hundred bucks from a platinum salvager so you definitely want to do it properly.

But, it won't gain you much HP. The exhaust system in the Focus is pretty damn good for a stock car.

Most people I know that do the punch-out on the cat are buffoons who already have something wrong with the car - their CEL is already on - and they go to Autozone and get the free OBD test, which tells them the cat is bad - and since they don't have the $200 for a new cat, nor the education to realize the problem isn't actually the cat it's something else, or tools to actually troubleshoot the car properly - they punch out a perfectly functioning cat then spend the next year trying to convince themselves that they "increased performance"
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