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Jerry_Williams69 t1_iw3x2oo wrote

Your car is going to run like total crap. The exhaust back pressure will be too low and the O2 sensor(s) will over compensate. Probably will recirculate too much through the EGR valve. Will get terrible fuel economy and the engine will be very weak. Not sure that it would damage the power train, but you will notice a difference.

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Rare_Message_7204 t1_iw53txm wrote

Wrong wrong wrong. The only job of the secondary or downstream o2 sensor (after the cat) is to tell the ecu the cat is working. The upstream o2 sensor does all the fuel adjustment... The car will throw an emissions CEL for the missing cat but it is 100% harmless for the engine to drive without a Cat on a prius as long as the upper resonator and upstream o2 haven't been cut out.

For the future inspection, OP should shop around at exhaust shops for pricing. Possibly even find a shop that allows bringing your own part, saving extra cash.

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Jerry_Williams69 t1_iw5d3j8 wrote

Yeah no, but I agree with your second paragraph. The engine will run like crap without the cat. I never said it would hurt the engine. Can you make an engine run well without major components? Yep. Going back to the OP, it sounds like the question is what would happen with a straight pipe and no other changes.

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Rare_Message_7204 t1_iw5ewsq wrote

Have you seen a prius header and downpipe? There is a resonator then a straight section with the upstream o2 ... As long as that hasn't been cut out it should have enough back pressure. It will be loud and throw an emissions code but won't run like crap.

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Jerry_Williams69 t1_iw5g3zq wrote

Yeah no. Have you seen the EGR system on Prius engine? When the O2 sensor(s) doesn't see what it should, it is going to try to send a lot of exhaust gas back to the intake. That won't happen without back pressure. Air fuel ratio will be off. Will cause sluggish performance and reduced fuel economy. Will also see increased combustion temps that can cause the engine to overheat.

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lipsticktracer t1_iw43xhy wrote

Not accurate. You can fuck things up by putting on way too small of a cat so you’re actually creating excess back pressure, but there’s no such thing as too little back pressure and unless you’re totally choking the exhaust, the computer will adjust and you won’t notice any difference.

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Zane42v2 t1_iw48xfz wrote

Yes, there is absolutely such thing as too little back pressure.

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Jerry_Williams69 t1_iw45uzj wrote

Of course you can over-restrict the exhaust. Engines are designed to operate in a back pressure range. Too high or too low, and they don't work right. Especially if the engine has an EGR system. You are flat out wrong about not noticing a difference.

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lipsticktracer t1_iw4cd8v wrote

I'm saying you can overrestrict, but you can't underrestrict. And if you buy a normal-sized aftermarket cat, you aren't in danger of overrestricting. These things are not that picky.

FWIW, I spent a decade fucking around with the exhausts of seven different POS lemons racecars. I've fucked around with every possible cat-delete configuration on a bunch of japanese and american cars. Modern emissions systems are really really tolerant and have enough map range to accommodate almost anything an amateur mechanic can throw at it. These are not high-strung race cars operating at the edge of their envelope. They're grocery-getters built to keep running fine under all kinds of environmental and maintenance conditions. Chill.

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Jerry_Williams69 t1_iw4e0qd wrote

Cool story bro. I've designed engines for CAT and Cummins. I know a thing or two about emissions systems and engine back pressure. You just don't realize that your "tuned"/deleted engines aren't running the best they could. You also aren't giving enough credit to the Prius's power train. It is pretty advanced for a mass produced auto product.

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