Catalytic Converter Removal Consequences.

So, out of curiosity and due to someone telling me this earlier, what exactly are the pros and cons with running a fat, catless pipe from the headers back on an older 2.5GT Leggy?

 

The reason this is coming up is because someone had told me that running a straight, catless system wouldn't harm anything but the environment. Yet, when I went catless years ago with my 2.5GT, I noticed an immediate loss of power and I was burning through my downstream O2 sensor and cats like crazy. I was told that my downstream O2 sensor was sending back bad data and the upstream was trying to adjust to compensate for the unfriendly amount of richness... After this, I was told our O2 sensors don't adjust our air/fuel flow at all. Is this true? What the hell would the point be in O2 sensors if this indeed true?

 

Also, is there any worry at all about lack of back pressure from having no cats and/or resonators? I've heard this can also cause some rather serious problems with engine internals. I just want to make sure I have all of this clear.

 

Thanks!

First off, you would never get an inspection sticker if the inspection station saw that. Second off, the fine you could potentially get from the EPA would be somewhere in the ballpark of $15,000 or more. Third of all, you would be burning through your downstream O2 sensors like crazy, and the one after the cat would constantly be reporting rich readings. Fourthly, the O2 sensor DOES feed data to the ECU which determines how much the ECU should trim the fuel injectors. The O2 sensors do directly assist the ECU in deciding how much fuel the engine needs by determining if the exhaust gas is rich or lean. Fifthly, performance would decrease, and you could potentially risk increasing wear on your exhaust valves if the pipes are too big, or there isn't enough backpressure. This usually only happens when running without headers, but it's also possible with very little backpressure. Lastly, you will get pulled over on a regular basis if you run a straightpipe, trust me, I've been pulled over 46 times, lost my license once, taken driver retraining programs, and then almost lost my license again, the majority of these incidents were a result of running cars with no exhaust. And speeding.

If you run a totally straight pipe from the headers back, I think the biggest problem you will experience will be the blood running from your ears and the perminent hearing loss. Yeah, its loud.

 

As for the loss of power, I can't answer to that. I've never experienced a loss of power by allowing more exhaust flow. Then again, I've never run totally straight-piped either, because it sounds like shit on any engine. I've always run at least one muffler. I would say that if you ran a 2 1/4" straight pipe you would be fine.

 

You aren't going to damage the engine as long as you have a reasonable length of exhaust pipe between the exhaust port and the open air (just the headers would be fine). If you were to, say, run with no manifolds/headers at all, the cold air hitting the hot exhaust valves in between pulses would cause the exhaust valves to crack or "burn" along the edge of the valve seat, which would cause them to leak.

 

Oh, and running without a cat will make your car smell like an old carbureted car that needs a tune-up. It stinks. I'm running without a cat right now because the headers I got replaced the entire exhaust right up to the catback, and I fully intend to have a HFC welded in sometime soon.

Smaller engines do suffer from power loss with no backpressure. If you were to run no cat and a full exhaust with some type of muffler/cherrybomb it wouldn't be too bad, infact it might just increase horsepower and torque, but you'd get that horsepower and torque at a higher RPM. No cat and no pipes from the headers back will decrease overall torque and horsepower, especially at lower RPM's. On larger engines backpressure is less important, but this is only really the case for engines 3.8 liters or less. Even engines larger than 3.8 liters with 4 valves per cylinder could suffer from pressure loss with reduced backpressure because of their increased ability to get rid of exhaust gasses.

I was once told that the whole "backpressure" thing is a myth, but the general idea of not running stupidly fat pipes remains...

 

Supposedly after a pulse from an exhaust valve it is followed by a vacuum that will soon be filled with the pulse from another valve. Have too much room for the exhaust to escape reduces or eliminates this vacuum making the engine actually have to do MORE work to push the exhaust out. Also, there's the whole issue with exhaust valves fluctuating temperatures between pulses.

 

But I really don't know if this is true and I can't even remember where I heard it.

It is somewhat true, but i know for a fact that backpressure does hinder performance. I've run many vehicles with small engines without cats and noticed a decrease in performance. I've also run larger engines without cats and there was no noticeable difference.

Thanks a ton for the information, fellas. I figured this much. I just wanted to be sure I wasn't going to talk crap to someone before I knew whether I was losing my mind or not.

If you run without a cat, it is not so much the reduction of backpressure from the cat that is killing the performance of the car, it is the fact that the EGT has dropped reducing the flow of the exhaust.

 

My suggestion is to wrap the header and delete pipe, it will bring the EGT back up and more often than not improve the performance, but you will still have the 02 sensor issue, which you can get rid of by pulling the 02 sensor out of the exhaust stream, by welding in a new taller bung or getting an extensoin of some sort.

 

These are just suggestions as i have never done this before, i have always installed a larger cat. Well in any case i hope that helps a little.

OBD II cars, the second 02 sensor relays info regarding the cats performance. the only one that dictates how the car runs is the first or front sensor. this post has hit he subject right on the head and sums it up well