All I'm looking for is a little bit of knowledge and understanding here. I see so often how bad it is to remove all 4 converters on the 97-03 f150 trucks because they will throw a CEL and run bad. Now what I would like to know is the front set of sensors to my understanding control the fuel/air mixture and all that and the rear sensors are for nothing more than to tell the truck that the converters are there and working. Why do so many people say the truck will run fine with just two converters and gotts 02 mod but it will damage the motor if you remove all 4?? If the front sensors are the ones that control all the fuel and air and everything? I don't understand how removing the pre cats and having zero cats makes a difference if you have the gotts 02 mod.

 

Dunno why it would run any different with 2 cats, 1 cat or no cat. Get a tuner to shut off the second set of sensors or just deal with the cel. Some have used spark plug anti foulers to trick the sensor.

 

I have the spark plug "gotts mod" now with the rear two cats gone I just don't understand why everyone says it will run worse than it does now if I take the pre cats off to.

 

Try it and find out??

 

If the front O2 sensors are installed in the exhaust system and connected to the computer the motor will run fine.. anybody that says different just doesn't understand modern EFI systems. But at the same time there is no good reason to delete all of the cats, it doesn't matter how you try to justify it the facts are they do not hurt performance but they do clean up tailpipe emissions.

 

Oh I didn't say I was gonna do this I was just looking for some knowledge as to why everyone says the truck will run fine if you cut the rear two out but will run horrible if you cut all 4 out

 

The reason why is that when the automaker designs the engine, they factor in how the cats will affect the flow, and they set up the cam accordingly. The amount of overlap, the amount of lift, the exact duration, the amount of advance, etc. If you remove the cats entirely, the engine goes into what is known as over-scavenging under various conditions, but basically what people don't realize or understand is that "backpressure" as they understand it is actually a myth. Basically the velocity of the exhaust as it exits the cylinder creates a vacuum that pulls the spent exhaust out (the piston never pushes it out) which creates room for the fresh intake charge. There is a brief period where both the intake and exhaust valves are open at the same time known as overlap. If the low pressure in the exhaust is too low during this time it will pull the fresh fuel air mixture from the intake into the exhaust. This has several obvious side effects. Firstly, thats a waste of fuel. Secondly, since that mixture contains unused oxygen, the O2 sensors detect it as a lean mixture, and the computer will add more fuel to compensate.
Thirdly, since the fuel and air that was wasted is not used to produce power, your vehicle produces less output than it otherwise would, and in all likelihood you will use more throttle to compensate, further hurting your fuel economy.

 

What confuses me most is isn't the first set of 02 sensors right behind the manifolds responsible for all that information to send it to the computer? The first set of sensors don't have any idea if the truck has 4 cats 2 cats or 0 cats do they?

 

No, the first sensors don't have a clue, but thats not the point. All they do is report oxygen concentration. So as I pointed out earlier, when you get unburned mixture in the exhaust, the O2 sensors pick up on the extra oxygen and the computer attempts to make tuning changes based on that. But even if your vehicle ran without oxygen sensors at all, it would still run better with the stock cats on it. This would be the case until changes to the cam timing to optimize the engines performance and efficiency without cats are made. So what I'm saying is your engine is designed to run correctly with cats.

 

Does going cat less cause a decrease in power and gas mileage or are you talking real engine damage by doing that? I just figured like what was stated above that as long as you have the front two 02 sensors on there your truck would be good. These newer trucks are so confusing to me.

 

As I stayed earlier yes, you will produce less low end power because you are pulling an unburned mixture past the engine and wasting it. It has to be burned to make power. And yes, you are wasting fuel.

If the engine had no O2 sensors it would not be harmed, since it would just waste fuel. Since it does have sensors and they will read an incorrect mixture, your truck would run even richer than it should, and long term it would damage the engine.

Your thinking this is caused by complex electronics, it has to do with the cam. The engine was designed to run with cats, and if you run without them, the engine over-scavenges.

 

What's confusing me here is how the front sensors would read wrong if the truck was cat less and the rear sensors were turned off due to gotts mod or a tuner.

 

Think about it, what do O2 sensors read? They do not read mixture, they do as their name implies, they read oxygen levels.

The cam is set up so that it has a specific overlap period that takes into effect that the cats are present and will offer at least some resistance to the exhaust gases as they exit. Don't think of it so much as back pressure as it is a natural induction driven by inertia. If you eliminate the cats, the engine over scavenges the cylinders, in other words during the overlap period the exhaust pulls the intake charge into the exhaust.

So the sensors reading differently us because a change was made that causes over scavenging to occur. The only way to correct the over scavenging is to either run cats, or to change cam timing.

 

How are so many people running catless and there trucks are running perfectly fine with no damage for miles and miles with no motor damage? My neighbors 99 f150 is catless and he has almost 300,000 miles on original motor that way. Are these people just lucky?

 

I doubt he put all that 300,000 miles on it in that condition, and yes, he can be lucky, but an engine will continue running with engine damage for a long time, and hows his fuel economy (probably terrible). I'm sure he's had lots of repairs and parts failures over the years too, some people like to talk about how great their ride is and how long its lasted, but are often selectively forgetful about the issues they've had.

Look, the maximum power you could gain by running without cats is around 5 hp, and that would occur at the top end of the powerband. Its not worth it. I recently bought a '95 Chevy 2500 5.7L and it was from a non-emission area, had 250,000 miles. I put a cat on because I live in a smog testing area. Performance difference before and after? Completely indistinguishable, 5 hp is not something you are gonna notice, thats the peak output of many walk behind lawnmowers, and they will bog down on tall grass. And in exchange for what? Reduced fuel economy, a decrease in low end torque (most of your typically driving and cruising happens at lower RPM where scavenging and valve overlap have more of an effect), and increased fuel consumption at idle.

I own a lot of cars, all with cats, some have run without them at some point. There are no advantages to running without.