What would a car owner remove a catalytic converter?

Would removing the converter add or diminish power to the vehicle?

Add. The less restriction in the exhaust system, the more power. The only problem is that it’s against the law to remove it and you’ll most likely not pass an emissions test without one.

Next to zero effect. The loud sound makes people think they have more power. And it’s illegal. And it’s just the wrong thing to do.

Race cars have a completely unrestricted exhaust. Does that answer your question?? But race cars do NOT have ECM’s (computers) that depend on many imputs to monitor and control the engine. Removing the CAT on todays cars will do NOTHING other than making your dashboard light up like a Christmas tree…

If you’re referring to an older car, here’s what the chain of events usually was: Owner gets rid of smog pump/feedback carb/some other piece of inscrutable 1970’s-80’s emission control technology and gains a few dozen HP’s. Then a few months down the line, the converter plugs up. Finding how much a new one costs, owner replaces converter with straight pipe. This was the reason why they invented emissions tests.

Or alternatively, even if the owner didn’t intentionally tamper with the smog equipment, converters for old cars are expensive and it may merely have gone out and not been replaced

It’s not that the converters themselves reduced the power much, it’s just that the other stuff the car needed to keep the converters working properly effectively detuned the engine. Nowadays, they’ve figured out how to get an engine to run well AND keep the catalytic converter running, so there’s no easy gains to be had by messing with the emissions controls. Plus they’re so integral to the FI system that you generally couldn’t even if you wanted to. And yet we still have emissions tests… hmmm…

The cat converter is not the restricting element that prevents power. removing it willl have zero impact…and if your car has an OBDII system it’ll cause the CEL light to glow forever, making it useless as a warning device. NOTE if you remove the upstream oxygen sensor too when you pull the cat you’ll have fuel metering problems.

The converter can readily handle the amount of flow that moves through your engine. Typically the intake ports, the exhaust ports, the valves, the amount and length of time they open, and restrictions caused by turbulance and other passages are the things that allow only so much flow to occur. The exhaust including the converter is designed to handle the output of the manifolds without adding further restriction.

Only if you began to address these other issues would you then need to addaress the exhaust system.

Pretty much every race car with the exception of NASCAR race cars indeed have ECUs. F1 cars do, Indy cars do, LeMans endurance race cars do,etc. There’s even been talk of NASCAR finally adopting fuel injection, which would mean ECUs would be mandatory there as well.

It varies greatly, especially on cars with forced induction. On someone’s stock Civic removing the cat won’t make much difference. But for someone with a Supra running 30 pounds of boost, having catalytic converters on the car creates quite a bottleneck.

Sure, but remember the cars Oldsfan is considering - 80’s Eldorados, that kind of thing.

I have to confess being guilty of removing a catalytic converter. On my 1976 Ford Granada with a W351 V8, the driveability problems were so severe (constant stalling) that we took it off, it still had a regular muffler and a small rear sound reducer (resonator), so the car ran quietly, We also disconnected the infamous smog pump which never worked well.

The overall result was better driveability, and we had to set the idle lower, since the converter back pressure was gone. Gas mileage did not suffer, but the car felt more powerful, mostly because it was less sluggish in accelerating.

I read Thoreaugh’s “Essay on Civil Disobedience” before making the disconnect, and reasoned that if industry could not produce a driveable car, I had the right to make it driveable for safety’s sake.

In '76 we could do that stuff without creating operating problems. Those days were fun.

I doubt that ol’ Henry would have been offended.

I removed the catalytic converter on a 1975 Monte Carlo. I also noticed the two main jets at the bottom of the carburetor were narrower than the main jets from previous years. So I drilled them out and restored the power and increased the gas mileage on the car. I recall increasing the jet opening by 2 thousands of an inch. I believe 1975 was the year the car makers had to go to a 14.7:1 stochiometric fuel ratio. Prior to that the Air/Fuel ratios tended to be 10% richer in fuel than 14.7:1.

I had several extra sets of jets so I experimented. When I drilled them out further, the power kept increasing while the mpg dropped further. At one point, both the power and mpg dropped.

As seen elsewhere if you have a cat it is required to be there. But there is no law saying you cannot put in a bigger cat(precisely). Really you would do better to dyno tune the system as it is. You might have a bad control that is hurting performance and the older systems need to run on a dyno to tune some of these issues