Reader Question: Deleting Old Cats?

Here’s the latest reader question, along with my reply! 

Doug asks: Do you know anything about whether gutting or cutting out catalytic converters would effect fuel mixture and ignition? Suppose I had a beater 2003 Land Rover Disco with clogging cats. Not really worth the replacement cost but it’s an unstoppable beast otherwise and I’d like to keep it in the rotation. It has O2 sensors fore and aft and I wonder if it’s going to try to compensate somehow. Disclaimer: Of course, this would be very much against the law and I would never consider actually doing it. I’m just looking for background information for the novel I’m “writing.”

My reply: Once upon a long time ago, removing the cats – with a “test pipe,” you could actually buy them at auto parts stores – was well worth doing for the performance and mileage improvement obtained thereby. But that was because these old style cats – especially the “pellet” cats GM used back in the mid-late 1970s and into the early ’80s – were horrendously restrictive and also because the engines were designed decades before catalytic converters were even conceived of.

My 1976 Trans-Am, for instance, runs immeasurably better with a free-flowing 2.5 dual exhaust system and no cats than it did when it was choked by a 2.25 inch single exhaust feeding into one converter.

The 455 under its hood dates back to the mid 1950s; this ancient carbureted engine was never meant to be catalyzed.

But modern cars with engines designed for cats are a different story – and so are modern cats, almost all of which are the lattice/honeycomb type that are not hugely restrictive and (in V8 applications) are usually two – so as to not plug up the exhaust. The exhaust is a system now and integrated with the engine’s other systems, including the fuel system, via 02 sensors. There is also a certain amount of backpressure built into the system on purpose. If you remove the cats, the fuel system will likely go into open loop mode and the affect of the change in backpressure may actually reduce mileage/performance.

You can get around the “check engine” (OBD) light coming on via installing just the right resistor in the 02 harness but the engine may still not run better and may run worse.

If it were me, I’d get a set of high flow cats and mufflers, which will improve the vehicle’s performance and mileage. I’d seriously consider tube headers while about it, too.

. . .