Even if your vehicle is already heavily modified, you might be missing out on some power via the exhaust flow. Just as you won't get as much power from your engine if it’s struggling to get air into it, the same is true for the exhaust gasses. This is where a high flow catalytic converter comes into play. However, many wonder if high flow cats are worth the investment -- here's the breakdown.
What Do High Flow Cats Do?
A regular catalytic converter works by having the exhaust gasses flow through a ceramic honeycomb structure that is coated in various rare metals. These materials are mainly Platinum, Palladium and Rhodium. As the gasses come into contact with these metals, they undergo a chemical reaction that essentially "re-burns" the exhaust to eliminate nitrous oxides, sulfur oxides, and carbon monoxide as much as possible. This, in turn, makes the exhaust gases quite a bit cleaner.
The problem with regular catalytic converters is that they don’t always allow the gasses to pass through as efficiently as possible. Part of the problem is that the catalytic converter isn't always designed with your specific vehicle performance in mind, and part of the problem is that some catalytic converters aren't flowing well because they're old, they're cheaply constructed, etc.
If, for example, you've got a discount after-market catalytic converter installed, the original OE converter that's years old, OR you've got an engine that makes significantly more power than stock, a high flow cat might be for you. While not exactly "cheap," high-flow catalytic converters are specifically designed to meet or exceed OE quality and performance standards without sacrificing flow rate.
Do High Flow Cats Increase Power?
Yes and no. Yes in the sense that they allow for the more efficient flow of exhaust gasses, removing restrictions that can impede performance. Vehicle owners who've significantly upgraded their engine power output will notice more power by upgrading. Same goes for any vehicle with old or cheap catalytic converters that are impeding flow.
However, if you have a stock engine with no modifications, and relatively new OEM catalytic converters, you won't see much of performance increase going with high flow cats. The catalytic converters on a stock vehicle (especially a newer stock vehicle) just aren't that restrictive until there have been significant performance upgrades.
Put more simply: If you've made major upgrades to your engine, which are defined as increasing power 20-30% (or more), then high flow cats are for you. If you've just added a cold air intake or performance engine tune, your money is probably better spent elsewhere.
Are High Flow Catalytic Converters Legal?
This is where you need to be careful. Different states have varying regulations when it comes to emissions, so you will need to make sure that what you’re buying will pass where you live. High flow catalytic converters can usually keep your vehicle's emissions output below the accepted threshold set by most air quality regulators, but some states specifically have rules against them anyway. So check with your local vehicle emissions authorities to be sure.
Are High Flow Cats Worth Buying?
If you have a bone-stock vehicle with a relatively new OE catalytic converter, you only want to buy high flow catalytic converters if you're planning for major engine upgrades. They're just not that important on stock engines with newer, fully functioning catalytic converters.
If you have an older car (8 years+) or a car with a cheap after-market catalytic converter, you may find that upgrading recovers some power you're currently losing to a clogged or inefficient catalytic converter. There's a testing procedure you can use to determine if your vehicle has a clogged or inefficient exhaust explained here you might try. That way, you can see if you need to replace your existing cats...and if you're going to replace a catalytic converter anyways, you might as well go with a quality performance replacement.
But if you have a modified car with significantly more engine power than stock (20% increase or better), you will almost certainly see improved engine power switching to high flow catalytic converters (especially top-end performance as measured on a dyno). If you've added a supercharger or turbocharger to your vehicle - or substantially increased the boost of an existing supercharger or turbo - you'll likely find high flow cats a good investment in terms of maximizing vehicle horsepower.
Finally, bear in mind that high flow catalytic converters often boost fuel economy on performance vehicles. While it's hard to say how big of an improvement you'll get, they could very well pay for themselves in reduced gas bills.