Emissions Tuning Fundamentals: Catalytic Converter Construction & Selection

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Catalytic Converter Construction & Selection

07.44

00:00	Common catalytic converter construction involves a core matrix that's either attached to or floating captive in an outer shell made of stainless steel.
00:08	Depending on a vehicle's engine emissions output, packaging options and associated emissions regulations at the time and location of certification, catalytic converters can vary a great deal.
00:19	Some of the biggest factors are size, cell count, loading, core material and placement.
00:26	Why size matters is pretty straightforward.
00:28	The bigger the catalyst housing, the greater the oxygen storage potential, the more room available for core surface area and the more clean up work a cat may be able to do.
00:38	In addition to core size and shape, density matters too.
00:43	Cell count refers to how many cells or small passages the cat core has per square inch.
00:49	The higher the cell count, the greater the surface area of the core for its size and the greater the obstruction to airflow.
00:56	The obstruction to flow can be offset by having a larger overall core size though.
01:01	Specifically, you'll often find a larger diameter cat on high output engines.
01:06	US federal law requires that cats remain effective for at least 8 years, backed by a mandated 8 year warranty but that hasn't stopped automakers from delivering exciting platforms.
01:17	The Dodge Hellcat Redeye and the 2025 Chevy Corvette ZR1 are a few examples of 1000 plus horsepower factory vehicles with cats designed to function long term.
01:28	Careful selection and combination of size, shape and cell count is key to long term durability.
01:35	Aftermarket performance cats are often 100-400 cells per inch, while factory catalysts for road use are generally 400-900 cells per inch.
01:45	If higher cell count is required, you might feed dual 2.5 inch pipes into dual 6 inch diameter cats to avoid impeding flow.
01:54	While the core's structure itself plays a role in oxidation, the materials placed on it are critical as well.
02:01	The term loading refers to the material loaded onto the cat core during production.
02:07	This will vary somewhat depending on the manufacturer and their own process but often a base coat is applied, then a wash coat.
02:15	The base coat focuses on improving oxidation and it helps the wash coat adhere better.
02:20	The wash coat contains precious metals which are involved in the chemical processes that result in harmful emissions reduction.
02:28	Which precious metals are used and the amount of each applied is the single greatest factor in component cost.
02:34	The core matrix material is usually ceramic or metal.
02:38	Coatings adhere to ceramic better so ceramic is commonly used in OEM applications.
02:44	Metal core cats are mostly used in the performance aftermarket and hold up better against violent exhaust pressure waves but generally do not hold coatings as well as ceramic.
02:54	Hopefully you've noticed we've been using words like usually, generally and often.
02:59	Like many things, there's a range of innovation and processes in the marketplace.
03:04	Some manufacturers have built more durable ceramic cores and found ways to securely adhere them to outer shells.
03:11	Others merely capture the core by stamping 2 layers of steel together around it.
03:16	Eventually, through wear, the core may start banging around inside the shell or even shift so that the core isn't aligned with the exhaust flow.
03:24	This can cause a sudden and extreme obstruction to exhaust gas flow.
03:28	Placement of the catalyst can be just as critical as size and loading.
03:32	As I discussed earlier in the course, start up and initial warm up are a precarious time in engine operation from an emissions standpoint.
03:40	If the cat doesn't get warm enough to be effective right after start up, a vehicle may not pass emissions testing on that basis alone.
03:47	Placing a catalyst very close to the exhaust ports gets it heated up and working more quickly, resulting in better emissions performance during that critical time.
03:56	A downside of that location is increased exhaust restriction, back pressure and heat when the engine is operated at high load and engine speed.
04:04	One workaround is using 2 catalytic converters in a single exhaust bank or 4 total cats if the engine has 2 banks.
04:11	A cat which is less restrictive due to size or cell count may be placed closer to the engine and contain fewer precious metals.
04:20	By using less or no precious metals on the first cat, that cost can be saved for a second cat, sometimes called the clean up cat, which may have a higher cell count and is located further down the exhaust stream in a cooler and safer location.
04:34	Since precious metals and coatings play a role in catalyst performance and they break down and are worn away over time, cat performance also degrades over time.
04:43	While manufacturers have to warranty cats for many years of acceptable operation, regulatory bodies account for an approved amount of cat degradation.
04:52	For that reason, emissions test results are graded on somewhat of a curve with emissions calculated to increase a certain amount over the useful life of the catalytic converter.
05:01	Correction of the results based on catalytic converter age is achieved by applying a deterioration factor referred to as DF for short.
05:10	Cat age is measured in miles and the impact of ageing varies per emission type.
05:15	For example formaldehyde increases more significantly over a cat's useful life than non methane organic gas, non methane hydrocarbons, carbon monoxide or NOx.
05:26	You'll find some links to example deterioration factors used for various emissions categories below this module if you'd like to read further or review the actual deterioration factor calculations.
05:37	We're providing links rather than displaying all the values here because these do change over time.
05:42	It's not the sort of thing that happens once a month but if you're working towards a formal emissions certification for a product, it's good practice to check for carb and EPA updates or those applicable in your area and use the most current figures.
05:56	If you're in an area that allows catalyst replacement or if you're working on a vehicle old enough to be exempt, be sure to get as much information from the cat manufacturer as possible on their efficacy.
06:07	Start with the basics like cell count, dimensions and see what they'll tell you about loading.
06:13	Some brands like Jesse have worked with partners like Cobb Tuning to generate solutions that achieve specific power and emissions goals while some cat manufacturers may only mention the core material being ceramic or metal and how much power they think that you can put through it.
06:28	In the USA, regulation on replacing a catalytic converter within the emissions warranty period has been shifting.
06:35	Regardless of the emissions performance of an aftermarket replacement, it may or may not be allowed so if you're considering creating or using an aftermarket catalytic converter, be sure to check the latest regulations in your specific area.
06:47	To sum up, the biggest factors in cat performance and reliability are size, cell count, loading, core material and placement.
06:54	Modern cat technology and proper sizing has allowed for emissions certified vehicles over 1000 horsepower so even OEM cats aren't an impossible barrier to horsepower.
07:05	OEM cats usually use ceramic cores and heavier precious metal loads which make them more costly.
07:11	Aftermarket performance orientated cats tend to use metal cores and fewer or no precious metals to reduce costs.
07:18	Catalytic converter emissions performance degrades over their useful life but EPA determines acceptable deterioration factors and applies them to lab test results like grading on a curve.
07:29	Many modern vehicles use two catalytic converters per engine bank with one close to the engine and one further back in order to achieve excellent results during cold start and warm up operation.