Inline 4, parallel cat converters in 4-2-1

[BLSTIC]

Emissions, ain't they grand?

Anyway many modern I4 engines use thin walled tubular manifolds with a super close cat converter. Works great for emissions because the total mass of the manifold and cat converter heats up in seconds. Works not so great for performance because nothing about an uneven 3-10" long 4-1 header with a cork at the end is right. No blowdown protection, no inertia, no pulse tuning...

What if I were to run two motorcycle cat converters (or really whatever fits and flows enough) at the end of the primary junction in a 4-2-1 manifold? I could get slightly longer much more even primaries, but the big gain would be the 180 degree blowdown interference would become 360 degree blowdown non-interference.

Would this be worth doing on an engine that still needs to have good emissions? What kind of VE changes would happen on a typical low-overlap modern 4v 4-pot?

[Schurkey]

The big question is how many tens of thousands of dollars is it going to take to get your proposed system past an EPA-certified emissions lab?

I'm not talking about some gas-station testing facility that opens the hood and looks to see that everything is in place before scanning the computer looking for "codes". I'm talking about the laboratory the manufacturers use to verify emissions compliance--which is what you'll need to get an EPA or CARB waiver.

No waiver = emissions tampering, as far as the EPA is concerned = $10,000 fine. Cat converters have to be located in the same place, and the same number of converters as the vehicle was certified with. You're changing both the location and the number of converters.

[3B]

What is EPA, something you eat? Must be one those Trumpland problems, we don't have that in the free world.

I'm not sure placing the cats after the primaries is so good idea. I was thinking using one cat after the final collector and extending it's casing to form a termination box. It places the cat little further back but should still be close enough to get it lit up and pass the local emissions testing. And still get the full benefits of 4-2-1 headers. The question is does it matter where the cat honeycomb is placed inside the termination box? Just after the collector exit or as far back as possible? Or somewhere in the middle?

[MadBill]

I'd say from a performance standpoint, as far back as it takes to allow the unrestricted portion upstream to match the desired box volume. For emissions though, too far back/too cool could be the final straw re failing; it would depend a lot on the test parameters and the engine calibration in 'test tune' mode.