[NathObeaN]

As per title, I’m curious if anyone has any tests/evidence that show some real-world results of installing a high-flow cat while keeping the OPF in place.

I ask because in the UK (which inherited EU laws on emissions), installing a high-flow cat is legal. But, removing an OPF is illegal. So, I wondered if it made any sense to install a high-flow cat while keeping the OPF in place.

Has anyone tried it? Any results you can share? I did some searching but couldn’t find anything online!

[doughboy]

Installing a highflow cat is still illegal as hiflow will not meet the cars type approved emission spec. It will bring up dash warning light to so you'll still have to map that out or you'll fail mot for that too.

https://www.gov.uk/government/public...h-implications

But most MOT test places aren't bothered about modified petrol exhaust if it passes the sniffer test and looks well maintained. The sniffer test is so easy to pass as it's based on levels from the early 90s.

I run decat downpipes with milltek high flow mid cats in place of OPF.

Works great and passes MOT twice now, just warm it up first. The cats actually look like opfs too.

There is no test for an OPF, only by visual inspection that its present. My hiflow cats fill the void left by the deleted opfs, even if the tester tech even noticed.