Catalytic bypass pipes

THe problem will then be ... will your car just fail the next MOT on emissions?

Also sprach "Mark W" :-

Why?

Also sprach "TTT" :-

Not necessarily. Some cars were fitted with cats before they were legally required.

To allow exhaust gases to flow more freely. Early Celica GT4 turbo's are fitted with cats. Removing the guts of them, or fitting a straight through pipe is a common practice for those wishing to tune for increased power. They can still comfortably pass the emission specs for pre '92 cars. No doubt the same applies to many other cars of a similar age. Mike.

Also sprach "Mike G" :-

Oh - a power thing. How dull.

Would you like it better if I said it's really an efficiency thing?

'course, most of the people who do it just want to use the extra efficiency to go faster...

Cheers,

Colin.

Well you did ask. Humph!. No pleasing some people. Mike.

An interesting piece of information

I used to have a Cavalier 2.0, which could be relied upon to deliver 45mpg on a long motorway journey.

I later had a more modern version, which would manage/scrape 38mpg, or just about 40 if very careful, under the same circumstances.

Difference? - the later model had a cat, the earlier one did not.

I have a suspicion that the Cat increases fuel consumption by about 10%

Just another reason why the original legislation for the compulsory fitting of cats was a mistake. Mike.

Also sprach "R. Murphy" :-

True, but removing the cat won't improve this much - perhaps slightly by reducing the back pressure, but not a lot. The bulk of the increase in consumption comes from the engine being set up to provide a slightly richer exhaust stream for the cat to burn - cats don't work properly in a lean-burn exhaust. Since all the metering and measuring happens upstream of the cat, removing it doesn't affect this.

I too found an increase in fuel efficiency of about 10% when I removed the catalyst from my 1992 Nissan Pulsar (Sunny) GTI-R. And being a Japanese import its emissions were far lower than the UK requirements for a car of that year - even without the cat.

However - more modern car fuel maps are designed with a restrictive exhaust in mind, so won't necessarily benefit from removal of the catalyst. In fact removal of the cat in the primary downpipe on my current car (twin turbo Subaru Legacy) can result in damage to the primary turbocharger, as it'll overspeed without the restriction.

Early 1990s cars (particularly turbocharged ones) can benefit a lot from removal of the catalyst though - but you do need to be sure it'll still pass the relevant emissions regulations.

Rich.

Which amounts to - engine tuning being modified to use more fuel, in order for the Cat to work!

Also sprach "R. Murphy" :-

Exactly.