Decat

[360bhp]

Was under the impression that having a decat/highflow cat/test pipe (whatever you wanna call it) burns more fuel...but came across some sellers on ebay, that claim fitting one helps to increase fuel economy! Just wandering, are they chatting crap?

[kalniel]

Why would it burn more fuel to have a freer flowing exhaust?

I can only think of a couple of possibilities - either your car was relying on backpressure for something (turbo diesel?), or simply the fact that a freer exhaust means your engine won't choke as much at high revs so will maintain fueling levels at the very high end, encouraging you to change gears later and thus causing a drop in economy.

On the other hand a freer exhaust *might* make combustion a little more efficient so you get more bang for your hydrocarbon, hence go further on the same amount of fuel and economy increase.

Back in the real world I don't think it'll make any noticeable difference except perhaps right at the very high end, which either your engine will be prevented from reaching electronically anyway, or by the time you're there you can completely forget about anything to do with fuel economy.

[GoNz0]

depends what car you have, if it does need the back pressure you may bun the engine out doing that.

petrol turbo, hell yeh do it

[360bhp]

2.0i non turbo engine, but should have a turbo running about 6-8psi in about 6 months. Had a decat on my last car, but was just wandering if it effected fuel economy for te better or worse.

[GoNz0]

better normally, may have to get it retuned or even chipped to take advantage.

[Rave]

Less back pressure equals fewer pumping losses which should equal better fuel economy all else being equal.

[Konan555]

Being on the cross over to catalytics my car came in both varieties in the same year so it's good for a comparison. The cat version is rated as 5bhp less and is poorer by about 3mpg.

Most cars fitted with cats will run a closed loop fuel system and adjust the fueling accordingly (over most of the range) once the bung is removed from the exhaust.

[dkmech]

This is slightly off topic but was reading an engineering magazine and apparently there is an increase in theft of catalysts for their scrap value (up to 100 quid apparently) - so watch that you don't decatted against your will!