can any one help me. need to know if i put a de-cat downpipe on my STi v3 will it use more petrol ?

thanks.

 

yes

 

Why??

It is still under closed loop control isn't it?

So the engine ECU still thinks there is a CAT?

Pete

 

coz he will love the noise & plant it more

 

Coorrrreeeeeccccttttttttt!

 

Until the roadside emissions check that is!!

Pete

 

Coorrrreeeeecccttttttttt!

 

when I first bought my Impreza back in '99 I could easily get 350-375 miles out a full tank of 95 NUL by just driving like Miss Daisy. I used it for work, so it was in my interest to get decent economy driving to/from clients.

Now, with a full decat I struggle (if that's the right word for an Impreza) to get 300-320 miles out of it with the same granny-like driving.

Don't know technically why, but yes it does. closed or open loop, you'll notice a decrease in economy.

Before any one starts, no, I never bought my Impreza for fuel economy. I fail to see why I shouldn't be bothered with some "reasonable" economy travelling to work at a pitifull 60mph in rush hour traffic every Monday to Friday.

Drive the t!ts off it and expect to need refuelling every 100 miles or so, but any performance car should give some economy when cruising.

Stefan

 

True, although how many of us have been stopped with a roadside check?

STI-MON,

If you're at all worried, keep the centre cat in place or fit a sports cat.

Stefan

 

With no cat, I doubt that it would pass the MOT.And it is a real pain to have to keep switching exhausts about.......

 

IF the map is adjusted to suit, you should find that the economy will be no worse, and perhaps fractionally better when cruising at a steady speed on the motorway - higher efficiency due to less back-pressure etc. This does rely on having the car mapped to suit, which is probably wise and certainly helps release the potential power.

However, when putting your foot down, you'd expect the car to produce more power - this doesn't come for free, and is very much at the expense of using more petrol! Therefore any driving other than sitting on the motorway all day or pootling along off-boost will be less economical.

 

I don't get it ....

I de-catted my Calibra way back in 1992 ..... it was a closed loop and didn't make a blind bit of difference to mpg.

All you are doing is making the exhaust 'breathe' easier .... the fuelling is taken care of BEFORE it reaches the engine and the info comes from the Lambda Sensor (which is still there) ........... the fuelling arrangements are the same CAT or no CAT!

Pete

 

Mine is using more since the de-cat but its down to me driving it a bit harder through the gears i guess, i never seem to get more than 280 miles per tank and i try not to work it out because its going to be shocking if some of you are getting 300 + then your lucky.

 

Well applying science...decat allows the turbo to spool up quicker and at lower revs. Meaning the fuel consumption will be alot more dependent and sensitive on your throttle movements....as the turbo will try to spin up and push more air (and thus more fuel) into the engine, you'll need to pay more attention and counteract this by closing the throttle more and keep the manifold under vacuum. Quite difficult to do IMO as the engine always wants to run at least 0.3 to 0.5 bar boost at low revs.

 

The difference Pete; is that when you remove the cat. from a Scooby, you allow the turbocharger to 'spool' up sooner, as a result its much harder to drive off-boost and this affects MPG adversely.

I read that in the library during my teabreak.....


Simon

 

I see that someone has already said that.....

Simon

 

Pete, de-catting a car! surely this is unheard of and a revelation! especially with his stance on the matter

 

Correct, it wouldn't pass an MOT. Takes minutes to swap over a catted centre section or even less if your friendly MOT station stuffs one on the end of the tailpipe.