car drinking petrol after changing exhaust?

1. AdamDougal Established Member Joined: Jun 2010 Middlesbrough Posts: 1673 2002 Quicksilver

   #1 11th August 2010, 22:44

   i changed my twin exit exhaust with a cat on it for a straight through decat standard pipe from the manifold connector to back box,
   now all of a suden im geting rubbish MPG?
   anyone help me out as im lost here?
   i have no idea what could be the problem as i have no idea about exhaust problems etc.
   
   help would be great thanks.
2. Lewiswhitlock Frequent Poster Joined: Jul 2010 southampton Posts: 591 1.4 8v 52 plate icelandic grey furio

   #2 11th August 2010, 23:29

   Lose the cat lose the mpg, keep the cat keep the mpg. Well i believe thats the common way it works? Correct me if wrong someone
3. user_35312 Established Member Joined: Jun 2009 Coventry CV6 Posts: 2477 Slowda Octavia vRS SOLD - Cam'd Chip'd VTR

   #3 11th August 2010, 23:35

   Quote:

   Originally Posted by AdamDougal i changed my twin exit exhaust with a cat on it for a straight through decat standard pipe from the manifold connector to back box, now all of a suden im geting rubbish MPG? anyone help me out as im lost here? i have no idea what could be the problem as i have no idea about exhaust problems etc. help would be great thanks.

   is it a mk1 or mk2? you havent left out a lambda sensor blank or something have you? doubt it though it should be in the manifold.. strange one. my car was a tiny bit better on fuel after de cat cuz its less restrictive but i hooned around more so it drank the fuel lol x
4. AdamDougal Established Member Joined: Jun 2010 Middlesbrough Posts: 1673 2002 Quicksilver

   #4 11th August 2010, 23:37

   Its a mark 1 but mark2 exhaust?:/ weired i know lol
   And the lambda sensor i have no clue? Lol
5. montsta1 L-plater Joined: Aug 2010 Felixstowe Posts: 40 MK1 Fully color-coded Blue VTs

   #5 12th August 2010, 00:44

   i found the same thing mate, my decat lost me a few mpg, but the sound make up fr it
6. stevyj Regular Poster Joined: May 2010 skipton Posts: 401 metro rio grande pug 306 dt corsa 8v sri tuned

   #6 12th August 2010, 09:24

   sponds better decat didnt notice that much difference in mpg get 35 to 40mpg staedy away. make sure lamdas connected and no blowing on joints
7. Steve Saxperience Hardcore! Joined: Mar 2005 Newcastle Upon Tyne Posts: 19910 CW Honda Integra DC5

   #7 12th August 2010, 10:40

   i got better mpg with a de-cat.
   
   You not rev'ing it driving it harder to hear the exhaust more lol?
8. atspeedracing Frequent Poster Joined: Aug 2008 ESSEX Posts: 727 106 GTi Turbo

   #8 12th August 2010, 10:45

   the problem is usually because people drive different because they like the sound of the exhaust.
   
   the ecu doesnt know what exhaust you have, so doesnt change anything.
   
   if the engine is breathing better, you should be able to apply the same power, with less throttle.... but as i say, people usually apply more throttle because they like the sound lol.
   
   - colin.

   1 user thanked this post: Steve
9. geoffgti88 Regular Poster Joined: May 2010 Shitty Redditch! Posts: 445 MK1 VTR Silver, 205 GTI track car, 306 S16

   #9 12th August 2010, 16:18

   I didnt notice a change in mpg, but did notice that it sounded more sexual (well in my opinion) lol

   1 user thanked this post: Steve
10. Boyes Regular Poster Joined: Apr 2008 Sheffield Posts: 276 Peugeot 106 GTi, Clio 172

    #10 12th August 2010, 16:59

    Possibly blowing?
11. user_15516 Saxperience Post Whore Joined: Apr 2007 Posts: 6640 Integra DC5R & VTS

    #11 12th August 2010, 17:30

    Quote:

    Originally Posted by Steve i got better mpg with a de-cat.

    Ditto - both my Saxos have been the same. Can't understand how people often experience the opposite haha.
    
    Check stuff like tyre pressures, give the car a service if it hasn't been done in a while etc.
12. Barry123 Saxperience Hardcore! Joined: Oct 2005 Aycliffe Posts: 32205 Saxo VTS

    #12 12th August 2010, 17:36

    It'll be the increased-noise-effect. Basically the decat makes the car sound meatier and you're probably giving it some more often that you realise.
    
    Secondly the decat is going to make the car run a touch richer for the first couple of hundred miles whilst the ECU adjusts the fuel trims to suit the adjustment in back pressure.
    
    decat should give you a bit more mpg if you behave yourself with the right foot.
13. bibbles L-plater Joined: May 2010 Posts: 60

    #13 15th August 2010, 01:15

    well, do u stick to a certain gear-change @ specific rev, for instance, every time my rev's reach 1.5, i change gear, up until im at 30mph in 5th. i get 80 miles out of a tenner, with the total excellium gear, which is roughly 54mpg. so maybe it could be something completely irrelevent. have u checked the tyre pressure recently? im mad for MPG and this is usually the cause. 0-60 in 9.35sec and top speed 117 my little sexo VTR OH BTW i have no CAT and i have a shitty standard £100 exhaust jobby while i wait for my powerflow to be fitted x
14. Cammy Established Member Joined: Jul 2009 Aberdeen Posts: 2211

    #14 15th August 2010, 01:27

    Hmm, i used to get quite good MPG with my Mani + milltek on, full tank would do me about 400 miles doing long runs... but i sat at 60 the whole way as allot of my driving was up north and back most nights.
15. user_21226 Saxperience Post Whore Joined: Feb 2008 wirral Posts: 6348 Icelandic vtr turbo

    #15 15th August 2010, 09:32

    Quote:

    Originally Posted by bibbles well, do u stick to a certain gear-change @ specific rev, for instance, every time my rev's reach 1.5, i change gear, up until im at 30mph in 5th. i get 80 miles out of a tenner, with the total excellium gear, which is roughly 54mpg. so maybe it could be something completely irrelevent. have u checked the tyre pressure recently? im mad for MPG and this is usually the cause. 0-60 in 9.35sec and top speed 117 my little sexo VTR OH BTW i have no CAT and i have a shitty standard £100 exhaust jobby while i wait for my powerflow to be fitted x

    30 in 5th wouldnt that be puttin strain on the engine/box
16. AdamDougal Established Member Joined: Jun 2010 Middlesbrough Posts: 1673 2002 Quicksilver

    #16 15th August 2010, 12:40

    thats what i thought lmaoooo
17. Gareth_R Established Member Joined: Mar 2010 Basildon, Essex Posts: 1042 Citroen AX VTS

    #17 15th August 2010, 13:00

    anything less than 30 does, but 30 in 5th is fine
18. axsaxoman Saxperience Forum Bum Joined: Jan 2003 Scotland Posts: 3380

    #18 16th August 2010, 20:36

    reasons simple --presuming driving is same
    you removed cat and now have no box where cat was --Iam guessing no centre box at all .
    the placing of the cat /first silencer -expansion box is important ,as that is the tuned length of the exhaust --so by removing the cat and having no box you have lost torque
    making a tuned system is all about the primary +secondary lengths up to the FIRST box --after that as long as it is not smaller it makes little difference as long as its not smaller dfia
    -,I am guessing it also makes a "bumming noise at around 3-4k if you have no center box --then goes quieter above + below that rpm band
    
    you have screwed up the tuned length and therefore lost efficency
19. stedixon Infrequent Poster Joined: Nov 2008 middlesbrough Posts: 146 saxo (gti) ;) berlingo ford fiesta savo vtr

    #19 16th August 2010, 21:14

    i used 2 gett good mpg on the vts bu fuked dowt with it tht muck im lucky to get 20 mpg now