MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGF Technical - Hello Pussy

The CAT.
Has anybody relaced the CAT with a plain tube on a 1.8vvc year 2000.
1) Will the emmisions fail at the MOT.
2) Will the year 2000 car fail for not having the CAT as standard.
3) I gather the CAT is fitted after the lamda sensor in this exhaust system and so does not effect the engine management. Is that the case?
4) Is a 4 bhp increase obout what is expected.
5) At the MOT will it be tested as a non-cat car or as a CAT car even though it has been eliminated.
6) Who supplies a good CAT bypass tube?

Lots of challenging points for you knowlegable MGfers.
A big thanks to all who reply.
Chris
Chris Jones

Chris

Not 100% on Construction & Use Regulations but if the car is post-CAT introduction date then it must have one. My man insists that a Cat-bypass would fail MOT on emmisions. Most people have to refit a CAT each year just for the test and then take it back off again. Seems a hell of a lot of work for just 4 bhp, which is the sort of figures that I've heard. Also, must notify insurance company who will possibly load as 'modofied'.

MS list a Sports Cat - may be worth speaking to Mike before scrubbing pussy.
JohnP

Hi Chris

I recently put mine through an MOT with only half a working CAT installed (IE. Half of the internal ceramics are missing)Its a long story :-)

And was amazed it still passed..

I did try to run without a CAT but it was just too noisy !! I would set car alarms off as i passed parked cars!!

Stu
Stu Dickens

Chris, got a 1.8VVC with CAT bypass.
Failed the MOT, but only just.
Only takes about 30mins to change if you've fitted stainless nuts/bolts when fitting the bypass.
Don't nkow about the lamba sensor.
4bhp is debateable. I've fitted it for more roar from my phoenix exhaust. They're may actually be a case for power loss with it fitted. Can't say I've noticed a diff eiter way.
MOT needs the CAT
MGF Centre, Mike Satur, (just remember to get the nuts and bolts as well)
HTH
Richard P

Agree with Stu, cat bypasses are just too noisy for road use (unless you really don't gie a **** about upsetting the neighbours)
As Huey Lewis says to Marty McFly's band in Back to the Future:
"Sorry kids....you're just too darn loud"
Cannonball Bob

>(unless you really don't gie a **** about upsetting the neighbours)<

Oopps! Guess I'm a bad boy ;-)
Richard P

1) Yes
2) Yes
3) Yes
4) No. VVC likes backpressure and removal of cat can have a detrimental effect on power. My 98 VVC showed -2 BHP when we dynoed it - I run with a sports cat now.
5) Suppled with cat as OE, so tested with cat. Non-negotiable.
6) Usual suspects - Mike Satur, Brown & Gammons, et.al. - I've got my old one in the garage if you want to make me an offer ... ajg@NOSPAMgrid-itc.com (remove the NOSPAM otherwise it won't work).

If you do go down this road, make sure you get stainless nuts & bolts as you'll be swapping it over every year for the MOT (and in the unfortunate circumstances that you are stopped and subjected to a spot emissions check).
AndyG

Right, first off - you say your F is a 2000 model year?

In this case there is a good chance that you won't be able to fit one anyway.

Mk1 Fs are as you describe, lambda sensor is pre0cat in the exhaust system. MY2K Fs run OBD2 compliant MEMS and this means (among other things) an additional post cat sensor (to detect it's presence).

If you decat a MY2K F or TF the MEMS gets very confused and defaults to 'safe mode', robbing you of most of your power.

So the first thing you need to know is whether you have a cat sensor or not. If you don't then the cat will be a bulging straight length of pipe. If you do then the cat will be a bulging length of pipe with a 90 degree bend on one end - the cat sensor is mounted in this bend.

Assuming you have a Mk1, here are the pros and cons:

Cons

1) Car will fail an MOT, but only on the cold start and then only just. 1.8 K engine is a modern design and is practically emmissions compliant without a cat.

2) This means that to pass an MOT you need to re-fit your catalyst each year - providing you use stainless steel bolts and something like Mike Saturs cat saver nuts this is a very easy job that takes between 15 and 30 minutes. Alternatively you could leave the cat alyst in the boot and have a word with your tester. Something along the lines of "The car is fitted with a cat bypass, the catalyst is in the boot if you need to fit it" 9 times out of 10 the car will just be passed - depends on how lazy your MOT tester is, how many cars they have to test that day and how much he wants to charge you extra. :-)

3) Technically the car isn't road legal.

4) Worth -2bhp on a VVC

5) Bypass tube will amplify the noise of your exhaust. Not a problem with a standard set up, but some sports exhausts can become deafening with a bypass fitted.

Pros

1) Bypass tube will amplify the noise from your exhaust! :-)

2) Bypass tube will prolong the life of your cat. Note you still need one to pass your MOT, but as it will spend 90% of it's time on your shelf you aren't wearing it out. Cats have a 6-10 year lifespan after which time the precious metals inside break down and loose thier effectiveness. New cats are around Ģ250.00, although deals can be had from places like the MGF Centre.

3) Bypassed car will pass a roadside emmissions test - i have passed two now. De-catted F only fails on the cold start, roadside tests are always done with a warm/hot engine. :-)

4) Worth +2bhp on an MPi F

5) Increased gound clearance (cat is one of the lowest points under the car, cat pipe removes this low point).

Conclusions

Debateable if this is a good idea or not - up to you how you proceed.

HTH

SF
Scarlet Fever

Hi all,
donīt agree that -2 bhp is static on a VVC when taking off the cat. If the fuelling is taken care of (the original regulator squeezed in a vice OR a FSE unit fitted)the more free-flowing bypass cat-tube is a clear benefit. Together with a de-burred exhaust mainfold it is all well worth the work.

BR, Carl.
Carl Blom

Yep, the +2bhp MPi and -2bhp VVC figures are a little debateable, and very surprising.

Here's how we did the tests:

Andy Gillhooley's VVC F
My MPi F

Same rolling road, same day, both systems were fitted with an MS Daytona rear silencer as this was the best performing aftermarket system on the previous exhaust tests we carried out.

We did a 'with cat' run on each car, then swapped over to the bypass tube and did a 'bypassed cat' run and compared the results. These can be found on Rob Bell's website.

At the same time we also did a run on each car using a generic 'off the shelf' cat with flanges welded on (cheaper alternative) and also a sports cat (expensive alternative).

All the results are within + or - 2bhp of standard.

This is about as good as you can get from a rolling road perspective. Rolling roads are notoriously unreliable ways of measuring power output, but providing all you are interested in the differences rather than overall power figures, you do a 'control' power run, the runs are done independantly of any manufacturers (people with a vested interest in the result) and the runs were done same day, same rolling road, same operators etc then you can get a reasonably accurate result.

However the results were confusing. We were expecting a result of around +5bhp for the bypass on both cars, this we felt was a conservative estimate based upon manufacturers (wild) claims. So when the MPi returned only +2bhp we were a bit taken aback. And when the VVC returned -2bhp we realised we had a problem.

The tests were intended to be published in MG World magazine, but because of the VVC result and the fact that we couldn't explain it the article was binned and now only appears on the web.

So yes, despite our best efforts to remove as many anomolies as possible, frustratingly there is still a question mark over the power results, but to date they remain the only independant data on the subject and therefore are a lot more believable than anything a manufacturer quotes.

The only theory we could come up with on the day (Roger Parker came up with this IIRC) is that the VVC MEMS, being a lot more concerned with the breathing of the engine than the MPi MEMS, is probably more sensitive to back pressure in the exhaust system - again, this is only a theory, but it does make sense and would explain the result. The other possibility is that the tests were flawed somewhere, either way the results remain ambiguous, but until someone repeats the test and gains conclusive, credible results they are all we have.

SF
Scarlet Fever

Hi all,
if ever there will be a second test it would be interesting to go the other way around. A VVC that have had itīs bypass tube for some time ( the adaptive VVC system needs TIME to re-adjust !) and after a test-run on the RR this car is fitted with a cat and run again. Probably this will give some interesting figures and also tell if there is any significant "back-pressure" that robbs bhp with the rather large cat.... At the same time a test with the tweaked fuel-regulator could be performed. Something for Total-MG ?
BR, Carl.
Carl Blom

This thread was discussed between 16/04/2004 and 19/04/2004

MG MGF Technical index

This thread is from the archives. Join the live MG MGF Technical BBS now