Technical About to decat (back pressure issue?)

[Funkstar De Luxe]

But before I do, I'm looking for some advice. I can get the job done for about £70, which isn't bad considering the quality of their work.

The problems is, I don't know if it'll actually give my any gains. I've been reading about back pressure being necessary on some NA engines, and removing the cat also lessens the back pressure - thus leading to a BHP loss.

Anyone decatted their B and can speak from experience? Would love some advice on this.

Thanks guys

Tony

 

[docorangepeel]

KC fitted a sports CAT a while back and he said it gave him a few more BHP but I don't know anyone who has taken the CAT out completely. If you are looking at a better gas flow you should also look at changing the manifold over as well. Supper Sprint do a replacement manifold and KC and myself can vouch for that upgrade.

 

[Funkstar De Luxe]

Well, from what I can gather a sportscat and decat have roughly the same affect on performance, the only difference is the price and emissions.
What about the lambda sensors? Am I going to run into problems there?

Would love to change the manifold, but I'll get a custom when I do.

 

[docorangepeel]

Sensor is on the down pipe infront of the CAT so no problems at all.

 

[HP]

There's a lot of guff mumbled about backpressure, but what happens in a properly designed system is that the exhaust gas velocity is kept high enough to effectively 'pull' the last of the exhaust out of the cylinders and allow more clean air and fuel in for the next cycle.
If the pipe is too small, the exhaust gas won't be able to get out fast enough - think blowing through a straw - and the charge in the cylinder won't be cleared as well as it might.
If the pipe is too large, the exhaust gas will be travelling too slowly to create the 'pull'.

Trick is finding the balance between too large and too small. A catalytic converter will add resistance to flow, so a high flow sports cat or a decat with a correctly sized pipe can add a noticeable amount of power.

 

[Funkstar De Luxe]

How do I calculate a correct size? Will the guys in the shop do this any way? They seems to have a great reputation...

 

[beacon]

Will it affect emissions at the MOT at all? I decated my last 3 bikes (always leaving the lambada probe in) and it made a real difference, but on bikes there is no emisssions test.

Chris

 

[Funkstar De Luxe]

Yes, in all likely hood it will fail the emissions test. Easy enough to fit the cat back on, or slip the MOT guy a fiver.

 

[HP]

How do I calculate a correct size? Will the guys in the shop do this any way? They seems to have a great reputation...

Go for a proven system like Supersprint or the like. These guys put a lot of time into designing the systems. Calculating the correct pipe size involves fluid dynamics type stuff, that tends to make my head hurt.

If you are looking at just removing the cat and keeping the rest of the system standard, as I'd guess you might be from the £70 bill, you will gain a bit - but not as much as if you replace the rear silencer with a less restrictive design at the same time.

 

[Funkstar De Luxe]

I'm already running a SuperSprint back box. I'll take her down to the garage and see what the guys say. Thanks for all the help, really put my mind at ease

 

[HgtBill]

If you decat this engine you lose torque and create flat spots in the mid range.

 

[Funkstar De Luxe]

If you decat this engine you lose torque and create flat spots in the mid range.

If that's true, then wouldn't a sports cat do the same thing? I'll just leave it in the mean time, I really need to be sure it won't mess things up. I don' fully understand why some methods of increased exhaustion are benefitial, while others are not. And the size of the decat pipe, surely it's not that important as the exhaust will still be as restrictive as the smallest part, no? I mean, I can under stand how a huge tube will pool exhaust gasses and a narrow one will restrict flow, but a happy medium should be, common sense dictates, no larger or smaller then the rest of the system it's immediately connected too. All those classes of fluid dynamics haven't paid off

 

[Mad Matt]

First of all let me say I'm no expert!

I've heard from various people that what HGTBill says is often the case. However a re-map can fix that.

In general a de-cat will be beneficial as the improved flow brings more benefits than losses to a car like the Barchetta. This is based on discussions I've been part of on other forums and talking to people who "race" prepare cars.

If I was in your place I'd sort out the air intake, de-cat the exhaust and then get a re-map. I wouldn't be too worried about it making things worse.

On the other hand a re-map, plus a GSR kit or CDA and cat-back replacement seems to get you to about 153bhp. I'm not sure that removing the CAT will get you any further. Perhaps some fast road cams would be the next step if that's what you want.

As I say, I'm not an expert this is just what I've picked up from other peoples experience over the years.s

 

[s130]

Yes, in all likely hood it will fail the emissions test. Easy enough to fit the cat back on, or slip the MOT guy a fiver.

While I see "your tongue in cheek" I have to complain slipping anybody anything to elude something, is ONLY something non Italian car drivers, chavs and boy racers would do.

Last thing I want to be hit by is any wheeled lethal weapon not fit for the road.

However, asking the tester if they can use their testers discretion and lift the idle revs a little to see if the emmisions become acceptable is perfectly valid and legal.

I've a long standing relationship with my local council MOT testing station over many years. Unlike my local Fiat garage who I take cakes etc. at Christmas my poor old MOT testers/station remain totally devoid of such good will gestures. We chat, we laugh, we even almost cry but neither side crosses that 'boundary'.

If they (as in council type MOT test stations who have nothing to gain from a fail) fail me then I KNOW it is a valid fail, and even if it was borderline then I take it as a job well done and a potential life saver.

Emissions are a real problem, especially with my Strada Abarth which would automatically fail current MOT standards for its year of manufacturer. Here the homolated figure % CO exceeds the MOT test level. I've managed to tune the car below the MOT level but doing so has it problems. Initially the MOT station (guys who knew/know their beans) understood that a twin sidedraft setup would be almost certain to fail the CO idle test. However they had the legal discretion to slightly lift the normal idle revs. If the CO level dropped below then that was OK. I had to go through this "discretion" loop many years ago. Since those days I carefully our 130TC to pass 'asis'
as presented tests.

Decent MOT stations are there to help you and not make money at your expense (beyond standard test fees).

Play clean, play safe and actually you will be better off, learn a shed load from the testers and be legal.

 

[Funkstar De Luxe]

Haha, it was indeed tongue in cheek. I doubt anyone would risk loosing their job and business for £5 Nor am I sure I would trust any MOT station that would take a back hander.

I don't think I will go ahead with the decat - I was never completely convinced it would work anyway. Would still like a few more horses from the B, but I might have reached the point where increasing it any more will cost serious money.

That was a good read though, Nick, thanks.

 

[afroking]

slipping anybody anything to elude something, is ONLY something non Italian car drivers, chavs and boy racers would do.

I hope that is meant as 'tongue in cheek' too!

Funkstar De Luxe, has your car been remapped yet? If not I would suggest trying it, you will notice a difference.

 

[docorangepeel]

As far as I know you can only map the older 95-99 ECU B's but is this really true? Does anyone with the newer Hitachi ECU have a new map?

 

[Funkstar De Luxe]

Would love a remap, but I'm not sure how possible it is. I've heard a few guys mention it's not possible, but I'll look into it.

I'm sure someone here has a remap done.

 

[Funkstar De Luxe]

To be on the safe side, I wouldn't run a decat as you never can guarantee that you pass emissions.

Currently the cat isn't a legal requirement in the UK.

If you have one Lambda sensor you won't have any trouble with the ECU. If you have a pre and a post cat sensor you can run into ECU problems. In this case you should get a sports cat.

If you have the cat in the manifold (depending on age) you benefit from a sportscat/decat. The manifold would need replacing and the cat has got to go further down.

If the cat is in the collector pipe it won't give you a great deal of an improvement to replace it by a sports cat/decat.

The main restrictions in most of the Fiat group exhausts is in the back box.

I hope it answers your question. If not, feel free to ask me with any more specific question regarding the exhaust.

Pmed another forum member...

 

[s130]

I hope that is meant as 'tongue in cheek' too!

Yes, tongue in cheek to the first part, but absolute defiance to the other part.


Now...let me see....which part is which ....

Lest there be any confusion of to where I am or stand for then here it is.

1) Legal - YES
2) Fraud - NO
3) Fun - YES - YES
4) Legal, insurance registered (where required) - YES

On the remapping....no problem. Go for what you can afford including any insurance premium increases.

Just be aware that it is now standard practice for insurance companies examine ECU maps, along with other mechanical modifications to check that what they have insured is what you were driving at the time of a claim. For the same reason that people don't decalre their modifications to save cost is the same reason insurance companies now check that you are not trying to cheat on them.

What may be a cool modification now could cost you dearly for the rest of your life, and also destroy your family's life, as well as that of your victim, at the same time! Taking a life of anybody, let alone a child, with a fully insured vehicle will devastate you emotionally. Doing it with an uninsured vehicle will, if not you, kill your family.