De-cat

[Tony]

From the Geometry up i am blind to the modifications you guys do!.... but often i hear people mention de-catting to increase exhaust flow.... then i hear the price..... me being not so educated in this field i wonder this... 'Why not just remove' the existing cat and smash the ceramic out of it?... all that would remain is the hollow shell.... the cost £0... isn't that a de-cat?

[Adie]

You'll still need to put the cat back on for MOT time.

[Tony]

You'll still need to put the cat back on for MOT time.

Seeminly not.... i know of a few from LOC who have had a de-cat and their cars still pass the MOT.... just!

[Bazza]

You'll still need to put the cat back on for MOT time.

Seeminly not.... i know of a few from LOC who have had a de-cat and their cars still most the MOT.... just!

 

most from LOC have only removed the pre-cats , the main cat remains, and will only pass the MOT if tested whilst hot

 

the pre-cats are there to act as a primary cat untill the main cat reaches operating temperature

[discount tyres dan]

you can sometimes manage to scrape a car a through a proper emitions test without a cat, but some places will get your car through an emitions test by using another car to get the readings, if you ask nicely enough ££ .

 

usually the cars that have a cat on the manifold and one on or near the front pipe sail through because the manifold cat does most of the work any way.

 

the reason a straight bit of pipe is used is that a hollow cat sounds terrible, raspy and rattly because the gases cant flow smoothly through the pipe.

[Tony]

You'll still need to put the cat back on for MOT time.

Seeminly not.... i know of a few from LOC who have had a de-cat and their cars still most the MOT.... just!

 

most from LOC have only removed the pre-cats , the main cat remains, and will only pass the MOT if tested whilst hot

 

the pre-cats are there to act as a primary cat untill the main cat reaches operating temperature

I understand that...(just) but what pre-empted this thread was some MR2 SW20's that only has one cat... this made me wonder since the de-cat must has the same uniform shape as the origonal to fit.... To my mind there is nothing mechanical about an empty box so why pay the price for a modifyed unit?

[jon]

I understand that...(just) but what pre-empted this thread was some MR2 SW20's that only has one cat... this made me wonder since the de-cat must has the same uniform shape as the origonal to fit.... To my mind there is nothing mechanical about an empty box so why pay the price for a modifyed unit?

 

Imagine aerodynamics inside a pipe. The stock cat on an MR2 turbo comes off the turbo and enters the manifold. To fit a cat in this somewhat small space they had to make the pipe bulge like a balloon to fit all the ceramic in there.

 

Taking the actuall cat out of this bulbus pipe would certianly greatly increase spool times on the turbo cars as the exhaust gasses would have a much smoother flow, however due to the non uniform shape of the chamber there would be a large amount of turbulance (or dirty air).

 

A cat replacement pipe (as i have sitting infront of me waiting to be fitted) has a VERY smooth path for the air to flow as it never had to accomodate a cat resulting in near zero back pressure.

 

To summerise cat out is good, replaced pipe is better. Tuning the exhaust on turbo cars is all about getting the gasses out as quickly and as uneventfully as possible, the replacement pipes aid this.

 

Regarding the legalities, pre august 1995 MR2 Turbos don't have a model report for the MOT testers to match to so they go through the very lenient "default" test. Shame that

 

HTH a little.

[Tony]

I understand that...(just) but what pre-empted this thread was some MR2 SW20's that only has one cat... this made me wonder since the de-cat must has the same uniform shape as the origonal to fit.... To my mind there is nothing mechanical about an empty box so why pay the price for a modifyed unit?

 

Imagine aerodynamics inside a pipe. The stock cat on an MR2 turbo comes off the turbo and enters the manifold. To fit a cat in this somewhat small space they had to make the pipe bulge like a balloon to fit all the ceramic in there.

 

Taking the actuall cat out of this bulbus pipe would certianly greatly increase spool times on the turbo cars as the exhaust gasses would have a much smoother flow, however due to the non uniform shape of the chamber there would be a large amount of turbulance (or dirty air).

 

A cat replacement pipe (as i have sitting infront of me waiting to be fitted) has a VERY smooth path for the air to flow as it never had to accomodate a cat resulting in near zero back pressure.

 

To summerise cat out is good, replaced pipe is better. Tuning the exhaust on turbo cars is all about getting the gasses out as quickly and as uneventfully as possible, the replacement pipes aid this.

 

Regarding the legalities, pre august 1995 MR2 Turbos don't have a model report for the MOT testers to match to so they go through the very lenient "default" test. Shame that

 

HTH a little.

 

Elloquently explained to a 'no brainer' like me in this area... nevertheless a valid question though! don't you think.

[jon]

A valid question indeed.

 

A simpler answer would be anything slowing down the air exiting the engine / turbo is bad, and a replacement pipe minimises obstructions.

 

Of course i'm sure someone without a turbo will chirp up about engines needing back pressure to make good torque but i'll address that if it comes up (it's false)

[Tony]

Governor...... your thoughts?

[Jap Devil]

For a forced induction engine (and not so critical for N/A) the best policy means getting in as much COLD air as possible, meaning efficient induction and forced through a quality intercooler. Followed by removing the exhaust gasses through the least restrictive pipework while maintaining an acceptable noise level.

 

The most efficient exhaust will use a manifold of equal length pipes (to equalise back pressure on each cylinder) and a decatted rear system.

 

The power gains achieved by decatting can be very cost-effective in terms of bucks-per-bang!

 

Word of caution: Replacing the stock exhaust with an unrestrictive system will screw up the fuelling; you'll fuel too rich at low revs and lean out at high revs. It's well worth having a remap done at the same time.

[TDIPLC]

I understand that...(just) but what pre-empted this thread was some MR2 SW20's that only has one cat... this made me wonder since the de-cat must has the same uniform shape as the origonal to fit.... To my mind there is nothing mechanical about an empty box so why pay the price for a modifyed unit?

 

Imagine aerodynamics inside a pipe. The stock cat on an MR2 turbo comes off the turbo and enters the manifold. To fit a cat in this somewhat small space they had to make the pipe bulge like a balloon to fit all the ceramic in there.

 

Taking the actuall cat out of this bulbus pipe would certianly greatly increase spool times on the turbo cars as the exhaust gasses would have a much smoother flow, however due to the non uniform shape of the chamber there would be a large amount of turbulance (or dirty air).

 

A cat replacement pipe (as i have sitting infront of me waiting to be fitted) has a VERY smooth path for the air to flow as it never had to accomodate a cat resulting in near zero back pressure.

 

To summerise cat out is good, replaced pipe is better. Tuning the exhaust on turbo cars is all about getting the gasses out as quickly and as uneventfully as possible, the replacement pipes aid this.

 

Regarding the legalities, pre august 1995 MR2 Turbos don't have a model report for the MOT testers to match to so they go through the very lenient "default" test. Shame that

 

HTH a little.

 

 

On the MR2, the cat doesn't "come off the turbo and enter the manifold", it's between the turbo and downpipe. I think Mr Toyota made the cat the shape it is to allow for a sufficient volume of catalyser material to fit in such a small space, which is a shame because the shape of the outer shell (and the replacement de-cat pipes) have an angle of convergence which is disruptive to air flow, so there is no real difference (from a performance perspective) between removing the catalyser material from the standard cat or using an aftermarket de-cat pipe. I think the main difference is the potential hazards to health from removing the catalyzer material, and the inability to refit the cat for any purpose if needed. Additionally, removing the material can be quite difficult (especially if the cat is fairly new) so if you are paying someone to do it for you it could potentially be as expensive as buying a de-cat pipe. Personally I would go for a de-cat pipe every time to keep my options open.

[jon]

On the MR2, the cat doesn't "come off the turbo and enter the manifold", it's between the turbo and downpipe.

 

Blonde moment, sorry.

 

I think Mr Toyota made the cat the shape it is to allow for a sufficient volume of catalyser material to fit in such a small space, which is a shame because the shape of the outer shell (and the replacement de-cat pipes) have an angle of convergence which is disruptive to air flow, so there is no real difference (from a performance perspective) between removing the catalyser material from the standard cat or using an aftermarket de-cat pipe.

 

You obviously know a fair bit more than me (i'm still at initial book stage) but we both agree the initial turn off the turbo isn't exactly healthy for airflow and both agree the chamber after this again, isn't ideal. A replacement pipe, not having this chamber (i'm happy to post pictures of mine) WILL help airflow out of the turbo so will help, however small.

[Tony]

On the MR2, the cat doesn't "come off the turbo and enter the manifold", it's between the turbo and downpipe.

 

Blonde moment, sorry.

 

I think Mr Toyota made the cat the shape it is to allow for a sufficient volume of catalyser material to fit in such a small space, which is a shame because the shape of the outer shell (and the replacement de-cat pipes) have an angle of convergence which is disruptive to air flow, so there is no real difference (from a performance perspective) between removing the catalyser material from the standard cat or using an aftermarket de-cat pipe.

 

You obviously know a fair bit more than me (i'm still at initial book stage) but we both agree the initial turn off the turbo isn't exactly healthy for airflow and both agree the chamber after this again, isn't ideal. A replacement pipe, not having this chamber (i'm happy to post pictures of mine) WILL help airflow out of the turbo so will help, however small.

Post the pictures! The likes of yourself, jap devil and TDIPLC are familiar with the conversation but people like me and any others watching this thread are not, and need as much help as possible to understand.

[TDIPLC]

For a forced induction engine (and not so critical for N/A) the best policy means getting in as much COLD air as possible, meaning efficient induction and forced through a quality intercooler. Followed by removing the exhaust gasses through the least restrictive pipework while maintaining an acceptable noise level.

 

Different cars react differently to tuning methods (otherwise r&d establishments wouldn't be necessary), but generally speaking, the exhaust efficiency is the primary consideration before the intake.

 

The most efficient exhaust will use a manifold of equal length pipes (to equalise back pressure on each cylinder) and a decatted rear system.

 

That's not really correct as the "equal length pipes" issue is more to do with pulse tuning than back pressure.

 

The power gains achieved by decatting can be very cost-effective in terms of bucks-per-bang!

 

That is true in many cases, but not all.

 

Word of caution: Replacing the stock exhaust with an unrestrictive system will screw up the fuelling; you'll fuel too rich at low revs and lean out at high revs. It's well worth having a remap done at the same time.

 

That is an over generalization as different models (even different years of certain models) react differently to tuning methods. Without doubt, recalibration of the fuel and ignition parameters can be beneficial or even essential in some cases.

[Jap Devil]

Here's a pic of the 3" turbo downpipe for the MR2 SW20 Rev3 and above. The pipe bolts onto the turbo at the end with the 6 boltholes and the bend at the other end replaces the cat chamber and bolts directly onto the (de)cat-back exhaust.

 

[jon]

And here's part of the pipe it replaces

 

 

http://cgi.ebay.co.uk/Toyota-MR2-Turbo-CAT...1QQcmdZViewItem

[Tony]

And here's part of the pipe it replaces

 

 

http://cgi.ebay.co.uk/Toyota-MR2-Turbo-CAT...1QQcmdZViewItem

Ultimately i have to ask to what end! Is the De-cat really worth the expense as a stand alone modification... what BHP would it add if it was?.... TDI indicated that to enhance the de-cat other areas would need to be addressed to finalize the modification.... to me 'the novice' this makes sense because i cannot understand how 'one' modification offers a complete solution.... feel free to educate me!

[Jap Devil]

Ultimately i have to ask to what end! Is the De-cat really worth the expense as a stand alone modification... what BHP would it add if it was?.... TDI indicated that to enhance the de-cat other areas would need to be addressed to finalize the modification.... to me 'the novice' this makes sense because i cannot understand how 'one' modification offers a complete solution.... feel free to educate me!

 

Decatting is just part of the process of helping the engine to breathe more efficiently. Using the MR2 as an example, the first stage would be:

 

1) Induction

2) Downpipe & decat

3) Cat-back exhaust

 

After which, the big bucks need to be spent:

 

4) Upgraded intercooler/chargecooler

5) Bigger turbo

6) Standalone ECU / remap

 

Right now, I have 1,2,3 & 6 and have dyno'd at 295bhp (fly) compared with the standard 245bhp. Future plans are a TD06 turbo and Greddy intercooler to give around 300rwhp

[Tony]

 

Ultimately i have to ask to what end! Is the De-cat really worth the expense as a stand alone modification... what BHP would it add if it was?.... TDI indicated that to enhance the de-cat other areas would need to be addressed to finalize the modification.... to me 'the novice' this makes sense because i cannot understand how 'one' modification offers a complete solution.... feel free to educate me!

 

Decatting is just part of the process of helping the engine to breathe more efficiently. Using the MR2 as an example, the first stage would be:

 

1) Induction

2) Downpipe & decat

3) Cat-back exhaust

 

After which, the big bucks need to be spent:

 

4) Upgraded intercooler/chargecooler

5) Bigger turbo

6) Standalone ECU / remap

 

Right now, I have 1,2,3 & 6 and have dyno'd at 295bhp (fly) compared with the standard 245bhp. Future plans are a TD06 turbo and Greddy intercooler to give around 300rwhp

 

A comprehensive list indeed.... are these mods specific to the MR2?... can any areas be 'fine-tuned' or bypassed to cut out the middle area.. I wonder if i could own a stock MR2 and add 5+6 and dismiss the rest?....be gentle I'm just looking at the posabilities through the eyes of someone on a budget.

[jon]

In your example you'd probably not want to fit an uprated turbo without the 1, 2 and 3 supporting mods as the inlet side couldn't get enough air to the turbo and the outlet side wouldn't be able to expel gass quick enough.

 

A good rev3 turbo engine can dyno upto 280bhp with just breathing mods, much more power after that isn't productive on turns

[Tony]

In your example you'd probably not want to fit an uprated turbo without the 1, 2 and 3 supporting mods as the inlet side couldn't get enough air to the turbo and the outlet side wouldn't be able to expel gass quick enough.

 

A good rev3 turbo engine can dyno upto 280bhp with just breathing mods, much more power after that isn't productive on turns

There is always a catch.... seemingly you cannot push more out unless the car breaths more in... 'ratio' is it 50/50 what is the expenditure?

 

And i must ask....'Mapping' how personal is that?

[Jap Devil]

In your example you'd probably not want to fit an uprated turbo without the 1, 2 and 3 supporting mods as the inlet side couldn't get enough air to the turbo and the outlet side wouldn't be able to expel gass quick enough.

 

Exactly......

[Pip]

No Tony it would defeat the purpose. You can have the biggest turbo and intercooler in the world but if you are taking in air through a straw and passing it out through a a hole no bigger than than your backside it aint gonna happen.

 

If you were on a budget sort the breathing out first as Mark pointed out.

 

P.S I have just read the second page of the thread, doh!!

[Tony]

In your example you'd probably not want to fit an uprated turbo without the 1, 2 and 3 supporting mods as the inlet side couldn't get enough air to the turbo and the outlet side wouldn't be able to expel gass quick enough.

 

A good rev3 turbo engine can dyno upto 280bhp with just breathing mods, much more power after that isn't productive on turns

There is always a catch.... seemingly you cannot push more out unless the car breaths more in... 'ratio' is it 50/50 what is the expenditure?

 

And i must ask....'Mapping' how personal is that?

I am still very curious about 'Mapping'.... can anyone explain this for me?