To decat or not to decat? this is the question

Level 6 User Joined: 26 Jan 2005 Posts: 159

Cheers, now Im less decided than before! cat, decat, cat, decat..........as I pull petals off a daisy

Site Subscriber Joined: 07 Oct 2004 Posts: 2281

The general concensus is to go for it mate, but hang on to your original CAT as you'll need it for emissions test on the MOT.

Level 6 User Joined: 26 Jan 2005 Posts: 159

Ok, my brother has finished digging and blocking a 8 x 3 x 5-6 foot deep pit so I can do things like oil changes and decating without a fuss

Forum Moderator Joined: 02 Mar 2004 Posts: 19978

Gentle Ben wrote:

Bad explanation, here's another:

The catalytic converter unit places slight restriction on the flow of exhaust gases as they exit the car. Replacing the cat with a section of pipe eliminates this obstruction and the exhaust gases exit the car at a higher velocity (higher gas flow).

Yes a CAT imposes a SLIGHT restriction to exhaust gas flow. The temps are high as well which helps to make sure it doesn't drag its feet through the system. Obviously a lower cell count means less restriction but cat body shape also has a lot to do with it

If the flow of gas through an internal combustion engine is increased, then an increase in power can be felt and a higher top speed can be attained. The downside is that exhaust back pressure is reduced. Back pressure in the exhaust keeps the air/fuel mixture of the next engine cycle in the cylinders longer, thus allowing more torque to be evolved since the burn is kept in longer.

Exhaust back pressure is not wanted on any engine. Its a restriction that increases pumpung losses, lowers EGT's (lower the temp falls the slower the gas moves) and causes reversion into the cylinder diluting the mix thus reducing output. If adding back pressure makes the car quicker at a point then its due to the CAMSHAFT(s) having to much over lap at that engine speed causing over scavenging. Also the burn time doesn't change ever.

The time it takes to burn at 1krpm is the same as it would at 18,000rpm. Keeping the burn in longer doesn't do anything. I have yet to see a properly set up car or a standard car actually continue ignition when the exhaust valve opens, to do this the timing would have to be retarded way back to some silly angle.

What does change is mixture density. At 1krpm at idle theres not much air and fuel in there so theres less shove on the crank during the combustion cycle. At full throttle theres a lot more air and fuel in there so theres a harder shove. Its an effect of valve timing wether or not some of all that air and fuel go straight out the exhaust, if the mixtures not there and can;t burn it doesn't make power - simple.

In short decat = higher top speed but acceleration (0-60 times) will suffer.

only if you short chamge up to 60, you will be at wot (I mean what really) and high revs so its not an issue.

The choice, is yours! (In my best early 90's quiz show voice).

Site Subscriber Joined: 07 Oct 2004 Posts: 2281

Well what can I say to that exactly!

Yes I know you know your stuff Chris but fundamentally there's nothing wrong with my explanation.

Site Subscriber Joined: 29 Jan 2005 Posts: 1108

I've got a de-cat AND a cat on my car, look...

Posted: Tue Jul 05, 2005 4:15 pm

Bet that purrs along nicely

Site Subscriber Joined: 13 Oct 2004 Posts: 4722

wow, that cat is identical to my mates..

Level 10 User Joined: 22 Mar 2005 Posts: 621

Very similar to mine too, but mine looks more like Sylvester.

Forum Moderator Joined: 02 Mar 2004 Posts: 19978

Gentle Ben wrote:

Well what can I say to that exactly!

Yes I know you know your stuff Chris but fundamentally there's nothing wrong with my explanation.

how is there nothing wrong with your explaination? Its wrong. The answer is roughly right but the way you got to it is wrong!

Its like beatign stephen hawkign at a maths quiz cos you pressed the wrong button on the calculator and got the right answer!

Mr Quoter-vator Joined: 16 Feb 2004 Posts: 7405

good analogy.

no offense to anyone, but this is why the internet is full of misleading shit, because some ppl "think" they know what they are on about (whether they genuinley think so or are just bullshitting) and start preaching "this is the way it is"

then ppl who dont know better start taking this advice as gospel and spread it or do it...l

my opinion, if you dont know just say so, its much more respectful to say your not sure and ask than it is to preach and bullshit in a manner that says u know everything.

this is my opinion. like i said, no offense.

Forum Moderator Joined: 02 Mar 2004 Posts: 19978

To quote someone else, who I don't know, maybe I'm making it up

theres nothing more dangerous than an ill informed idiot!

For example lowering is a good point. slam it and it will handle loads better and grip like a racecar.

What tosh, it might be better on a track but thats the only place!

Site Subscriber Joined: 28 Jan 2005 Posts: 3547

Chris H wrote:

Its like beatign stephen hawkign at a maths quiz cos you pressed the wrong button on the calculator and got the right answer!

PMSL

Forum Moderator Joined: 02 Mar 2004 Posts: 19978

danlp6 wrote:

Chris H wrote:

Its like beatign stephen hawkign at a maths quiz cos you pressed the wrong button on the calculator and got the right answer!

PMSL

at my line or my spelling? lol

Site Subscriber Joined: 28 Jan 2005 Posts: 3547

lol! the coment, i could of corrected the spelling, but it wouldnt be original then!

Level 10 User Joined: 18 Jun 2004 Posts: 614

Chris H wrote:

Gentle Ben wrote:

Bad explanation, here's another:

The catalytic converter unit places slight restriction on the flow of exhaust gases as they exit the car. Replacing the cat with a section of pipe eliminates this obstruction and the exhaust gases exit the car at a higher velocity (higher gas flow).

Yes a CAT imposes a SLIGHT restriction to exhaust gas flow. The temps are high as well which helps to make sure it doesn't drag its feet through the system. Obviously a lower cell count means less restriction but cat body shape also has a lot to do with it

If the flow of gas through an internal combustion engine is increased, then an increase in power can be felt and a higher top speed can be attained. The downside is that exhaust back pressure is reduced. Back pressure in the exhaust keeps the air/fuel mixture of the next engine cycle in the cylinders longer, thus allowing more torque to be evolved since the burn is kept in longer.

Exhaust back pressure is not wanted on any engine. Its a restriction that increases pumpung losses, lowers EGT's (lower the temp falls the slower the gas moves) and causes reversion into the cylinder diluting the mix thus reducing output. If adding back pressure makes the car quicker at a point then its due to the CAMSHAFT(s) having to much over lap at that engine speed causing over scavenging. Also the burn time doesn't change ever.

The time it takes to burn at 1krpm is the same as it would at 18,000rpm. Keeping the burn in longer doesn't do anything. I have yet to see a properly set up car or a standard car actually continue ignition when the exhaust valve opens, to do this the timing would have to be retarded way back to some silly angle.

What does change is mixture density. At 1krpm at idle theres not much air and fuel in there so theres less shove on the crank during the combustion cycle. At full throttle theres a lot more air and fuel in there so theres a harder shove. Its an effect of valve timing wether or not some of all that air and fuel go straight out the exhaust, if the mixtures not there and can;t burn it doesn't make power - simple.

In short decat = higher top speed but acceleration (0-60 times) will suffer.

only if you short chamge up to 60, you will be at wot (I mean what really) and high revs so its not an issue.

The choice, is yours! (In my best early 90's quiz show voice).

As I've found out. more power around town with the cat on. make's for a better drive. De-Cat was good for top-end. 5k+