Decat - power loss

[ashdadrummer]

I have a 16v, i fitted a dummy cat and once Id done a few miles to wear it in it lost power, (but it did shoot a good 2ft flame) I fitted the cat back and the power came back, i also noticed a large mid range torque range increase. I recently had a few ignition problems and blew my cat to pieces, i fitted a straight through decat, which granted wasnt a perfect fit but was close enough, iv suffered a similar loss in torque and power again but now i get a cheeky 1ft flame.
its obviously because its running too little back pressure but i havnt heard of anyone else complaining about there decats.
its now running dangerously slow (roughly 115bhp compared to 150+ mid october) any ideas on that also?
just wondered if anyone had any similar experiences or cures?

 

[ashdadrummer]

any ideas?

 

[sammyhale22]

You will lose torque and it will have flat spots on RPM because the ECU is programmed to work with a CAT + smaller displacement engines work better with some back pressure as opposed to larger engines which do not.
The best setup is high pressure after exhaust manifold and low pressure near the exhaust exit to improve power and allow improved scavenging, this improves the gas flow and allows the exhaust gases to leave the exhaust more quickly. Removing the CAT will kill a small engine. Only 2 Litre+ benefit.
Its better to leave the CAT on and just get a better flowing exhaust system, one with a lower pressure backbox.

 

[Dunn]

ive seen nothing but bad things on my decat
an its for this reason im taking it off this week
as my ramps arrived today
leave the cat on as above or if you have the money go for a sports cat
much better imo

 

[mat-nos]

get a re-map to work with the free flowing exhaust/decat or get a sports cat on the standard ecu.

 

[ashdadrummer]

thanks guys.

I guessed it had affected the scavenging but Id only heard good things about decats on these engines.

What bad experiences did you have with your decat?

I would put my cat back on but its in pieces dont really fancy paying out for another one.

Any ideas if a cup chip would help at all?

 

[st3ve]

What engine are you running it on with 150bhp?

 

[ashdadrummer]

a standard clio 16v lump, little bit of diy to the inlet manifold and throttle body, magnex cat back system and huuuuuuuge pipercross with a bit of cold air to it, magnecor leads and iridium plugs, ooh and the power steering removed.

 

[ashdadrummer]

Ive just realised my car list says 1.2/16v ????? :S
Its a mk1 1.8 16v hense why i even considered a decat

 

[TimR26]

Ive just realised my car list says 1.2/16v ????? :S
Its a mk1 1.8 16v hense why i even considered a decat

That did have me confused lol. Maybe try getting it remapped.

 

[ashdadrummer]

i can get hold of a valver ecu with a cup chip off an old tarmac rallyed mk1
any chance that would be a worthwhile upgrade?

 

[mat-nos]

i had the cup chip with piper x ik, full magnex exhaust with decat, supersprint 4 branch, cams and adjustable pulleys, lightened/balenced flywheel and an uprated fuel pump made 165 ish bhp on a few different rr's. but because of the cup chip i picked cams that moved peak power up the rev range as you loose touque with manifolds for some reason and the cup chip has an 8k rev limit............addictive

so yes a worth the eprom upgrade if your going to ugrade other components too if not just dumps more fuel in and has a raised rev limit for no reason......

 

[kev182cup]

the back pressure thing is a myth. basically what a exhaust does is create a vortex of air that your engine pushes out but say if the diameter of the pipe is too large the engine will use too much effort to do so and will lose power, such as your case. if you had back pressure it would push the gasses back towards the engine and fight against air getting pushed out.
basically you need to work out the correct diameter of ya exhaust system. id say 2 inch would be a good size. 2.5 for a cs if any1 was interested.

 

[ashdadrummer]

well id like to put the ktec cams in at somepoint but ive got a few more bits before i get to the engine properly.

Iv got a 2" with one mid box and then 2" to the 2.5" back box

 

[sammyhale22]

Back pressure isn`t a myth. It is responsible for creating a vacuum in the exhaust manifold which in turn causes a vacuum effect through the exhaust valves.... which helps draw air into the cylinder chamber on the valve overlap. Hence it assists smaller displacement engines and can seriously cripple a smaller engine if it isn`t present e.g. removing catalytic converters etc.