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Discussion Starter · #1 ·
anyone had a problem with decats i did mine the otherday chopped out the cat and welded a pipe in its place but i seem to get flat spots sometimes now.
but when its working by hell theres a differance. ECU light is allways on also now :(
 

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Its gone into safe mod, basically the lamba sensors have picked up high concentrations of exhaust gases such as CO. The ECU cuts out and drops the power output (i think) to stop you doing any long term damage to the engine (if there was a geniune fault) you need to purchase an ECU eliminator from Janspeed for around 27 quid :)
 

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Also removing the cat has altered the exhaust backpressure and on NAS engine you need backpressure to help with exhaust gas scavenging.
 

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OK, I'll try!

Couple of things to start with, everything has mass, even air and once something is moving then it has momentum.

In your engine you have exhaust valves and inlet valves.

The petrol engine in our car is a 4 stroke engine so 1 complete cycle of the engine equates to 2 revolutions at the crankshaft.

1. Induction. The piston moves down creating a depression(suction) in the cylinder. The inlet valve opens allowing the fuel air mixture to enter the cylinder.

2. Compression. The piston now begins to rise up, compressing the mixture, the inlet valve is now closed to stop this fuel/air charge being pushed back out the inlet.

3. Power. Once the piston is at (or very near) top dead centre, then there is a spark to ignite the mixture. This resulting explosion then forces the piston back down giving the power.

4. Exhaust. Once the piston has reached the bottom of its travel, the exhaust valve opens and as the piston gets pushed back up so the burnt gasses are expelled.

Thats basically it. Now there are little things here that help. One is that if you open the inlet valve near when the piston has nearly finished pushing all the gasses out then the mass flow of this air going out the cylinder has caused a slight depression in the cylinder. This depression can be used to help pull in more fuel air mix so in fact there are times when you have both the inlet AND the exhaust valves open together. This process is known as scavenging where you use the inlet charge to help push the exhaust out (or if you like, the exhaust to help drag the inletcharge in)

On NAS (Normally aspirated) engines then a slight positive pressure on the exhaust side has been found to be beneficial in this process. It is to do with the fact that if you increase velocity then you decrease pressure and inversely if you increase pressure then you decrease velocity. It helps this rush on inlet fuel/air into the piston rather than straight out the exhaust valve.

With forced induction (ie Turbo/supercharge), it is the converse that is true. You need as little backpressure as possible as the inlet charge has positive pressure and so will scavenge the exhaust gases far better.

I dont know if that has helped any but exhaust gas scavenging is difficult to explain simply.
 

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Yes as it does take away some of the restriction in the exhaust system but if you reduce the backpressure too much then you will get less scavenging and can suffer flat spots and poor pickup.

Another drawback as well is that most cars have some sort of lamda sensor and some can monitor hydrocarbon/C0/C02/02 levels. Now that you have changed the whole exhaust perfrmance then you need to get some sort of lambda bypass and also get the ecu remapped on a rolling road to take advantage of this reduced exhaust backpressure as you have now changed the whole engine characteristics.

If you dont then you will have an engine set up for a catalytic converter running without one. You cant just take one off and expect a big performance gain without doing that.

Dont forget that it will all be totally useless as well if you dont improve the induction system as well. Both need to be improved at the same time. It is no use having a performance exhaust allowing all the exhaust gasses out nice and easily if the inlet is restricting the airflow in or is poorley routed. Also if you have a really good induction system, free flow with lots of cold air and huge ducting and plenium chambers and then have a system that restricts the exhaust, isnt balanced etc then you will have the same problems.

It is a VERY complex science. Look at F1, all those different teams all with thier own design of chassis, aerodynamics, suspensions etc. All bar Ferrarri buy thier engines in from dedicated companies like Ford, BMW, Honda and employ technicians from those engine companies to develop and maintain those engines. It is a VERY exact science and just chopping and changing things without testing can be very hit and miss. You might get lucky and make an improvement, but considering that the manufacturers have already done a lot of engine development/design/testing etc, you are just as likly to make things worse.

Neccessity is the mother of invention
 

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rite now your getting technical !!

Reet Ive got a Strait thru pipe were my cat used to be, I also have a stainless steal performance back box.

On the other side I have a 52mm throttle body and a Open K and N strait on to it... I have got the cold air feed running strait from the mesh bit at the bottom of the front bumper to about 2 centimeters away from the air filter.

but ...

I aint had my ecu remaped or an eliminator fitter for the engine management lite.

Is that ok or would i need other shit dun ??

Also do you know how you have a small muffler after the cat ? can u remove that and fit a striat pipe there too ? will that do n e thing ?

cheers
 
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