Plus and minus points for removing cat
- Thread starter precurser
- Start date
Precurser,
As per your parameters, we will ignore the legal requirements on noise and emission. But the primary role of the exhaust system is to facilitate the flow of exhaust gases from the engine header with the highest velocity, and/or with the least amount of backpressure. Hence you cannot view this mod in isolation, as the exhaust gas will only flow as well as the greatest restriction.
If say you have a poor pipe or muffler design then it will choke the flow no matter how good the rest of the system is designed. On the other hand, if you are employing a well-designed cat, there is very little excess gained power from de-cating. What matters is the construction of the cat, where abrupt angles from which the gas enter and exit make huge differences.
One more thing to note - trying checking if there are high-performance cars de-cating (I am talking overseas, over various forums)? I might be wrong, but I suspect the rarity, since there should be very little power difference from de-cating in these cars with well-designed cats.
Hope this helps. Cheers.
As per your parameters, we will ignore the legal requirements on noise and emission. But the primary role of the exhaust system is to facilitate the flow of exhaust gases from the engine header with the highest velocity, and/or with the least amount of backpressure. Hence you cannot view this mod in isolation, as the exhaust gas will only flow as well as the greatest restriction.
If say you have a poor pipe or muffler design then it will choke the flow no matter how good the rest of the system is designed. On the other hand, if you are employing a well-designed cat, there is very little excess gained power from de-cating. What matters is the construction of the cat, where abrupt angles from which the gas enter and exit make huge differences.
One more thing to note - trying checking if there are high-performance cars de-cating (I am talking overseas, over various forums)? I might be wrong, but I suspect the rarity, since there should be very little power difference from de-cating in these cars with well-designed cats.
Hope this helps. Cheers.
The cats are always the largest restriction in any halfway good exhaust sytem.
Many take the stock cats off. They are high cell count, highly restrictive units. Few take off race cats because they are bought with the intention of fitting into rulesets or quieting a guilty conscience.
Any exhaust system that is contructed for an application with the most basic rules in mind, will not have any restriction along it that is anywhere close to what a stock, performance, or race cat for that matter. Look at any type of cat and you'll see. You don't even really need to know the cell count because the best ones are still restrictive. If it weren't it would not even be a cat because there is not enough surface area for the reactions to take place.
Many take the stock cats off. They are high cell count, highly restrictive units. Few take off race cats because they are bought with the intention of fitting into rulesets or quieting a guilty conscience.
Any exhaust system that is contructed for an application with the most basic rules in mind, will not have any restriction along it that is anywhere close to what a stock, performance, or race cat for that matter. Look at any type of cat and you'll see. You don't even really need to know the cell count because the best ones are still restrictive. If it weren't it would not even be a cat because there is not enough surface area for the reactions to take place.
Hey Allan! The pre-cats are purely for emissions. Cats need to get up to the right temps to start converting right. Cold starts are the worst for emissions because 1) cat(s) are not up to temp and 2) cold engines have MUCH worse emissions vs a warmed up engine. Having a small and low mass cat near the exhaust ports allows quick heat up of the unit, so it can function properly and convert as much of the bad gasses into less harmful ones.
A large single cat near the exhaust ports would not heat up quickly enough because of the large mass sinks too much heat (just like a large mass rotor absorbs lots of heat before saturating, except in this case you WANT quick heat saturation). Packaging that close to the engine might also be a problem.
Cheers :wavey:
A large single cat near the exhaust ports would not heat up quickly enough because of the large mass sinks too much heat (just like a large mass rotor absorbs lots of heat before saturating, except in this case you WANT quick heat saturation). Packaging that close to the engine might also be a problem.
Cheers :wavey:
Re: Plus and minus points for removing cat
Hi Precurser, if you look around the exhaust threads, you'll find some info on losing backpressure vs stock, the points made in those posts should answer your question since the issues are the same.
precurser said:
Hi Precurser, if you look around the exhaust threads, you'll find some info on losing backpressure vs stock, the points made in those posts should answer your question since the issues are the same.
piggyboyz said:
np man.
Determining which would be better to de-cat would need you to take at the cross sectional area of the cells, cell density, and length of both the pre-cat and the cat. With that you will know how much drag each causes and which one will yield a larger increase in flow when eliminated.
Intuition says that the larger main cat will have much more length to it, but then again, the cross sectional area of the pre-cat will likely be smaller, so inspection is necessary to decide. What is for sure though, is that flow will not be good till both cats are removed. Stock cats are just performance killers - whether 1 or 2 in series.
The backpressure issue has been discussed in one of the recent exhaust threads. I think Eric, Rodney, and I, posted our views on it. Ok found it.. http://www.bmw-sg.com/forums/viewtopic.php?t=2866&start=15
Hi Shaun,
I read some forums, some saying that the earlier we merged the twin-pipe to one, you will get very good low-end torque ? true ?
BMW 6 pots has two 3to2 header and the 2 pipes will go into cat, mixed in there and than out 2 pipes again to center box, mixed again, 2 pipes to rear box.
I read some forums, some saying that the earlier we merged the twin-pipe to one, you will get very good low-end torque ? true ?
BMW 6 pots has two 3to2 header and the 2 pipes will go into cat, mixed in there and than out 2 pipes again to center box, mixed again, 2 pipes to rear box.
NTY
Well-Known Member
De-cat Performance
Hi guys,
Let me share my experience on this. I had done de-cat by:
a) gutting out the entire cat content. (Pre cat, post cat and both)
b) replacing the cat section with straight through pipe.
c) full set of performance exhaust.
The gain comes from (C). Both (a) and (b) have no performance gain based on gtech timings and terminal speed.
Hi guys,
Let me share my experience on this. I had done de-cat by:
a) gutting out the entire cat content. (Pre cat, post cat and both)
b) replacing the cat section with straight through pipe.
c) full set of performance exhaust.
The gain comes from (C). Both (a) and (b) have no performance gain based on gtech timings and terminal speed.
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