catalytic converter, catalytic converter Options

[redceli]

im getting my exhaust system done at a shop and someone told me if i take the catalytic converter out you will loose some torque. is this true? if it is then how much horse power would a performance catalytic converter give you.....thanks

[Galcobar]

Cars do not need backpressure -- it's a myth.

Big pipes have lower initial backpressure, but quickly create turbulence (a form of backpressure) because they let the gas expand, cool and slow down. Exhaust therefore cannot exit the engine as quickly and efficiently, and energy that should be going to turning the crank is instead being used to push exhaust gases out.

Backpressure is an inevitable consequence of not having a variable exhaust size. The harder the engine is working, the more exhaust it needs to get rid of, and the ideal situation would be for the exhaust pipes to grow as the engine pumps out more fumes.

Since we don't have that technology, what we do is pick a compromise size. It's not as efficient at low revs as a small pipe, because the exhaust gases have room to expand, cool and slowdown, reducing the scavenging effect and forcing the engine to expend energy on moving what is now heavier gas. At high revs it's not as efficient as a larger pipe, because the gas bottlenecks. Either way, you get backpressure. The compromise aims to minimize that backpressure over the band, and preferably have the most efficient exhaust extraction (high velocity, lowest backpressure) at the RPM point where the engine spends the most time.

Really big pipes are good only if you're producing enough exhaust to be efficiently using them. This is where FI and NO2 come in, since they effectively simulate greater displacement and therefore a bigger engine.

The muffler is part of that compromise. Removing it alters the volume at which the exhaust system is most efficient at extracting exhaust gases from the engine.

Don't confuse free-flowing (aka high velocity) with high capacity. A small straight pipe is more freeflowing than a large serpentine pipe for good reason. And once you get the piping as straight as possible, you're back into the issue of matching size to exhaust volumes.

[Negative]

Cars do not need backpressure -- it's a myth.

The rest of your statement does not support your opening statement. But we have all heard this before.
The fact is the rest of your statement above backs up what I said above. Besides experience and trial and error prove facts better than just making statements and since I'm on my 5th Celica and I have had a variety of exhausts I do have a wealth of experience. Some of the exhaust systems I put on my previous Celicas actually decreased the performance. The method above is the best setup I have had so far - not to say that there might not be room for more improvements.

[Galcobar]

The rest of your statement does not support your opening statement. But we have all heard this before.
The fact is the rest of your statement above backs up what I said above. Besides experience and trial and error prove facts better than just making statements and since I'm on my 5th Celica and I have had a variety of exhausts I do have a wealth of experience. Some of the exhaust systems I put on my previous Celicas actually decreased the performance. The method above is the best setup I have had so far - not to say that there might not be room for more improvements.

Actually, my entire point is consistent -- backpressure is never needed, it is always bad.

There are two consequences of backpressure. The first is energy is wasted as the engine has to push the exhaust gas out of the combustion chamber, meaning less energy is available to drive the wheels. The second is the imperfect evacuation of exhaust gases from the combustion chamber, meaning there is some left to foul the incoming air-fuel mixture and therefore decrease the combustion efficiency. Either way, you get less power to the wheels.

If we could design a system with no backpressure, we'd have an ideal exhaust system.

It's entirely possible to put an exhaust system on that decreases performance -- just like it's possible, to a certain point, to improve performance with a better design. These improvements are of course relative to the other exhaust systems tried. A one-inch pipe on our cars produces backpressure by lack of capacity. A four-inch pipe produces backpressure by lack of flow velocity. Either way, you have backpressure and less than the best possible performance.

Your exhaust system is not ideal. It is however, the best you can do for your style of driving with this particular engine. The best exhaust system we can do minimizes backpressure at the point where you want your engine performance to peak, because backpressure is always a drag on engine performance. Somebody else might prefer a different setup because their driving style means they prefer to maximize available power at a different RPM -- perhaps with the aim of improving low-end power. They can do so by choosing an exhaust setup that is most efficient at that chosen RPM and therefore minimizes backpressure at that RPM.

This post has been edited by Galcobar: Feb 16, 2006 - 9:10 PM