Header/HF Cat/Cat-back explaination

[caverman]

I was asked by a member to give an explanation of what the HF cats and Cat-back look like and how they hook up. I’ll explain it to the best of my knowledge. If anyone else wants to comment or correct where I’m wrong please feel free to do so.

I took a picture of the LG system to point these out since they have the most complete image.

You can see that I marked off what the Long Tube (LT) header is.

Then you would connect either a High Flow (HF) catalytic converter (cat) or an Off-Road connection pipe (pipe without a cat).

From there your cat-back system will connect up. This can either be your factory cat-back or an aftermarket cat-back like Corsa, Borla, Magnaflow, etc.

So, if you already own or plan to buy an aftermarket cat-back or if you want to use the factory cat-back they will all connect up to either the HF Cat or Off-Road connection pipe. In the picture they show the exhaust with the Off-Road connection pipes hooked up but if you were to run the HF cat you would replace that connection pipe with the HF cat (they put them next to the exhaust to show where they would be installed).

You'll want to purchase the HF cat if you live in a state/county that requires them for inspection. The cat is part of the emissions system and is used to burn off excess gas in the exhaust in order to have cleaner emissions coming from your car. They do this by using a honey comb looking material that heats up to extremely high temps in order to burn off the excess fuel.

The factory cats have a 600 cell honey comb inside. That means there are 600 small openings to allow air to go through. HF cats typically have a 200 cell honey comb. As you can imagine if you have 600 cells vs 200 cells in the same amount of space (the inside of the cat) you'll have more material to slow the flow of air thus causing a significantly more restriction in your exhaust. HF cats have less cells or less material inside which allows for more air flow. Those cell numbers may not be exact but in my research those were the numbers I was given.

Then there is the Off-Road connection pipe and there are 0 cells in them. That means you get complete "un-restricted" flow for that amount of space.
The downside to running Off-Road cats is that since the excess fuel is not being burned away you’ll have more of a stronger smelling (gas smelling) exhaust from the outside of your car. This should have no effect on the inside of the car.

The downside to running HF cats is their cost compared to Off-Road pipes and a horsepower of two loss from the slight restriction in air flow.

[caverman]

Here are some close ups of the factory cat vs HF cat vs a cat that has had the inside remove (this is the same as off-road connection pipe)

You can see how the factory cat has very small area to allow for exhaust to flow. Then compare that to the more air that a HF cat has. Then compared to an Off-Road or no cell pipe.

[caverman]

There is also another option. That is for those who want to swich out the factory cats (sometimes called mid-pipes) with a set of HF cats. This would be used with the factory manifolds (headers) instead of LT headers.

The same concepts apply. You would be recplacing the section that goes from the factory manifold to the the cat-back. They would include one HF cat vs the factory mid-pipes having two cats.

Here is a pic of the factory mid-pipe and then a SOLO HF cat mid-pipe.

[somewhatdamaged]

Awesome summary! I wish this was around when I first was exploring my exhaust options!

[Smirnoff]

Caver great write-up.....I will also add that w/o Cats, there is a real-real nasty smell that gets everywhere.....