The theory of exhaust systems ?

all very interesting guys, can someone please just tell me the exact exhaust i should have installed, its a 1st gen, tah

[dylmah]

this was a sweet post. thanks for the ideas

[SABBAi]

Another area to consider, is the cooling of the exhaust gases throughout the length of the exhaust.

It's a good rule of thumb that you want to make the exhaust closeest to the turbo, as big as you can. This is why you will see dump pipes that come out the same size as the exhaust wheel and jump up in diameter drastically. Not gradual. The idea behind this is that it creates cavitation which sucks the gases out of the exhaust housing. This also being the hottest area.

The larger tubing, usually around the 3" mark, will give the gas less restriction. This is a common aftermarket upgrade. 3" dump and front pipe. If you are going down the street leagl route, a high flow cat is the next step. This area of the system would see temperatures dropped a bit as well. The larger piping whens less restriction, meaning less friction, equaling less heat.

Many of the aftermarket cats that are 'high flow', flow well for a cat. Think of 1000's of tiny square tubes that run the length of the cat. They are designed to heat up to silly temps to burn excess fuel/oil/etc to comply with emissions. a 3" high flow will not flow the same as a 3" pipe of the same length. chances are, that it will flow similar to that of a 2.25" to 2.5" piece of tube of the same length. this is the reason that the cat converters are always scorching hot.

This is always the first point of restriction in an aftermarket system. Having this in mind, you could jump up to a 4" cat. The flow of a 4" cat would be similar to a 3" pipe of the same diameter. This is all fine and good, but now you have another localised heat section. If you do a sudden drop in pipe size back to 3" then you will have the problem of bad flow.

Realistically, with a 3" front section and 3" cat, a 2.5" cat back with a resonator and a semi straight through, or even a full straight through rear muffler will give you a heat more improvement over the standard system. Keep in mind it will be a lot louder.

But it is all a a significant cost. If you are going to design the system to best suit your needs, you will not get out of it for a couple of hundred. The other thing to think about is whether or not you run a mandrel bent or crush bent exhaust system. Mandrel keeping the tube size the same through the bend. Crush bends of 2.5" bring the area inside closer to that of 2".

[boki82]

nice one SABBAi i agree on what you have said

[Cromax]

I think some of it's right ... some of it's wrong though.

[SABBAi]

Originally Posted by Cromax

... some of it's wrong though.

Which parts do you think are not correct?

[Cromax]

I am starting to get very sceptical about what people are saying about exhausts ... as I know that what you are saying would be good if applied to a car that does track - but not so much for a car which is stuck in traffic...

... apologies about saying some of it was wrong without stipulating exactly what it was ...

.. the cat does not simply burn off excess fuel, it actually changes the structure of the molecules for Hydrocarbons (HC), Carbon Monoxide (CO) and Nitrous Oxide (NO) into DiHydrogen Oxide (H20 ... aka water), Carbon Dioxide (CO2) and Nitrogen (N2) ... as these emissions are essentially benign. You're right that they do operate at high temperatures, but reducing the operating temperature of a cat below about 450 degrees celsius (this is from memory) means it is not performing its job, and therefore you might as well just replace the cat with a straight pipe.

I hope I make sense.

[SABBAi]

I was talking about a system for a turbo car. I wouldn't run a 3" system on any N/A car that was for the street.

This type of exhaust system is fine for a car that is used for street work. Stop start traffic. Being stuck in traffic has no effect on how an exhaust system works, when you consider that the engine is not far off idle, just for long periods of time. Sure engine temps may rise, but that is due to lack of airflow through the radiator. Not the size of the exhaust.

The info you have supplied about the working of a cat is, as far as I know pretty accurate. I was happy not to include as much info, for the sake of simplifying.

Cheers Cromax.

[NicoEvo]

Hey does ne1 know the stock exhaust size?

[SABBAi]

On what car?