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can some1 tell me the difference between 4-1 headers and 4-2-1????? im looking to install some on my 04 civic ex coupe but i have no idea which ones to choose

 
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4-into-1 headers are simpler to make, but are designed to work at constant high-midrange to high rpm -- racing. Can make more power than a similar 4-2-1, but will take it from the midrange and move it higher. Usually cheaper as it's easier to fab. Least favorite headers are 4-1 shorties, not great unless you're running a turbo.

4-2-1 will typically boost upper-low-rpm and midrange more than high rpm power, but can also lift the entire powerband up without taking too much from the top, if it's from a reputable maker. Way better for street cars, and your PCM's stock maps will like it better, I think. More expensive due to tri-Y design.

To me, even for racing a 4-2-1 is a superior design for an exhaust, as it makes the engine easier to control. However, tuning is more than an exhaust... do your homework.

 
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thanx for the info..... do you think dc headers will go good.... i have a aem v2 cold air intake, throttle body spacer, and a skunk2 catback exaust on my 04 civic ex?

 
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When it comes to makers... you may want to ask your question in the Basic Performance forum, since that's what it's for, and the experts on these matters will be helping there.

I would never install a 4-1 on my EX... unless I could get a 4-2-1, may as well stay stock

 
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the biggest issue is the 4-2-1 deletes the O2 sensor, so you will be screwed during smog ( if your state requires it)

 
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^True. Though doesn't some maker weld bungs onto a D17 4-2-1? I know they make Escort 4-2-1s with a sensor bung, so there has to be one for the D17... otherwise the aftermarket for the 7th gen is a lot weaker than I thought. :P

 
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DC sports makes a 4-2-1 with sensor bungs. It deletes the cat though!

 
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What do you guys recommend?

 
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oh, yeah, i mixed sensor and cat :P

one that does not delete the cat - the ones that do are described as so, non?

 
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So the ideal street exhaust I'd buy with my own money... would be a freer-flowing, 4-2-1 that does not delete the cat. DC's one is obviously off-road-only. Does such an animal exist? Seems like it could be bolt-in, if they used the length in the head pipe as the third 'Y' (like I thought the stock system was ). Cat could then be used... but I've read here someone mention the stock cats clog fairly soon. :/ Some welding required.