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Hey! So I have an 07 Si Coupe and I essentially want to get the full skunk2 exhaust package (Header and Megapower Cat Back).

However I'm a bit confused and overwhelmed by all of the different info out there and I can't seem to get a clear explanation of what I need to get. Forgive me if I sound like a noob when it comes to talking about car parts... BUT in photos of other headers I've seen that some come with 2 pieces, and/or they are longer than the Skunk2 Alpha. I know that getting the cat back exhaust will get me from the CAT to the end of the exhaust (tail pipe), but is there a pipe in between the end of the cat back and the start of the header? I know that area of the exhaust is where people "straight pipe" their cars and I've also heard that there are potentially 2 versions of the Alpha header... one that comes with a straight pipe and one with a high flow CAT? (maybe that's wrong?) As far as I know there are also 2 versions of the cat back that have different sized piping... is that something I need to consider for fitment? As you can tell I'm very confused about all the possibilities.

Bottom line, I know I want to get those 2 parts (Skunk2 Header and Cat Back) But I'm confused on what all the options are and if I'm missing anything! I also don't want to get the wrong parts. Any insight on this would be much appreciated! (For the record I do already have an intake and flashpro)
 

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it can be a confusing topic. Depending on where you live be careful messing with the CAT. Test pipes and high flow cats work well but modern cats are not nearly as restrictive as those in the past.

As to size of the pipe there are some differing opinions.

Exhaust is pulled out your pipe via a vacuum created by the exhaust pulse . Each time a cylinder fires an exhaust pulse races speed-of-sound-like out the pipe, If you have an oversized exhaust 3 or more inches and you dont have enough exhaust volume (running nat aspirated for instance as opposed to supercharged) you may run into back pressure issue where you have the exhaust pulses backing up on each other due to the large pipe not allowing for the creation of an effective vacuum . As the pulses back up so does the exhaust hurting performance. Cross and H pipes were created to take advantage of exhaust pulses.

2.5 in is a great size pipe for a 2 litre high revving N/A motor like the k20. I think 2.5s sound better than 3s on a N/A civic. Much less fart can, more superbike.

Now turbo and supercharged motors are all about moving the most air/fuel/exhaust so bigger pipes are called for .

So n/a I would stay 2.5. Better sound, better exhaust scavenging, and generally better performance as long as its kept N/A. Even upgrades to cams, intakes, and Hondadata a 2 litre N/A motor will not push enough exhaust to overwhelm a 2.5.

You cant go too wrong with bigger intakes, headers, injectors and shite but too big a pipe and your exhaust pipe may become a performance killer.

I am sure you will hear differing ideas about this issue but many of these people are unaware of what actually moves the exhaust down the pipe. They think it is the force of combustion that pushes the exhaust out. Not the case. All pulse action and vacuum.
 

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Okay this is something I had to learn the hard way.

There are 2 styles of headers for these cars.
Oem style and aftermarket style is what i say.
OEM style headers are very short. About as long as your forearm, they are usually called shorty headers and are seen on our cars as 4 to 1 configuration. And do Not have a cat attached.

Aftermarket headers are very long, usually 4 to 2 to 1 config.
and can come with the cat attached to them.

The OEM cat from the factory is basically a big and long 3 inch downpipe that reaches up to the shorty header closer to the engine

aftermarket cats are very short and only bolt onto the end of long aftermarket headers at the bottom of the car
 

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If I remember 4-2-1 is better for midrange and 4-1 is top end.

Unequal length headers (think Subie WRX) have a more stataco sound than the more rhythmic equal length. Once again the exhaust pules is the factor. If the pulse has to travel further on one side of the motor than the other, it will sound discordant (but still cool). Pules' traveling along equal lengths sound smoother to our ear and are generally what you find in most vehicles.

For the money I would only get the header and then make sure the gains are focused mid-range where the K20 needs it. There are noticeable gains with certain aftermarket headers but the value vs gain prospect falls off precipitously with the addition of the CAT back on N/A motors. Stick with 2.5 (unless you turbo/supercharge) and spend the money you saved on the CAT back for a set of mild cams or pillow ball bushings for your compliance bushing and strut mount.

Some upgrades are knee jerk these days. Rear camber kits are a good example. If you are not going lower than 1.5 inch's on your drop and you cant heel-toe shift then you can skip a camber kit. Lots of other stuff to spend on and get a better return in go-fast or dynamics. So keep in mind the original intent/vision you have for your car and purchase accordingly
 

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Ya off of the headers you'll have your flex pipe and cat and then your exhaust. They do have high flow cats if you get one get one that's a 100 cell as the higher the number the more restriction there is. I ended up just gutting my cat so it's a straight pipe essentially but will look they there's a cat to pass inspection. 4-2-1 is the way to go as they are more free flowing. For my 06 si I went with a plm header as they are known to be good headers and sibce I did a k24 swap they are made to not hit the firewall like other headers are known to do. I also went with a skunk2 mega pro series 3" catback exhaust and it's a great exhaust if you can find it so I highly recommend that. I've heard a lot of good things about the alpha headers as well. If you do put some headers on I recommend wrapping then with some exhaust wrap as it'll help keep your temps under the hood down a bit, especially if you have a short ram intake. If you have emissions tests or anything like that I recommend getting the version with the high flow cat so you will still pass the test if you don't have emissions or anything like that just go with the straight pipe version so you'll gain a few extra ponies. The straight pipe is also known as a test tipe. If they don't have the skunk2 mega pro series 3" or 76mm exhaust in stock I suggest getting the Invidia 3" exhaust. Don't go with the 2.75" exhaust only go with a 3". Hope this helps man.
 
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