Header & Exhaust

The header and exhaust is for a 1999 civic si. I'm looking for the best hp
gain. Which company would be best?

 

Sell the Honda and buy a Ferrari.

You will see virtually zero HP gain unless you change all the other aspects
of your engine, such as:
1) Camshafts
2) EFI map
3) Ignition curve
And then you have to still conform to your local emissions regulations...

Horsepower is not something that magically appears out of the blue when you
add a low restriction exhaust to a vehicle that might not have been
strangled to begin with. Has it occurred to you that you might get LESS
horsepower if you muck things up?

Do some Googling before you waste your money and become a Rice Boy.

 

The exhaust is only a part of what I want to do. The car will be getting
cams and a forced induction system along with a ton of other things.

 

If you do some searching you will find tests between different headers
and exhausts and what their power gain was.

Generally you should use a 2.25" pipe exhaust for your civic for a little
bit of gain. If you throw a really wide fart can on it (2.5"-3" pipe),
you will end up loosing power.
With Headers, a DC Sports (although not the best, but is adequate) 4-2-1
header will be a good match to a 2.25" Exhaust.
As far as numbers go, dont expect to make a lot more power.
With an Exhaust/header/Cold Air Intake combination, you would probably
look at gaining 10hp within the powerband.
When I did this to my old 95ex I could feel a little bit more pull when
accelerating, especially when Vtec kicks in.
Always keep in mind, your Civic wasn't desinged to be a race car. It is
(for all intents and purposes) an economy car.

The first thing you should ask yourself is what your goal is as far as
uprgrading your engine.
Are you looking for conservative basic bolt-ons, or would you like to add
some forced induction in the future?
A major NA rebuild of the engine or an engine swap.
It all depends on what you want and how much money you can spend.
Also add in downtime while working on the car, and I would highly suggest
finding a reputable technician that can help work on your car.

Dont get sucked into the Rice thing. Spend your money on the engine
before spending it on looks.
Do it right and think things over.
If you find yourself buying something just to satisfy the urge of doing
something to your car, dont do it.
Plan everything out first.

I would suggest spending sleepless nights reading everything at
www.honda-tech.com
That website contains an invaluable amount of information.