B18A and B18B engines
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The first step to getting more horsepower is getting your engine to breath better. An intake, a header and an exhuast system are the keys to doing that, often referred to as I/H/E. Remember, you get what you pay for, cheap stuff may save you some money but in the end it hasn't usually had the research and development of the more expensive stuff and wont make the same power

Intake

AEM, Arc, Fujita and Injen are all good choices. If you are looking to make more power go for a cold air intake (CAI) over a short ram (SRI). My personal favourite is the Injen CAI. It's a good unit for the price, its CARB legal and it's features a lifetime warranty. Although if money is no object the ARC induction box would be a good choice.

Header

Out of I/H/E the header will make the most difference, it will also determine how much power your future modification will make, so dont cheap out, invest in a good one. Skunk2, Fujitsubo and 5zigen are all quality headers.

Exhaust

In terms of making power, most exhuast will offer little difference from one another, the main thing is the sound. The only thing you want to be careful of is that you choose the right size. Go 2.25" - 2.5" for a small build ie just I/H/E, go for 2.5" or above if your planning on going further ie camshafts, headwork etc.

Tuning

By now your probably noticing that your car is a little bit faster, but get it well tuned and it will be totally transformed. Find out what your local tuner prefers to tune on for your best results. However in any Honda your can't go past a Hondata ecu the AEM Engine Mangement System is also great if you tuner knows how the tune it. Once tuned go out and have some fun with your car!

Throttle Body and Intake Manifold

These will net you a little more power but are particularly important if you are going to go to the next step i.e. modifying the head. The Skunk2 is an excellent choice for a mildly modified car, for an extensively modified car the Edelbrock VictorX is an excellent choice.

In terms of throttle body Skunk2 is again a good choice, I would also recommend Professional Products throttle bodies, they are cheap but still do the job. Choose something a little larger than your standard one, there’s no point bolting a 70mm throttle body onto a car that previously had a 62mm. A 65mm or 68mm would be a better choice and would make more power, unless of course you are going to go further with camshafts etc.

Now that these are done get a tune! This is about as much horsepower as you are going to get from bolt-on products, to go further you must open up the engine.

Before you go to the next step there are a number of things to consider.

Is your drivetrain up to scratch? Believe it or not upgrading the drivetrain will actually make you go faster. Many people have noted that a car with an upgraded clutch, lightweight flywheel and higher final drive will be quicker it a straight line than one with an intake header as exhaust.

Are your brakes up to scratch? Safety first people, more power means you need to be able to stop quicker too!

Is your fuel system up to scratch? If you don't know ask your tuner they should be able to tell you if your car is nearing/reaching its limit or still has plenty of room before you need to upgrade.

Next step: Opening up the engine

So, you’ve done all the basic bolt on modifications and are ready to crack the engine open ( if you haven’t please do these first, the engine wont be able to breath properly with them!) You first need to decide on a budget and consequently determine whether you are only going to modify the head or your going to build the bottom end too. This is important for many reasons the main being that certain camshafts make more power at certain compressions. You can’t install the biggest cams available if you are only going to run standard compression. The first thing to do is to determine your compression if you haven’t already done so. Here’s a list of some Honda engine’s and their respective compression ratios. Please also note that the higher compression you go the less room for error you have for tuning, one mistake could cost you the engine, but don’t let this deter you, any good tuner will have experience with this so it shouldn’t be a problem.

Camshaft choices for a Mildly modified head
Crower Cams (403's), bisi stage 2 cams

Required: I recommend you upgrade your valvesprings and retainers

Highly Recommended: Adjustable Cam Gears

Camshaft choices for a Medium modified head:

Crower Cams (404's)

Required: Adjustable Cam gears, Valve Springs, Retainers

Highly Recommended: Increased compression (11-11.5+)
Recommended: Head Porting

Adjustable cam gears are recommended to get the most power out of your cams but remember to get them adjusted when your car is getting tuned otherwise you will not see any benefit. Generally best to match your cam gears to the same brand cams although it’s not a huge issue. Otherwise AEM cam gears are great value and a superb product. It’s impossible to tell you what settings to dial your cams gears in at as every motor is different, once again your tuner should know how to set them.

Valve Springs and Retainers:
If you are revving higher than your engine was designed to these are a must. If the valve springs can’t compress and retract fast enough you could get valvefloat at high RPM which could be catastrophic for your engine. See this youtube video if you want to see it happen Aftermarket retainers are also important as they will hold up to higher RPM driving much longer, the stock retainers simply aren’t up to the task. In addition aftermarket retainers are usually lighter meaning your engine revs much more freely and you’ll hit the point of maximum power much sooner.

Bottom End

Pistons & Rods

Upgrading your pistons to higher compression equivalents is an essential part of increasing the compression of your engine. The upsides of higher compression are that you will make more power, especially with larger camshafts and the engine will be overall more responsive, the downside being that there is a smaller window for error when tuning, however any competent tuner should be able to tune with them no problems. Most aftermarket pistons are also forged, making them stronger. For forced induction more people choose to run lower compression pistons as these setups are harder to tune and for more reliability, However it is very possible to run a high compression ratio on a turbo car with extensive tuning, the benefit being a much more responsive engine. Some people also choose to run larger bore pistons to increase their displacement and consequently torque, this is definitely worth considering keeping in mind that if you race it could put you in a different class. Get a set of rods that will fit your new pistons and are lightweight and strong for reliability.

Pistons: ITR pistons, JDM pr3, JDM P30, CP pistons

For naturally aspirated applications I strongly recommended investing in the lightweight crank (only $100-$150 extra) for reduced rotational mass and to allow the engine to rev much more freely.

Sleeving your block is highly recommended for forced induction and encouraged in bigger naturally aspirated builds for reliability. A properly installed sleeve kit will prevent warping of the cylinder walls during high RPM driving. Stock Sleeves on a B18a and B18b can hold 300 hp with a good tune, but if you are planning to use forced induction then re-sleeving is recommended.

good info...hopefully this will help with the ppl searching now

great write-up.