Intake to Exhaust Flow Comparison
For a normally aspirated engine the exhaust flow needs to equal about 75% of the intake flow. When nitrous enters the equation, the exhaust needs to flow more. How much more depends on how much nitrous you're planning on using. As a general rule of thumb, the exhaust flow needs to be increased about 5% for every 20% increase of power from nitrous injection. In other words, if 40% of your engines power is made from nitrous, the exhaust flow needs to be equal to about 85% of the intake flow. This is of course limited to how much the head can be modified.

Intake Port Work
NOS Kits add so much oxygen that getting oxygen in is no longer a problem. A large intake port is not needed or desired. The larger the port, the more surface area it has and the intake charge will have lower velocity. Slower moving nitrous has more time to turn from a liquid to a gas, so a large port will have less liquid nitrous getting in the cylinder. As nitrous turns to a gas it will expand, taking up room in the intake and reducing the amount of normally aspirated air. More surface area will give the nitrous more area to absorb heat, which will cause even more nitrous to turn in gas. The same goes for large intake valves. The intake valve is the hottest part of the intake system and when nitrous is involved you don't want excess surface area on the valve. For small-block Chevy motors, a 1.94" intake valve is more than enough for even all out 500+ hp nitrous motors. The exhaust is a different story.

Exhaust Port Work
All the extra exhaust has to be dealt with. The exhaust valves of a nitrous engine are almost always too small. When possible it is best to reduce the size of the intake to allow room for a bigger exhaust valve. A 1.94" / 1.5" Chevy head is a good starting point. 1.6 valves can be installed with no problems and even 1.65" are possible. The head of the exhaust valve should not have any sharp edges. It should have a nice smooth radius to allow the exhaust to travel around it as easily as possible. The valve job on the exhaust is the most important part, there will be so much more cylinder pressure when the exhaust valve opens which makes for a lot more gasses trying to escape through the valve at low lifts. Low lift exhaust flow should be your number one concern (up to about .300" lift). A good multi angle valve job is the best bang for the buck in a nitrous engine. The short side radius will usually benefit from a straight cut to the port floor. The area directly past the seat should be as wide as possible. The valve seats should be slightly wider also (.010"-.015") to help get rid of some of the extra heat in the valves that nitrous will make.

Combustion Chamber modifications
Usually you cannot do much chamber work without reducing compression and being forced to use a high dome that hurts power. With nitrous, a high compression ratio is not needed, so some work can be done in this area. Nitrous can make some very respectable power with compression in the 10:1 area. First step is to unshroud the exhaust valve as much as possible so the gasses can move around the valve easily. The next step is to polish the combustion chamber and remove any sharp edges. Sharp edges will be the first to get hot and cause detonation (as well as be the first to melt). Polishing the combustion chamber will help keep carbon build up to a minimum (a good idea for any engine).

Rocker Arm Studs
One area that is often overlooked is the rocker studs. The intake does not get any extra loads from nitrous, but the exhaust studs will get much more abuse. There much more cylinder pressure when nitrous is being used, so the exhaust valve will have to open against a great deal more pressure. It is not uncommon for rocker studs to break in nitrous motors, because most engine builders do not realize the extra loadings on them. Always use a quality exhaust rocker stud and where possible, use a larger diameter stud.

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