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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