Choosing a Camshaft
Optimum cam timing for a nitrous motor will be different
than optimum timing for that same motor off the bottle, so
you will have to make a choice as to whether you want the
most power with or without nitrous. Obviously if you are driving
the car on the street most of the time, you will want the
best power off the bottle. If you find that you can spare
some power to make your car faster at the track, picking a
camshaft to favor nitrous can make a substantial difference
when nitrous is in use. If course it is a trade off, but usually
the power that you make on the bottle, will be far greater
than the amount lost off the bottle.
Pumping Losses
Nitrous oxide adds oxygen, much of which is in liquid
form. So you can see that a large intake valve and port is
not required or desirable. Larger intake ports cause more
of the nitrous to turn to a gas and reduce the amount of normally
aspirated power, if the nitrous takes up more room, there
will be less room for air, reducing volumetric efficiency.
Also, you do not want or need long intake duration or a very
high lift, so the intake side of the cam does not need to
be any different when nitrous is used. The exhaust is a totally
different story. All that extra oxygen and fuel makes for
a substantial increase in exhaust. How can the exhaust valves
deal with this? It can't, pumping losses go out of sight.
Much of the extra power made in the cylinders never makes
it to the flywheel, because it is used to push out the exhaust.
Since making the exhaust valve large enough and the port flow
enough is impractical with most cylinder heads, we must take
other actions to cut pumping losses (which is actually just
a band aid fix).
Reducing Pumping Losses
The first obvious step is to use a dual pattern cam
with longer exhaust duration. Opening the valve earlier will
help by getting the valve open more and bleeding off some
pressure before the piston starts moving up the bore. This
does eat into the power stroke, but more power is freed up
than would be made by holding it closed longer (the best solution
would be a larger valve and better port). The blow down phase
(overlap period) becomes very important in a nitrous engine,
because the gasses has much greater velocity and can over
scavenge, closing the valve exhaust valve a little earlier
helps. Anytime you make more power by reducing pumping losses,
you are freeing up horsepower that already existed in the
cylinders. The engine will still experience the same loads,
but more power will be put to the flywheel and less will be
used to push out exhaust.
Camshaft Specs
As I said earlier, the intake needs to remain pretty
much the same, but the exhaust needs more duration, an earlier
opening point and an earlier closing point. To make this happen,
you need to use a dual pattern cam with more exhaust timing,
and a wider lobe separation angle. Cam's with 112-116°
lobe separations are common is nitrous motors. To keep the
intake timing the same, you must install the cam advanced,
usually 6-8° advanced. The good things about this are
that advancing a cam will bring more low-end (at a trade off
of top-end) when running without the nitrous and the wider
lobe center angle will also help idle and vacuum. Even the
most radical nitrous profiles are usually pretty tame on the
street. Ultra high lift cams are not need to make power with
nitrous. On the exhaust side, the low lift flow is the most
important thing, and must be dealt with much more seriously
than high lift flow.
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