NITROUS CAMSHAFT SELECTION

Posted: August 18, 2011 in Nitrous Oxide
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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. Of 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, due to the loss of air speed, and reduce the amount of normally aspirated power.  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 pressures and volume. 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.

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, referred to as cylinder blow down, 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. 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 in 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 cylinder blow down pressure is the most important thing and must be dealt with much more seriously than with the use of high lift camshafts.  Camshaft selection with a more aggressive opening ramp is one way to relieve these cylinder pressures faster in order to reduce pumping loss.
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