Nitrous Cam vs. naturally aspirated

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Nitrous Cam vs. naturally aspirated

Post  460bronco on April 14th 2012, 10:57 am

This is a VERY general question, but I was wondering what the main differences between a dedicated nitrous cam and dedicated NA cam are? Are there any trends like increasing or decreasing LSA, changing Intake centerline, lift and duration respectively???

Thanks in advance,
Jon
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Re: Nitrous Cam vs. naturally aspirated

Post  bruno on April 14th 2012, 12:15 pm

A cam optimized for nitrous (N) is quite different than a cam optimized for NA use. It will make a lot more power while on N, on the order of 10-15% of the nitrous boost, but lose quite a bit compared to an optimized NA cam off the juice. I want to take a minute to explain a little about this and offer suggestions about compromises that will work pretty well with either.

Using N poses a problem. An optimum head for nitrous use would have a larger exhaust valve at the expense of a smaller intake valve. When you add N, you are drastically increasing the volume of exhaust gasses needing to be pumped out. If you add a 150hp shot on to a 300hp motor, 50% more exhaust gas is created, for example. Not only due the pumping losses go up dramatically, there is more residual gas and contamination of the intake charge to deal with. The exhaust valve area would need to be 50% larger to fully compensate.

Not only is 50% more exhaust valve area completely impractical, it would severely compromise NA power. So, the cam must be used to compensate. The method of compensation is to open the exhaust valve earlier. As a rule of thumb, the range to look for is 10-20 degrees earlier with 150-300hp of nitrous. There are a couple of ways to open the exhaust earlier. We can increase the LSA, increase the exhaust duration, or advance the cam. As with all aspects of cam design, there are always tradeoffs. Increasing the exhaust duration will also increase overlap when typical cam profiles are used. If we had exhaust lobes specifically designed for nitrous available, which most of us do not, just the opening side could be lengthened. Increasing the LSA opens the exhaust earlier, but decreases overlap. As with a wider exhaust lobe there will be a downside if the overlap was correct in the first place.

This leaves us with advancing the cam. Let's look at some valve events by starting with a typical aggressive SBC HR cam: 294/300 on a 110LSA installed 4 degrees advanced. The valve events are IO/IC-EO/EC 41/73-84/36. This gives us 77 degrees of overlap. We want to use a 100hp shot and compensate by opening the exhaust 10 degrees earlier. The valve events we want are 41/73-94/36. To get this, we can open up the LSA to 112.5, increase the exhaust duration to 310 degrees, and install it on a 106ICL (6.5 degrees advanced). We now have valve events of 41/73-94/36 with 77 degrees of overlap - just what we wanted. This might lose 20hp or so compared to the first cam off the juice, but gain an additional 20-30hp on it.

The compromise position is somewhere in between. Open up the LSA a couple of degrees, advance the cam a couple, and bump up the exhaust duration a bit. The choice is yours - but keep in mind that most cams are so far from optimized in the first place that a properly chosen nitrous cam is likely to be better both on and off the juice.



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Re: Nitrous Cam vs. naturally aspirated

Post  Mike R on April 14th 2012, 4:41 pm

Good read, I feel smarter already Laughing Laughing Laughing

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