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Advanced Nitrous Tuning
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How To Become An Expert
Nitrous Tuner
This section is devoted to
advanced tuning theory. That’s
really just a fancy way of saying
we’re in the gray area where the
“science” of engine tuning
becomes art, or at the least a
high craft. Which is to say, not
even the experts can predict
exactly what’s going to happen.
They, like you, have to try it,
observe it as objectively as
possible, quantify it, and, decide
on the best course of action.
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What these experts have that you don’t is an ingrained, virtually instinctual model of the complex
mechanical and chemical processes happening in an internal combustion engine. It’s as if their
central nervous systems are branded by experience, bitter and victorious, so much so that they see
things other’s don’t—noticing the imperceptible hidden in the obvious. While we can’t give you an
expert’s experience, we at can least give you a rudimentary model to guide you on your way to
becoming an expert nitrous engine tuner.
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The Most Overlooked Tuning “Secret”
Let’s start things off with a concept that always gets lost in the search for peak power, more torque,
and achieving optimum nitrous-to-fuel ratios. This most overlooked concept is that, even with
healthy doses of nitrous, you’re only increasing the oxygen content of the charge by just a few
percent. That’s wild when you think about it. Just a scant few percent increase in the oxidizer
content of the intake charge and you get incredible power output.
This should show you just how sensitive gasoline and other fuels are to oxygen, which, by the way,
precludes using pure oxygen as an induction agent. If you did it would probably use your pistons for
fuel along with the fuel and the cylinder walls. And we don’t want to have to measure the Brake
Specific Aluminum Consumption of that combination.
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Think about this for a second. You’re doing a lot of work to set up a system to deliver super-cooled
nitrous oxide into your engine. The upshot of this is that you gain a few percent increase in the
oxidizer content, which allows you to react to more fuel more quickly to create more heat and
pressure to push down harder on the top of your pistons, which drives the wheels that win the
races. Really, that’s all we’re doing here. But it’s so easy to get lost in the forest looking at the trees
that you need to step back and just get the basics. And this is the most basic and most overlooked
concept regarding tuning a nitrous system.
Measured by volume, air is about 20.95 percent oxygen and 78.06 percent nitrogen, with the
remaining 0.99 percent taken up with trace elements like helium, hydrogen, carbon dioxide, etc. By
weight, nitrogen makes up 75.5 percent of the atmosphere’s mass, with oxygen contributing 23
percent of the mass; the remaining 1.5 percent is contributed by the aforementioned trace
elements.
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Of course, it’s the oxygen content that reacts with fuel to make power, so we’re interested in that
portion of the atmosphere. In a conversation with Ken Duttweiler at the ’97 NHRA Winston World
Finals, he noted those figures may be accurate for a global average, but in the environment from
which engines draw air, oxygen usually accounts for around 19 percent of the mass of the intake
charge. It does fluctuate, but he said 19 percent is a good working number, so that’s the one we’ll
use in this exercise.
Nitrous oxide, measured by volume is 33 percent oxygen; 66 percent nitrogen. Measured by
weight, 36 percent of its mass is oxygen with 64 percent nitrogen. When you first hear the
numbers you think you’re going to get a big jump in the oxygen content of the intake charge with
nitrous oxide. But you don’t get the whole 17 percent increase (36 percent minus 19 percent = 17
percent) because you’re not running the engine solely on the nitrous system. You're augmenting
the atmospheric charge with nitrous oxide, blending two oxygen sources, each with a different
amount of oxygen. How much of a blend is what tuning is all about.
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Complicating the tuning process is the usual motor mayhem of pulsing flow, changing rates of
volumetric efficiency through rpm range, combined with the flat flow rate of nitrous systems. Put
roughly, a nitrous system makes prodigious force at the torque peak because at that point in the
rpm range you get the highest percentage of nitrous oxide to air. At higher rpm, the engine still
makes more horsepower, but as the airflow demands out pace the flat flow rate of the nitrous
system, you get a lower percentage of nitrous oxide to air and power falls off. Unless, of course,
you have a staged system that increases nitrous flow as the engine demands. More on these
systems later.
Because of the rate and volumetric efficiency variations of piston engines, it is advantageous to
build on your understanding of nitrous systems, if we confine our discussion to the nitrous verses
air flow rates at the torque and horsepower peaks. By looking at the differing nitrous to air ratios at
these points, you’ll understand that these ratios change dynamically as the engine runs through its
rpm range. Let’s present a simple model in order to make the math and the concept as plain as
possible. Let’s have a look at the torque peak first.
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Modeling a Nitrous System
The gear heads at Super Flow, Inc., makers of some of the better dynos, flow benches, and other
power and flow measuring equipment on the market, say that, in general, an engine flows
approximately 1.25 CFM per horsepower at peak torque and 1.4 CFM per horsepower at peak
power. We’ll use these values throughout our discussions.
Assume you’re making 200 horsepower at the torque peak without nitrous. Using the Super Flow
constant, that cyphers out to about 250 CFM When you hit the button, suddenly the engine
thunders to 340 horsepower at the torque peak and the Super Flow constant no longer is valid.
The reason the constant (this goes for the peak power constant, as well) no longer is valid is
because nitrous has a greater oxygen content than that of air, so it takes less nitrous oxide to react
to a given amount of fuel. And if you've grabbed hold of the concept we've been repeating like a
mantra throughout this book: “fuel makes power, not the nitrous,”then this should make sense to
you. If not, call the tech gurus at NOS and meditate on the concept while on hold.
In a perfect world, or at least in a world where reality conformed to wishes, we should be able to
calculate a nitrous flow constant in the image of the Super Flow constants used above and just nail
the question of nitrous flow to the wall, shoot it through the heart, and end all these vague tuning
decision once and for all. We can’t exactly do that, but we can get close enough to be useful. And
in the immortal words of Mary Poppins, “enough’s as good as a feast.”
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If we just take the additional power gain, ordain its genesis as nitrous, and calculate what it would
take to make 140 extra horsepower at the torque peak, we can estimate how much nitrous we need
to run. This isn’t a completely accurate model of the reality within our engine. Reality is always so
much messier. The difficulty here is that not only do we have the ever-changing conditions of a
pulse device running through its rpm range, but now we have an oxidizing agent that’s active in two
other ways. First, it cools the incoming atmosphere, increasing its density and thereby its oxygen
content. Second, it also pressurizes the intake slightly, reducing the flow through the carburetor.
What’s a tuner to do?
The pros measure everything. They already measure the air and the fuel going into the naturally
aspirated engine—with savvy nitrous tuners doing the same with the nitrous and the enrichment
fuel. That’s the way to do it if you have a dyno and generate BSFC data. For the rest of us, we’ve
got to tune on the run, literally; but if you understand the relationships of the data from the dyno,
you’ll be able to make better tuning decisions at the track.
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Fuel / Nitrous / Airflow at Peak Torque
Since airflow constants aren’t practical when using nitrous, the only other constant we can use is
Brake Specific Fuel Consumption (BSFC). I’ve written about BSFC elsewhere in this book so I won’t
explain it here other than to say that BSFC is the ratio of fuel consumed to the power output.
Suffice it to say that a BSFC value of 0.50 is about average for a well-tuned engine with roughly a
12:1 air/fuel ratio. The reason why it’s a good value to use when working with nitrous is because,
“fuel makes power, not the nitrous.” If you know the total rate of fuel flow, you know, based on a
0.50 BSFC, approximately how much power that fuel flow rate will support.
To use our BSFC standard we need to convert the other values into like units of measure. Going
back to our model, 200 horsepower at peak torque is about 250 CFM, which if you reference ratio
and flow charts in section 7.8, we find 250 CFM is about 19.05 lb./min. That corresponds to a fuel
flow at 0.50 BSFC of 100 lb./hr. (To convert the lb./hr. to lb./min. simply divide it by 60.), which
converts to 1.67 lb./min. 19.05/1.67=11.4 a/f ratio. At the torque peak, an 11.4:1 a/f ratio isn’t too
far off for an engine tuned to accelerate. Most engines accelerate quicker if they’re slightly rich.
Peak steady-state power comes with a much leaner mixture, somewhere around 13.5:1 depending
on the combination.
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Now lets do the math when we have nitrous to factor in. The extra 140 horsepower equates, at peak
torque, to a naturally aspirated flow rate of 175 CFM, or 13.34 lb./min. for air; for nitrous,it’s only 50
CFM or 5.83 lb./min. at a N20/fuel ratio of 5:1 referenced to a BSFC of 0.50 for 1.17 lb./min. mass
fuel flow. We come by the 5:1 N20/fuel ratio via the NOS crew. It’s found that 5:1 is a good,
conservative, slightly fuel-rich beginning ratio. About the leanest mix it suggests is a 6:1 ratio, but
you can go leaner if you've got the nerve. According to David Vizard, the chemistry suggests that
stoichiometric, or complete reaction, would take place with a 9:1; however, that ratio won’t make the
best power. At the 6:1 ratio the figures break down like this:
61CFM or 7.00 lb./min.
Before we go on, let’s explain where we got the CFM values of the nitrous at lb./min. flow rates. It’s
basically an estimate. If you check the reference chapter for the physical constants of nitrous, you
see its specific volume at 1 atm. We’ve chosen the specific volume factor at 70°F of 8.726 ft3/lb.
Exactly what temperature and therefore volume it reaches in the engine before the intake valve
closes, we’re not sure, but this figure at least is a good estimate. We should also point out that as
the engine heats during a power run or dyno pull, the intake temperature increases, and as it does
the volume of the nitrous charge increases as its temperature increases as well. To recap, this is
what w'’ve done so far (see Chart 1).
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Chart #1
200 hp @ 11.4:1 a/f ratio = (250 CFM or 19.05 lb. /min.) + 1.67 lb/min fuel
+ 140 hp nitrous @ 5:1 n/f ratio = (50 CFM = 5.83 lb./min.) + 1.17 lb./min. fuel
OR
140 hp nitrous @ 6:1 n/f ratio = (61 CFM = 7.00 lb./min.) + 1.17 lb./min. fuel
Next, add the flow rates together to get the total:
200 hp +140 hp = 340 hp
Naturally aspirated: 19.05 + 13.34 = 32.39 lb. /min. + 2.83 lb. /min.
N2O @ 5:1 ratio: 19.05 + 05.83 = 24.88 lb. /min. + 2.83 lb. /min.
N2O @ 6:1 ratio: 19.05 + 07.00 = 26.05 lb. /min. + 2.83 lb. /min.
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Previous | Next
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This has been a sample page from
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How To Install and Use Nitrous Oxide
Injection Systems For Maximum Horsepower
by Joe Pettitt
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Includes information on nitrous basics and advance
nitrous theory. Written with the assistance of Nitrous
Oxide Systems
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Nitrous oxide injection is one of the potentially easiest, least
expensive, and fastest ways to substantially increase engine
horsepower. This new title, authored with the assistance of one of
the industry's largest manufacturer of nitrous equipment, provides
the latest technical information available regarding the proper
installation and use of this high performance, yet potentially
damaging equipment.
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Click below to view sample
pages from each chapter.
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"How to Install and Use Nitrous Oxide is filled with information
on nitrous, including the basics of advanced nitrous theory.
Photos, charts, and graphs accompany the text and illustrate
key points. Hands-on sections of the book cover how to plumb
a nitrous system and how to set up an engine to handle nitrous.
There's information on ignition timing, compression, wiring,
solenoids, octane, and fuel delivery." -- SPORT TRUCK, April
1999
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Chap. 1 - Introduction to Nitrous
Chap. 2 - How Nitrous Works
Chap. 3 - The Nitrous System
Chap. 4 - Installation Tech
Chap. 5 - Operating and Tuning
Chap. 6 - Basic Engine
Chap. 7 - Advanced Tuning
Chap. 8 - Nitrous Fuel Injection
Chap. 9 - Dyno Sessions
Chap. 10 - Real World Project
Chap. 11 - Chemical Reference
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8-3/8 X 10-7/8
128 pages
300 b/w photos
Item: SA50
Price: $18.95
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Click here to buy now!
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This is a great book that anyone using, or considering using a
nitrous oxide system will love!
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Other items you might be interested in
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How to Rebuild and Modify Carter/Edelbrock Carburetors
Author David Emanuel outlines carburetor types, gives a thorough look
at carb selection and carb function, and offers detailed information on
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Also features the history of Carter as well as the history of the AFB
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