| Mazda
RX7
| Running Nitrous
Oxide on a Mazda RX7 |
Nitrous Oxide Injection on a
Rotary Engine
By Sean Cathcart
Search for Nitrous Oxide Kits on eBay
Due to the high levels of misinformation I have seen being provided on
this forum about nitrous oxide, and its use on rotary engines, I
thought it was about time to clear up a few things.
I have seen almost every question imaginable when it comes to nitrous
on this forum. And really, the answer to all of them is: it depends.
The use of nitrous is always dependant on goals. How much power are you
looking to put out? How long do you want your engine to last? How much
money do you have to spend? What kind of modifications already exist?
In this article, I will review the basics of nitrous, to building
set-ups to work safely on ALL engines.
Nitrous
Basics:
Nitrous Oxide is a gas comprised of two nitrogen atoms bonded to a
single oxygen atom, and has the chemical formula N20 thusly. It is
common among the drag scene, with many common names tossed around for
it: juice, gas, NOS, laughing gas, spray, and nitrous.
***Automotive grade Nitrous Oxide is identical to medical grade nitrous
oxide (laughing gas), save one point: automotive grade Nitrous Oxide
contains sulfur dioxide, about 100ppm. Unlike medical grade nitrous
oxide, the automotive grade gas cannot be “huffed” for delusional
purposes. The sulfur dioxide is added for the soul purpose of
preventing abuse of the gas, and attempting use of auto-grade nitrous
oxide will result in extreme sickness. The additive does not affect
performance in an engine.
First off, it is important to understand the basics behind horsepower.
Think of an engine as an air pump: The more air you can move in, and
out, the more power you’ll make. As well, the easier it is to move the
air in and out, the more power you’ll make. This makes sense, as
removing exhaust and intake restrictions with higher flowing
replacements increase engine power, as does adding more air to the
engine via a turbocharger.
Nitrous oxide can be thought of as “liquid air”. When Nitrous Oxide is
heated to around 574oF, it breaks apart into two nitrogen atoms and a
single oxygen atom. This extra oxygen allows for more fuel to be added
to the combustion chamber, and thus more power to be made. In our air
pump analogy, it makes more power by adding more air, like a
turbocharger does. Nitrous Oxide itself is not flammable. However, when
heated, it breaks apart and separates its molecular bonds, and gives up
its oxygen atom, allowing for gasoline combustion.
The spare nitrogen atoms acts as a buffer to the combustion process,
slowing down the combustion reaction, and preventing detonation.
Detonation damages engines. This buffering effect is the reason pure
oxygen can not be added.
Nitrous Oxide is stored in bottles at approximately 950 psi as a
liquid. When released from this pressure, it quickly drops temperature
to approximately –127oF and comes out in a gaseous form. This cooling
effect does the same thing as an intercooler: it allows for a more
dense air charge, and thus more power. For every 10 degree reduction in
intake charge temperature, a 1% increase in power can be had. A nitrous
oxide injection typically lowers air temperature by 60 to 75 degrees.
Thus, on a 200 hp rotary, the mere change in intake temp level would
net 14hp.
Nitrous oxide systems are available in two different forms: wet
systems, and dry systems. Since nitrous is simply a method of adding
more oxygen to an engine, more fuel must be added with the nitrous to
prevent the engine from running lean and destroying itself, as well as
to make the extra power. A wet system adds the extra fuel by mixing
fuel with nitrous oxide gas through a “fogger” nozzle and spraying the
mixture into the intake tract. A dry system adds the extra fuel via the
injectors, by using a computer or a fuel pressure regulator increase
supply.
Nitrous itself will not increase the wear on your engine. As with any
increase in horsepower, however, they will come some extra wear and
tear on your engine. This is increase in wear would be the same no
matter where it comes from. In fact, one of the advantages of nitrous
oxide is that you only use it when you want, and thus, only put strain
on the engine when you feel it is necessary.
Typical nitrous kits come with 10 lb bottles. Nitrous is measured by
mass, not by volume, and a 10 lb bottle holds 10 lbs of nitrous oxide.
Other common bottle sizes are 2.5 lb, 15 lb, and 20 lb. The length of
time a bottle will last depends on your set up. The formulae for
calculating nitrous use is: 0.8 lbs of nitrous x 10 seconds = 100
horsepower.
The
Button:
We’ll get more to nitrous activation later. A system can theoretically
be activated for as long as there is nitrous in the bottle. It is
recommended, however, to never get on the system for more than 15
seconds at a time. I abuse that rule.
Nitrous should only be used at WOT. It is load-dependant in that rpm
point usage depends on the amount of load on the engine. You could
engage nitrous at 1500 rpm in first gear, but never below 3000 in
fourth. It should never be used above redline, or until fuel cut
occurs. Failure to follow these rules can result in nitrous backfire,
where the nitrous/fuel mixture actually ignites and backfires through
the intake manifold, commonly causing intake manifolds and related
houses and systems to explode. This can be very dangerous, and cause
bodily harm.
The
RX-7 and nitrous:
Wet Vs. Dry:
Which is better on an RX-7? The stock fuel injection system on RX-7’s
is being pushed pretty hard even at the stock ECU usage. Attempting to
push more fuel through the stock injectors can prove fatal when the
injectors simply cannot provide enough fuel for the engine. On the
other hand, dry nitrous kits are much less likely to nitrous backfire.
So, in this case, the answer is dependant on user needs and existing
modifications. For situations where 50 hp and lower tuning is being
used, the stock injectors should provide adequate fuel for nitrous use.
However, if your motor is severely ported, and extremely modified, you
already may be approaching your injector delivery capacity. As well, a
clogged fuel injector may just slightly lean out stock performance, but
could absolutely destroy a motor running on nitrous.
Overall, I completely recommend wet kits. It offers easier tuning, does
not rely on the already sketchy stock fuel injectors, and offers a
wider amount of power. A dry system should really only be used on a
minimally modified N/A RX-7 looking for no more than a 50 hp increase,
and have verified a perfectly working fuel injection system, complete
with recently professionally cleaned injectors. Turbocharged vehicles
should never run a dry system.
Seeing as how most users have already modded their cars and are looking
for 60-75 hp shot, it seems to make the most sense to concentrate on
wet kits from this point on.
Kits:
All nitrous manufacturers offer “complete” kits for our cars. Never, in
any circumstance, should these kits be considered “complete”. They
provide the minimal amount of items to get a system running, but hardly
ever enough accessories to make a safe system.
A safe system:
First and foremost, a nitrous inducted engine needs fuel. Every RX-7
running nitrous oxide must first upgrade their pump. The Walboro
drop-in pump is more than apt in this situation. In NA’s, this can,
however, present a problem:
The stock fuel pressure regulator is unable to compensate for the added
flow of a higher volume pump at idle and low load situations. With this
upgrade, the stock fuel pressure regulator cannot flow enough fuel out
as is coming in, and the fuel rail pressure increases, causing the
injectors to add more fuel than the ECU registers. Hence the car runs
rich at idle and low load throttle. This can cause hesitation and a
large lack of power on the bottom end of the cars, as well as compound
a carbonizing problem already existing in the lower rpm range.
Turbo cars already have upgraded pumps, and although they run the same
fuel pressure regulator, use a resistor pack to lower voltage to the
pump at idle and at low rpm points.
In order for an N/A car to run properly with an upgraded pump, either
the Turbo fuel pump resistor pack must be wired in, or an aftermarket,
higher flowing fuel pressure regulator must be added. The latter is
obviously better for overall tuning, but there is the added cost, not
to mention the change in fuel system plumbing. Regardless of method
chosen, this problem must be addressed.
Ignition:
The stock ignition on RX-7’s is already exceptional compared to other
stock cars. An upgraded ignition is obviously better, but not required
for nitrous oxide use. However, the ignition system should be retarded
under activation. For smaller shots, up to 75 horsepower, the stock
Timing maps should be fine. Above that power level, timing becomes
risky, and retard should be used to avoid detonation and harm to the
motor.
The amount of detonation depends on your horsepower jetting, current
set up, and various other factors, but as a well of thumb, the timing
should be retarded 1 degree for every 25 horsepower of nitrous jetting.
Thus, a 100 hp shot should be retarded 4 degrees, and a 200 hp shot
retarded 8-10 degrees.
Methods to retard are questionable. You can retard the entire ignition
at the crank angle sensor, but the power will suffer the entire time
during non-activation.
A better idea is to use a retard box. MSD is known to make a few, and I
believe even one set-up to work without a CDI box. They are wired in to
retard the timing a preprogrammed amount set via retard “pills”, and
only retard when the nitrous system is activated. This is the best
system, but again can add cost to a system build-up. A retard unit
should be considered essential above 125 hp shots.
The
Bottle:
The bottle should be maintained at a temperature around 70 degrees F.
Normally, a bottle heater is required to keep the temperatures this
high, however, in hot weather the temperature can actually increase too
much, and will need to be cooled with either a wet rag or some ice. Too
low a temperature and the system will run too rich and make little
power, too high a temperature and the system will run too lean and
detonate.
In the end, a nitrous pressure gauge should be used to monitor the
system. The system should be kept between 900 and 1050 psi. Any higher
and you should cool the bottle, and lower and you should heat it.
However, never at any point should flame be used to heat a bottle, nor
have the bottle left unattended with a bottle heater on. A pressure
gauge should be a required accessory to any system, but is rarely
included in kits.
As well, a safety release valve should be employed with any kit. This
safety valve should be plumbed to the outside of the car, and when the
bottle pressure gets to high, the valve will dump all of the contents
of the bottle. It prevents the bottle from exploding from excess
pressure.
The bottle should be mounted with the valve facing towards the front of
the car, and mounted at a 15 degree angle. The bottle should be mounted
as far back as possible in the hatch, or in the trunk.
It is possible that the nitrous solenoids could leak, and while the car
is shut off, fill the engine with nitrous. Upon start-up, this would
cause the engine to run extremely lean and detonate. Thus, when not in
use, nitrous oxide bottle valves should be kept in the closed position.
Most kits include a manual knob to open the valve, and most companies
offer a remote bottle valve that can be opened and closed
electronically at the flip of a switch. At around $150, this accessory
is not necessary, but a nice convenience, especially for those with
trunks.
When the bottle is close to being emptied, a surging effect is felt.
This is the time to bottle refilled. It can be tiring to be without
nitrous, so many users keep several bottles either swap in a spare
filled bottle, or get your single so that they always have nitrous
available.
System
Solenoids:
Each wet system has 2 solenoids which opens and closes to allow the
nitrous and fuel to flow into the engine. These solenoids are typically
rated at around 1000 psi operating range, but usually can withstand up
to 1500 psi without worry.
Different flow rates are available for different needs in both fuel and
nitrous solenoids, typically rated by horsepower values. Try to select
a solenoid most suited to your range of horsepower shot. Most solenoids
included in kits will flow enough to support up to 250 horsepower
levels of tuning.
Usually not included in kits, but should be considered a necessary
accessory, are filters. Nitrous filters should definitely be used to
prevent the nitrous solenoid from sticking in the open position (which
would allow nitrous to fill the engine after you turn the system off,
running the system extremely lean and blowing the motor). Fuel filters
are also a great idea, but not near as necessary.
Injection:
Typical wet system plumb into a “fogger”, which is just a nozzle with
two lines running into them, one a nitrous feed and one a fuel feed,
whose purpose is to mix the two to form the nitrous oxide-fuel atomized
“fog”.
Most basic system use one fogger. This setup should be mounted 4-6
inches in front of the throttle body, with the fogger outlet nozzle
pointing in the direction of airflow.
More advanced systems plumb a separate fogger into each intake manifold
runner, and such is called “direct port”. These systems allow for more
even distribution of nitrous oxide per combustion chamber, but are
usually only necessary when extremely large horsepower shots are being
used. A direct port setup usually costs 50% more than a single fogger
system, and involves a lot more plumbing and installation time.
Supply
system:
All nitrous systems should use quality braided stainless steel hose to
supply both the nitrous and the fuel to the system solenoids. Every kit
includes the necessary lines, and every manufacturer sells these lines.
These lines are typically rated to 2000 psi, and come in pre-determined
lengths.
A purge valve is an excellent accessory to your supply system. After
system use, the nitrous line is typically filled with nitrous oxide
vapours. When the bottle is opened, this gas is compressed, but still
exists at the front of the nitrous feed line. When the system is
activated, this gas, which is nowhere near as concentrated as the
nitrous liquid that is fogged in, causes the car to run momentarily
rich, and make a small “bog” until the nitrous liquid reaches the
fogger. A purge valve is a separate nitrous solenoid that allows the
nitrous vapour in the feed line to be “purged” out of the system. This
allows for more of a ‘hit” feeling when the system is activated. The
spare vapour is vented outside the engine, typically in front of the
windshield. An activated purge valve is what is causing the “steam” to
shoot over the windshield typically seen at the drag races.
Electronics:
As we all know, driving is nothing like the fast and the furious.
Nitrous oxide will not just come on when you push a button and then
hang on. Unless you use an on/off push button (forget what its called,
push once on, push again and its off), the nitrous would only be on as
long as the button is depressed. The method of just tapping the button
in TFATF would only allow the nitrous to come on for those few
milliseconds while the button was tapped. As well… the “too soon
Junior” line makes no sense. As they are claiming them to be 10 second
cars, hitting nitrous for 10 seconds at a time is fine, and thus, the
person hitting the shot sooner would be in a better position.
Most systems actually don’t use a button.
Every system should be wired into some form of WOT-only activation
switch.*** Most kits include a microswitch, which is simply a button
which completes a circuit whenever it is depressed, mounted to the
throttle body. Push down on the gas pedal to WOT, and the circuit is
complete. Let off the gas, and the system is deactivated. This makes it
impossible to blow your engine by accident with a part-throttle
activation, and turns off the nitrous oxide injection during shifts. As
well, it is possible to use a switch that uses the TPS to activate the
system, but knowing how finicky the TPS is already on our cars, I don’t
recommend this.
As well, a master switch should always be used, otherwise the system
will activate any time you floor the car. This is usually just a toggle
that is thrown to activate the system.
It is possible to wire in a button into the system, or as a replacement
for the master switch, but it must be wired inline with a WOT
microswitch.
Really, a toggle master switch “is” a button. Just throw the toggle in
whatever gear you want to activate the nitrous system while driving,
and as soon as you floor it, the system will become active.
A good accessory to add is a Hobbs switch. A Hobbs switch is a switch
that opens only a set pressure. In our terms, you would use it to
deactivate the nitrous system should the fuel pressure drop.
Another available accessory is the Nitrous Express (I think) air/fuel
system. Should the air/fuel ratio become to lean or too rich under
nitrous activation, this computer will deactivate the nitrous system. I
have no experience with this system, and hope it would employ a better
unit than the stock O2 sensor, but I doubt it. In this case, I imagine
the damage would already be done by the time the computer deactivated
the system. I’ll look into it further.
Getting really hardcore… progressive computers. This was the *** from
activation systems above. These allow you to program your nitrous
settings, such as rate advance, and part throttle activation. Only
should a system be activated at part-throttle if you have a progressive
controller. They are pretty expensive, but allow for the ultimate in
tuning. I’m in the process of getting one.
Building
your system:
-50 hp and less: If your injectors are verified as perfect, go dry. I
recommend Venom.
If not, use a single fogger mounted 4-6” in front of the Throttle
plates. Add an upgraded pump, plumb your feed line out of the banjo
bolt. Use a nitrous filter, and get a bottle heater and a nitrous
pressure gauge. 4-AN nitrous feed line. Safety Blow off Valve.
75 hp and less: same set-up as wet kit as listed above.
-125 hp and less: Wet only. Single fogger mounted 4-6” in front of the
Throttle plates. Manifold porting, TB mod, upgraded pump, upgraded fuel
pressure regulator. Nitrous filter, bottle heater, 4-AN nitrous feed
line, plumb fuel from a y-splitter off of the secondary rail. Upgraded
Clutch. Run a retard unit. Safety Blow-off valve.
400 hp and less: Wet only, upgraded external fuel pump and filters,
aftermarket fpr, y –splitter to feed nitrous solenoid. Nitrous and fuel
filters, 6-AN feed line, purge valve. Hardened Stationary gears, oil
bearing mods, higher rate oil pressure regulator. Direct port only,
consider using a progressive computer. Large engine porting, all
manifolds ported. Upgraded clutch, and probably transmission if your
are hitting a “high” shot. Consider Staging shots. Bottle heater, and
retard unit. Safety Blow-off valve.
Nitrous on a turbo: don’t do it if you need to ask. Even with a
massively ported S5 wastegate, you’ll still experience crazy boost
creep. Sorry to sound rude, but blown motors are already a prob for
these things. If you have specific questions and think you can handle
it, ask me.
Anything bigger… you shouldn’t need my help.
For any basic kit... add $500 for *required* accessories.
My system:
I have built a very extensive system for nitrous oxide injection on my
1990 TII. My mods are as follows:
- 7000 miles on Street Ported Rebuild
- electric fan
- electric water pump
- sumped stock tank to 100- micron stainless steel filter to 1200 hp
aeromotive fuel pump into 10-micron paper filter into braided feed
lines into Y splitter into rails, Y splitters off of each fuel rail to
aeromotive fuel pressure regulator, with a base setting of 40 psi. Each
Y-splitter off the fuel rails feeds one nitrous solenoid.
- Completely mirror polished and ported intake manifolds, mirror
polished and Modded Throttle body, polished intake elbow, all emissions
removed.
- no crank driven accessories save alternator
- MSD 7-AL3 CDI box with addition 2-stage retard on leading coil, 5
degrees + 8 degrees.
- intake, 3” downpipe and midpipe to horrible stock catback.
- Stock turbo with massive ported S5 wastegate.
- Boost gauge, fuel pressure gauge, nitrous pressure gauge
- 15 lb nitrous bottles with a remote bottle valve into a 6-AN feed
line to a purge valve, purging excess vapour out through the stock
intercooler, nitrous and fuel filters.
- First stage: Single fogger, currently jetted to 120 horsepower, in
intake elbow.
- Second stage: Direct port, one fogger per intake runner, each jetted
for 40 horspower.
- Bottle heater, separate circuits per stage wired to covered toggles
mounted in front of the shifter (where the ash tray used to be).
- Combined stages: 280 horsepower. However, using the fuel jettings to
compensate for added boost from the turbo creeping. I’m sure its
running rich… Its time for an upgrade.
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This Write-Up is For . . .
Mazda RX7 | 2nd
Generation (FC3S)
Submitted By:
scathcart
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