Nitrous Part II
More -----> TUNING TIPS
majority of racers fine-tune air/fuel ratio by changing the fuel pressure.
Because the fuel side operates at much lower pressures than the nitrous
side, very slight changes in fuel-system pressure can significantly alter
system performance. Raising the fuel pressure richens the mixture;
lowering the fuel pressure leans the mixture. It's also quick and easy out
in the field, and you don't have to carry a bunch of different jets with
you. Tune off fuel pressure once the SPREAD is established. However,plate
systems seem to like low pressures. With modern nozzles, 6 psi is a mean
starting point. You have about 1 psi to play with either way. Find out
what it likes: Get the pressure right, then play with the jetting. The
low-pressure fuel side is so sensitive to pressure variations that any
changes should be made in 1/2- or even 1/4-psi increments. Most dead-head
(non-return) pressure regulators tend to creep at idle, so it is common
practice to test actual fuel pressure under simulated running conditions,
or to use a data-logging computer that records the actual pressure during
Nitrous burns faster, so it requires less timing, but how much to
take out initially and how fast to put some of it back in are key
fine-tuning elements. Timing is a great way to control the system. Every
outside thing affects the nitrous, affects how it loads the
engine--vehicle weight, converter, clutch, gearing. The exact amount of
timing to pull out varies for every combo. Start off conservatively, and
put timing back in gradually. On a nitrous system, even a 1-degree change
in advance can make a huge difference. A safe rule of thumb with good race
gas is to pull out 1.5-2 degrees for every 50hp increase. If you push
timing to the ragged edge there's not much of a performance improvement,
but the engine will be much more prone to detonation. The timing can get
down as low as 10 degrees with both stages on a multistage system
activated. If you run a datalogger and have a window of tuning available
try taking a few degrees out before and after peak torque. Do Not use
timing as a crutch for trying to run too big a system. It's better and
safer on parts to run a smaller nitrous hit with just enough timing. The
exact timing may vary per cylinder. For example, cylinders 2, 4, 6, and 8
tend to run hotter on a big-block Chevy. Torque converter-equipped cars
may require even more retard. With the timing so retarded, the distributor
rotor phasing gets out of whack. One of the most overlooked contributors
to nitrous engine failures and ignition problems when retarding timing
electronically is rotor phasing. The rotor must always be in phase to the
lowest timing that the engine will see during nitrous use. If you will be
retarding more than 8 degrees electronically, we recommend that you use a
crank trigger so that the rotor may be phased properly.
Finally, there are the nitrous and fuel jets. On prepackaged systems, it is standard practice to run the jet combos
recommended by the system manufacturer for the power level desired. If
you're not making the power you think you should, majority opinion says
you should play with the jets only as a last resort once the car fails to
respond to fuel-pressure and timing changes. Others recommends leaving
pressure and fuel jets alone. "Keep fuel pressure at 53/4-61/4 psi, and
just change the nitrous jet" to compensate for a bad-air day. The nitrous
side pressure is so much higher than the fuel side that going up or down
one nitrous jet size is a much finer (and safer) tuning tool than changing
fuel jets. However, some tuners disagree, recommending size changes to the
fuel jet in the nitrous system to get the engine to perform at its best.
It's just like jetting a carb. Either way, be sure to keep careful records
of any changes and carefully read those plugs! If you have to change jets
more than two sizes, look for a restriction somewhere else in the
The two most common nitrous-motor catastrophic failures are blown
head gaskets or burnt pistons. A blown gasket can be hard to run down
because it can be caused by either the wrong air/fuel ratio or detonation
(or both, since the two can be related). If you just treat the symptom
with improved gasket sealing, there's a good chance the next failure will
be with the pistons. Piston failures are easier to diagnose. You'd think
going too rich is safer than too lean, but if liquid fuel trickles down
past the rings, the resulting distress can lift the rings upward. Many
nitrous failures are caused by running the engine excessively rich.
Contrary to popular belief, richer is not necessarily safer. On the other
hand, burning a trough down through the rings into the skirt usually means
you are both too rich and have too much timing in that cylinder. If you
burn a hole through the top of the piston, you are running too lean. If
you start the burn and there's nothing left to burn, everything's
gone--you'll just burn the aluminum.
in a sophisticated nitrous system requires a patient, methodical approach.
Do not get greedy. When you're going from a compressed liquid to a gas,
the density changes constantly, and you don't have as much control as
you'd like to.
Go conservative on timing.
Be careful not to run too
rich or too lean.
Use really cold plugs.
Learn how to read the
Institute a nitrous-system maintenance schedule. And hit the
nitrous at the top of each gear, not the bottom. If building from scratch,
design for high rpm--it's easier on a nitrous motor. Nitrous only
magnifies existing problems. Get the engine running right on the carbs
before you hit the bottle.
On a sheetmetal type intake. Change the jets in the carbs about 4 numbers leaner when
you switch to the nitrous. Then dial-in the nitrous oxide on one of the
recommended lower-power tune-ups. If it doesn't perform as it should,
don't step up to the next level until you've sorted out any existing
problems. Change only one thing at a time.
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