Nitrous oxide injection is the budget performance enthusiast/racer’s surefire route to big power. No other form of power generation comes close to deliver- ing so much horsepower for so little money. This chapter deals with results rather than going into system design detail. But first some basics.
Nitrous Oxide is a compound consisting of one-third oxygen and two- thirds nitrogen. By comparison, our atmosphere consists of one-fifth oxygen, with the rest mostly nitrogen. N2O is a gas at room temperature, but if (in the same manner as propane gas) it’s held in a container under its own vapor pressure it’s a liquid. Depending on temperature, the vapor pressure is typically 700 to 900 psi. When released from the bottle, the expansion and partial vaporization the N2O experiences cause it to super cool to –128 degrees F. N2O is a liquid at this temperature. Using electric solenoid valves to activate the flow, N2O largely in liquid form can be injected into the engine. Since N2O is 50-percent richer in oxygen than air, and it’s largely in liquid form, a great deal of additional oxygen can be delivered to the cylinders. The additional oxygen supplied by the N2O must have its own supply of fuel; otherwise, the now oxygen-rich charge will simulate an oxyacetylene cutting torch and melt the pistons.
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The additional fuel is supplied by tapping into the pressure side of the fuel line and is delivered on demand via a solenoid valve similar to that used for the N2O.
With both solenoids connected to a common activation switch, the system can be fired by simply pushing a button. Fig 11-1 shows the layout of a plate system that installs between the carburetor and intake manifold.
A nitrous system can take the form of a simple plate installed between the carb and manifold or injector nozzles installed into the manifold port runners. For high- output installations, both methods are used in the form of a two-stage system. The first system to be activated may bring in 200 hp, and the second system is acti- vated as RPM climbs. This is done to avoid extremely high cylinder pressures that would result from injecting large quantities of nitrous into an engine at low RPM.
Because we’re dealing with budget constraints, only plate systems will be considered. They’re less expensive than nozzle systems. This shouldn’t be viewed as a potential power limitation. A good plate system can produce more power than the bottom ends we can typically afford. Power increases of well over 300 hp are possible on a well spec’d motor, and this can easily bring the total power of a 350 to more than 800 hp. At these levels, bottom end and block integrity are the key issues rather than the capability of the N2O system.
Although the basic premise of an N2O system may seem elementary, the seemingly simple process of injecting it and the extra fuel into the engine is far from guaranteeing results. For a given amount of N2O, a poorly designed system may only produce an additional 75 horsepower, as opposed to as much as 175 hp for an optimal system. These dif- ferences are brought about by design detail. Such things as the fuel-to-N2O ratio, fuel atomization, the effective mix- ing of fuel, and N2O in the manifold and distribution are but a few of the factors that influence the production of power. These are design factors that nitrous system manufacturers should address.
During the late 1970s, a glut of nitrous system manufacturers appeared on the scene. Many failed to produce results anywhere near their claims. As a result of fierce competition, what we have left today are those companies that survived by delivering positive results. I use systems from NOS, Nitrous Express, and Zex. I’ve seen results from systems by these manufacturers that can be deemed satisfying by any standards.
The use of nitrous isn’t without potential reliability problems. Over a period of 20 years, I haven’t experienced an engine failure from the use of N2O other than experimenting on one engine to see how far I could go with Chevy’s best stock parts before mechanical fail- ure. For the record, the 350 concerned produced 1,027 hp at 6,250 rpm (863 ft-lbs) just prior to the block cracking along both banks of cylinders in the lifter valley. At lower RPM, more than 950 ft-lbs of torque was seen.
The biggest reliability problem is melted pistons, and the usual cause is inadequate fuel supply, closely fol- lowed by excessive ignition advance. Without the fuel needed for the oxygen content of the N2O, combustion temperatures of the now lean mixture go sky high. This results in detonation and temperatures far more severe than can occur in a non-N2O engine having too much compression and timing for the fuel octane used. The golden rule is to be sure to use the fuel pressure called for in the instructions. If the fuel pressure drops during use, problems will result. This will be your fault, not the manufacturer’s.
Nitrous, if uninhibited, speeds the combustion process considerably. As a result, we find that it becomes more of a necessity to retard the ignition timing for best results as power levels increase. This can be compensated for at lower power levels, eliminating the need to spend money on an ignition retard. Here is how it’s done. A typical N2O system will have the fuel side calibrated about 40-per- cent rich when at the recommended pres- sure. This serves two important purposes: first, the extra fuel acts as a coolant to handle otherwise excessive temperatures, staving off the possibility of detonation; second, the extra fuel slows the otherwise more rapid combustion of N2O/fuel mixes. This means that with most 100- to 150-hp plate systems, the same ignition timing can be used, whether or not the system is activated. But this situation can- not go on indefinitely. At somewhere above about 150 hp, an ignition retard of some sort will be needed.
Each manufacturer has its own pump pressure and jetting calibrations to take care of this cooling/ignition timing situation, so it’s mandatory to read and comply with all the instructions before use.
To get the best from a nitrous system requires know-how. Starting at the cylin- der and working back down the line, the first aspect to take care of is the spark plug heat range. Because most basic systems are calibrated rich, the plug will have additional cooling, but you still should check that the plugs shows no sign of overheating. Change to cooler plugs if there’s any doubt concerning plug temperature.
If the plugs run too hot, the motor can detonate and damage will result. Sometimes, but not always, overheated plug electrodes will burn off and the cylinder stops running before piston damage occurs. This is fine as long as the plug electrodes burn faster than the pis- tons. You can’t count on this, and it’s not the recommended way to determine if the plugs are running hot. Go to a cool- running plug if you have doubts about plug temperature range. It’s far easier to fix plug fouling due to running too cool than to fix pistons from running too hot!
Don’t use thin-wire plugs such as the Bosch platinum, because these have inadequate heat dissipation capabilities for a N2O-injected motor. For the record, those $3-a-pop Autolite race plugs get the job done very well and most decent-sized stores like Auto Zone have them. Failing that you can replicate the electrode form of an Autolite race plug (so long as it is of the right heat range for the job) on to a regular Autolite plug as per Fig 11-2.
Only use new and/or fully functional plug cables. Be sure the ignition system can fire the motor to well beyond the RPM required. If the ignition system drops a spark, the result can be a destructive backfire. If your HEI is equipped with an upgraded module as recommended in the ignition chapter, your ignition will be up to the job.
On the fuel side, install a pressure gauge and regulator between the pump and the nitrous solenoid. Check that the fuel pressure called for is delivered to the system throughout the full-throttle RPM range used. Using less is asking for trouble. Be sure to use adequate fuel octane. Don’t use fuel that’s been stored for a long time in a container that’s partially full or has been vented. The light, front- end hydrocarbons that nitrous systems like probably have long since evaporated. Don’t use high-octane aviation fuel. It has poor vaporization properties and doesn’t work as well as automotive race fuel.
Stabilize the bottle temperature. Under most circumstances the best results are seen with bottle temperatures that deliver 900 psi of pressure. A thermostatically controlled bottle heater is a really good add-on to your N2O installation. At 900 psi, the system doesn’t deliver a significantly different amount of N2O than at 700 psi. Although the temperature and consequently the pressure goes up, the density at the higher temperature drops and, for all practical purposes, compensates. The reasons most engines make more power at the higher bottle pres- sure is that mixing of the fuel and N2O is improved, and a more favorable ratio of gaseous nitrous to liquid nitrous for better combustion is achieved.
More Optimal Engine Specs
To get the best from a system, we need to revise some engine specifications. First, disregard what most successful Pro Mod engine builders do, which is jack up the compression ratio to 15:1 or more. Although the dyno shows more horse- power, they are doing it wrong and for my usual consultant fee I can tell them why and what’s better. For our purposes here about the best CR for both output potential and reliability with the parts we have to work with is 10:1 for a highly loaded motor (200 to 300+ hp) or 11 to 11.5:1 for a system rated in the 100-hp range. I should emphasize here that the horsepower rating of the system is only an approximation based on a typical application. If you spec your motor appropriately, you won’t have a typical situation, and therefore will see more power from a given number of pounds- per-minute flow of nitrous.
For example, the basic 100-hp system from NOS normally produces between 100 to 110 hp on a typical small-block Chevy, but will produce around 180 hp on exactly the same jet- ting even in a budget motor if it’s appropriately set up. A basic rule to follow is: Motors best respond to small amounts of N2O with high CR, and to large amounts best with low CRs. Other than limits imposed by detonation, the significant factor affecting the best CR to use is the flow capacity of the exhaust valve.
To get the best from N2O injection, cylinder heads need to be modified a lit- tle differently from the norm. Currently, no cylinder heads have been produced explicitly for optimizing N2O output. When N2O is injected into the engine, it produces an increase in exhaust volume in exactly the same manner, as would a bigger inlet valve and cylinder. Unfortunately, the exhaust flow capability of the cylinder heads generally available to us is limited. The amount of pumping losses seen on the exhaust stroke can increase considerably without additional exhaust flow capability.
A typical system delivering about 10 pounds of nitrous per minute increases the output seen in the cylinders, on the power stroke of a small-block Chevy by about 190 hp. So the now-restrictive exhaust increases the pumping losses seen on the exhaust strokes by about 80 hp. The result is a 110-hp increase at the flywheel. Retaining exhaust valve capability when such a nitrous system is used requires the exhaust valve to be increased from 1.6 inches to at least 1.8. Such an increase will effectively recover the 80 hp lost. In addition, the extra power will be achieved with a slight reduction in engine stresses. If higher- output race systems are to deliver their best, then an exhaust valve as much as 2 inches in diameter is required. It’s obvious that valve increases of this order are beyond the realms of practicality, but the situation does show how important exhaust flow is for a nitrous motor. At the very least we should make the exhaust port flow as well as possible.
If the heads can accommodate it, and most stock-type heads will, the intake valve size used can be the smaller 1.94-inch item. This then leaves enough room to install a 1.7-inch exhaust valve from a Pontiac V-8 (389 to 455 ci). This move has more effect on power than you may suspect because the most critical part of ridding the cylinder of spent charge is the “blow down” phase that takes place between exhaust valve opening and BDC. Because of this, the low- lift flow is more important than the high-lift flow.
The loss due to the use of a smaller intake is 10 hp at most and only comes about when the nitrous is not in use. On the positive side, when the N2O is activated, the increase due to the bigger exhaust is about 30 hp on a typical street system.
Although the bigger exhaust valve is the preferred method, there are limitations as to how far such a course of action can be taken. Fortunately there are other alternatives that help reduce the otherwise high exhaust-pumping losses seen when the nitrous is in use. By opening the exhaust valve earlier, the cylinder has more time to blow down. Also, the valve reaches a higher lift by the time the piston arrives at BDC. These two factors combine to make it easier for the exhaust to exit the cylinder.
In most instances, a high-lift 1.6:1 rocker on the exhaust is also of benefit, whereas it’s usually not needed or is even a hindrance to power on a non- injected engine. On a 10:1 350 with a 200-hp system, opening the exhaust valve 10-degrees sooner is worth 40 to 70 hp, so we’re not talking pocket change.
Checking out the specs of a functional nitrous cam may lead you to believe there’s more to it than just opening the exhaust earlier. Cam companies (the ones in the know, that is) will tell you that an N2O cam also needs to have wider lobe centerline angles and be more advanced in the engine. Although it may not be apparent at first, this is only due to the fact that the added exhaust duration is mostly on the opening side of the cam lobe. An example will clarify the situation.
Let’s start with a single-pattern 270- degree duration cam for a 350 with virtually optimal timing. This will be on a 108 LCA and timed in at 4 degrees of advance. This means the intake center- line will be at 104 degrees ATDC. This gives a timing of 31-59-67-23. Now let’s add 10 degrees of duration to the open- ing side of the exhaust lobe. Since the way the air is induced into the cylinder is virtually unaffected by the N2O, we shouldn’t alter the intake-event timing. All that needs to be catered to is the increase in exhaust volume, which is done by the earlier opening of the exhaust. Our nitrous cam will then have the valve timing at 31-59-77-23. All that’s changed is the point at which the exhaust opens. In changing this, we have, by virtue of the method by which we express the LCA and the resultant advance/retard of the cam, changed the LCA to 110.5 degrees and the advance to 6.5 degrees. From this you can see that any cam that has an extended exhaust duration and is on a wider-than-normal LCA will work as a nitrous cam if it’s installed in a more advanced position.
Crane had a number of cams with 110- to 114-degree LCAs that can be installed at 6 to 8 degrees of advance to good effect. This should bring their intake centerline to, or close to, the 104 required in a 350. COMP Cams has a series of five cams for nitrous applications. These have from 12- to 21-degrees more exhaust timing than intake. In a 350 they’re best installed with the intake centerline at 104 degrees.
When it comes to selecting a cam, it’s worth noting that a cam for a nitrous engine doesn’t need to have as much intake duration as normally would have been used. Because we literally are pouring in oxygen in liquid form, the intake valve can pass all the oxygen the engine can use without going to big intake lobes. When selecting intake duration for use with nitrous, figure on about 10- to 12-degrees less than you would have picked for a non nitrous application.
Using a nitrous-oriented cam for a motor that will have nitrous installed isn’t only about getting more power. The additional 40 to 70 hp seen is a convincing argument, but there are other important advantages if your motor will power a street cruiser. Because of the wide LCA and the reduced overlap of a nitrous cam, street manners are excellent. This means that not only will you see all that extra power when the nitrous is on, but also better street drivability than the equivalent non-nitrous cam when the nitrous is not used.
A Long Arm
Although its effect is much more limited, increasing the stroke on an exhaust-limited engine (which virtually all nitrous engines are) helps extract more from the nitrous that is injected. Assuming no other changes, increasing the stroke of a 350 to make it a 383 will produce about 10-hp more from a system rated between 100 and 150 hp. Since the added stroke is usu- ally worth about 15 to 20 hp from the naturally aspirated side, the combined effect of a longer-stroke crank means about 30-hp more without the use of more nitrous. Extra cubes from larger bores produce better results when the N2O is in operation, but the effect isn’t as noticeable as extra stroke. The main advantage of a bigger bore is that the engine makes more from its naturally aspirated charge.
We’ve looked at the results of installing a nitrous cam, and though it gives big power increases, it’s only a quick fix. The earlier opening of the exhaust valve reduces torque when the nitrous isn’t in use. If this mode of operation is important, opening the exhaust valve earlier can only be taken so far. By combining a larger exhaust valve and opening the valve a little earlier, these two moves are almost additive. A system that produced an overall engine output of 640 hp was increased to 756 when the heads and cam were more appropriate for the task in hand.
As more expertise in the use of N2O is acquired, there are benefits to fine- tuning the system and effectively injecting greater quantities of N2O. As has been mentioned already, most systems, as calibrated by the manufacturers, are set so the fuel-to-N2O ratio keeps piston temperatures within bounds and slows combustion so that the same timing with or without N2O works. As greater quantities of nitrous are used, it becomes necessary to retard the ignition timing. This not only allows the engine to produce more power, but also stops it from detonating itself to pieces.
Using the spark plugs as a guide to combustion temperatures, adjust the fuel jetting and ignition timing to produce better results. A basic rule here is: Always start with the mixture a little too rich and the timing a little too retarded. Then ease up on a more optimal mixture until the plugs say “no leaner,” then stop. At this point, go back to one jet-size larger on the fuel, and then turn your attention to the timing. Add in no more than 2 degrees at a time, and as soon as worthwhile gains are no longer seen, stop. Stopping just short of optimum will allow a useful buffer zone to accommodate those times when things get a little hotter than anticipated.
Nitrous power increases above 200 hp can become difficult to put to the ground. This is a nice problem to have, but it won’t win races. When the higher levels of nitrous-augmented horsepower are required, it’s worthwhile going to a two-stage system. Two-stage plate systems aren’t significantly more expensive than a single-stage system. Instead of overwhelming traction with excessive low-end torque, the amount of nitrous at low RPM can be reduced, and as RPM increases, the second stage can be activated. Not only does this make the use of nitrous more manageable off the line, it also allows a greater quantity of nitrous to be used as the amount the engine will tolerate increases with RPM. Because the rate of N2O injection is independent of RPM, we find, if we ignore internal engine friction for a moment, that the amount of additional horsepower produced is about the same regardless of RPM. This means that a system producing a 150-hp increase results in a torque increase of 315 ft-lbs at 2,500 rpm. At 4,000 rpm, 150 hp equates to 197 ft-lbs. At 6,000 rpm, that same 150 hp is only 131 ft-lbs. The increase in cylinder pressures obviously follows suit. Dumping in too much N2O at low RPM can produce gasket-blowing cylinder pressures.
Because of the high cylinder pressures at low RPM, it isn’t advisable to use anything other than a moderate system at revs below 3,000.
This big increase in low-RPM torque means that faster acceleration and quarter mile times are achieved by using fewer RPM than if the nitrous isn’t used. It also means that the amount of additional stall required in the converter is less. High-stall converters are made for engines that don’t produce any power until they’re up on the cam and revving well into their power band. If a nitrous motor is backed up by a converter with too little stall, it will be compensated for by the nitrous producing more low- end torque.
Because a tighter converter is more efficient, it allows a nitrous car to run significantly better MPH than if a looser converter were used. In many applications a converter no looser than a typical street unit is required. However, if a lot of torque is fed into a stock converter, it’s likely to balloon and/or break the fins. A heavy-duty converter will be required with any package making more than 400 total horsepower.
As the torque produced by the nitrous drops off at higher RPM, the engine can tolerate the use of more nitrous. By two-staging, we make the system more chassis friendly and also make more top-end power.
Although it may fall outside your budget, there are some tools that can help make a more sophisticated and effective nitrous system. MSD, Accel, and Holley all produce an ignition pack- age that allows the use of a fixed distributor with the advance curve generated electronically. All these units allow the triggering of the nitrous to retard the spark timing. Also, they allow for a nitrous shutoff a few RPM before the rev limiter is reached. In addition, multiple rev limiters can be used so that one limit is used for the burnout, another for launching, and the final one for high-end limit. All these units make for better use of the animal-like output that nitrous generates. Since there’s a relatively wide spread in cost, I suggest that you get a catalog from each of these companies, then determine which unit best meets your financial needs.
How Much Power?
There are two ways to use nitrous. It can be used to augment an engine that’s largely optimized for use without the nitrous. This means the nitrous power is simply an add-on and will give a good increase but not necessarily to its full potential. The engine in this instance could well be described as a nitrous- assisted engine. The fact that the nitrous isn’t giving its best is partially offset by the fact the engine, without it, is. However let’s strike a middle-of-the- road deal here and have the cam half- way biased toward nitrous use. In this instance the cam is an XR282HR COMP hydraulic roller (this is a post-1987 hydraulic roller block). The cam at 230/236 at 0.050 is relatively mild and ideal for street/strip use with a strong bias toward street use. This cam on a 110 LCA was set in at 6-degrees advanced. The chart on the left on page 149 shows the output of this motor with out-of-the box EQ heads and Zex’s cheapest perimeter spray plate system.
The jets for this unit are calibrated to produce a nominal 100, 125, 150, and 175 hp. In this instance I opted for the 150-hp jetting. At a total cost of under $5,300 as a turnkey deal (includes carb and full exhaust), this motor was a true budget street performer. A smooth 600-rpm idle and perfect road manners belied its ability to rip off mid-11- second quarter-miles in a 1970 Camaro.
If we’re building for maximum power with the nitrous in operation and leaving the power to fall where it may, some figures are possible that are limited more by component strength than by power production capability. For a drag- race application where finances dictate that the engine lasts at least 100 passes, it’s best to limit power to about 850. The chart below right shows the results of an engine that was conservatively jetted with the idea of allowing it to survive for a couple of season’s worth of racing. This engine made only a relatively modest 440 hp without the nitrous, but that went to 780 with the single-stage system activated.
This engine was a parts-collecting exercise if there ever was one. A new 750 mechanical-secondary Holley sat on top of a well-used swap-meet Victor Jr. intake. The heads (Dart) were extensively ported and utilized the usual 2.02/1.6 valve combination. With the flat-top Ross pistons, a 10.5:1 CR was achieved. A set of Isky 1.6:1 rockers, found at a swap meet, were actuated by a roller cam. This was acquired as a damaged part from a motor that suffered a rod failure due to excessive RPM. One cam intake lobe and three roller lifters were damaged. This package cost $30. The exhaust profile was 305 seat duration, ideal for the nitrous application. The cam was sent back to the company that originally ground it and a much shorter 284- degree profile was ground on the intake lobes. This fixed the damage and allowed the LCA to be widened to 112 degrees. For about $160 we had a roller cam and a set of lifters. This setup didn’t require heavy-duty valve springs because RPM involved weren’t that high. The crank was a stock GM forging ground 20 under and the rods were a set of Crower heavy-duty 5.7-inch long items that we had owned forever. A prepped stock pump and a budget Moroso oil pan took care of the bottom end. Ignition was by means of a hopped-up HEI with 28-degrees total when the N2O was in operation. Headers were 1.75-inch-diameter and brand new from Walker. A 15-inch collector and open exhaust were used. Fuel was courtesy of Unocal. This motor was a lot of work, but it cost only $2,900.
Written by David Vizard and Posted with Permission of CarTechBooks