Nitrous Tuning Tactics

Even novice nitrous users are aware of the big power potential of nitrous injection. They are also aware that past a certain base level, the engine will need to be "tuned" for the nitrous. Haul a kit from the box and the instructions make it clear that the number one tuning parameter is ignition retard (once the injection gets to a critical level). But that's all covered in depth, so we don't need to go there. Instead, we'll move right on to half a dozen tuning tactics that experts use, but that are unlikely to be in any installation instruction sheet.

Carb And Booster Selection
Because nitrous runs so cold, the fuel's light front-end hydrocarbons-which normally evaporate and more easily initiate the combustion process-simple don't evaporate. This means that the job of providing a suitably atomized mixture for effective combustion falls more heavily onto the carburetor. Even moderately over-large carbs or boosters without strong signals are to be avoided. This is good news for the street guy, because that slightly smaller carb is actually going to work better with the nitrous than one just a little too big, and it will also drive better on the street.

The booster design in a carb intended for a nitrous engine also has a strong influence on the mixture quality arriving at the cylinders. Annular discharge boosters are good at atomizing fuel, and frequently work best with nitrous. But you are not locked in here-a stepped dogleg booster (common with many high-performance carbs) runs a close second.

Bottle Heaters
Bottle heaters are a means toward consistency (by stabilizing the bottle pressure at a fixed value), and toward better combustion. And like carb and booster selection, it still plays a major part in mixture preparation. Most nitrous plates/ nozzles aim the stream of nitrous directly at the stream of fuel coming from the orifices. This considerably aids in fuel break-up and the mixing of fuel and nitrous.

Additionally, the heater increases the boil-off rate. If bottle temperature is too low, the pressure drops as the pass down the strip progresses. This results in less power from about mid-track on, where it could normally be used to good purpose.

Contrary to what you might think, heating the bottle does not result in a greater flow of nitrous. As temperature climbs, density drops, so the pounds-per-minute of nitrous flow changes very little. Nine times out of 10, the mixture is more homogeneous, and the power increases. On a nominally 150hp installation with 750 psi as a starting pressure, we have seen as much as a 20hp increase when the bottle was heated to deliver 950 psi. Not bad for a little warmth.

Intake Manifolds
If track performance is the only criteria, a no-compromise race-style intake manifold is best. If the car is a true street machine that needs meaningful speed at the strip, then the situation changes somewhat. Here, a good two-plane (180-degree) intake is needed. That said, the deal is hardly narrowed down to a single choice, as there are intakes for both warm and cold weather. If the car is to be driven predominantly in the southern part of the States, where winter weather is not that bad, then an air-gap-style two-plane is hard to beat. Edelbrock, Dart, Professional Products, and Weiand all make intakes that work well in this area (and we know from experience). If the car is driven in the cold winter climes of the northern states, some kind of limited exhaust heating of the intake manifold is mandatory-otherwise carb icing will result.

Initially, you might think intake heat would cause so much more of the nitrous to turn into gas in the intake manifold that the amount of air drawn in would be severely compromised. If this were the case, a large proportion of the power from the air drawn in would be lost, reducing the overall effectiveness of the engine as a nitrous power producer. To some extent, this is true with an exhaust-heated stock-style iron intake, since iron holds far more heat than aluminum. With a typical performance two-plane intake (with a smaller-than-stock heat crossover passage), the action of the nitrous quickly cools the intake to an acceptable level. If the car has insufficient traction off the line, the use of a heated intake can soften the initial nitrous punch to the point the car hooks up. If your car hooks up well, purge the nitrous through the engine rather than to the atmosphere, as this will quickly cool the intake to an effective operating temperature.

Heads for Nitrous
As soon as nitrous is a factor in the equation, the spec for the "best heads" changes. Getting sufficient oxidant and fuel into the cylinder is no longer the single biggest problem to overcome-getting rid of the exhaust is. Although juggling cam events helps, the real deal is to sacrifice intake valve size for larger exhaust valves. This is not always practical, but there are easier options available. For instance, a number of small-block Ford and Chevy heads have a 1.94 intake valve instead of the common 2.02. Such heads can easily utilize a 1.7-inch exhaust valve instead of the usual 1.6. One added advantage here is that the bigger exhaust has increased fuel economy potential, while allowing the engine (with a nominally 125hp nitrous kit) to make as much as 25 hp more.

When choosing heads for a nitrous motor, be aware that the port volume for best results is smaller than it is for a non-nitrous-injected engine. For the street guy, this is good because the smaller-volume street-style ports make for significantly better low-speed output. This means a tighter converter gets the job done, and the combination gives better gas mileage to boot.

If bigger valves are going in, it also makes sense to rework the exhaust ports for more flow. Even if the budget does not run to bigger exhaust valves, porting the exhaust is still cost-effective. Reworking a set of exhaust ports on a typical set of aftermarket aluminum heads is only a minor part of the overall cost normally associated with a full porting job. Figure a competent porter should get everything done in an hour and a half; multiply this by their hourly rate, and you can determine just what a set of high-flow exhaust ports will cost.

Nitrous Cams
When selecting a cam for a nitrous application, a decision has to be made that considerably affects the cam spec: Does the cam have to optimize output when the engine is running off the bottle, or on? If it's "off the bottle," just install whatever cam is appropriate for the spec of the motor. If it's "on the bottle," things get to look quite different, but in reality the change you need to make to the cam is simple.

Let's backtrack and consider heads for a moment. Most of the heads available have insufficient exhaust flow for a nitrous motor with any serious amount being injected (150-plus). To be sure that exhaust-stroke pumping losses are not high enough to cause a big drop, we need the cylinder pressure to blow down to no greater than about 2 atmospheres at BDC, just before commencement of the exhaust stroke. Think about this: If you have a 383 small-block Chevy, injecting about 200hp worth of nitrous generates approximately the same exhaust volume as a 570-inch big-block. That 1.6- or even 1.7-inch exhaust valve is just not going to cut it. So what we do is trade off a little power developed on the power stroke for a lot less pumping loss on the exhaust stroke. This is done by opening the exhaust valve sooner. There is no need to change intake opening or closing points, or to add extra overlap (which stays at whatever was optimal for an engine without nitrous).

Making a move as minor as opening the exhaust valve sooner drastically changes the spec of a nitrous cam from its non-nitrous counterpart. This is because of the way in which the cam specs are normally quoted. Other than lift, the duration, lobe-center line angle, and advance are the key cam spec issues. You've probably heard that a nitrous cam needs to be on a wider lobe centerline and timed-in for more advance, and it sounds like there should be a lot more to it than just opening the exhaust valve sooner. In reality, there isn't, and putting numbers to it shows that just altering the exhaust-valve opening point makes the cam look like an entirely different animal.

Let's say we have a single-pattern cam that was optimal in a 350 small-block Chevy for non-nitrous use, and its valve events were IO 36, IC 64, EO 72, EC 28. Basically, that is a 280-degree (advertised) cam on a 108 LCA (for 64 degrees of overlap), in at 4 degrees of advance (intake centerline at 104). If we add 8 degrees to the opening side of the exhaust (but leave all else the same), our cam is now 280/288 for duration and, with valve events of 36-64-80-28 (all we did was change the 72 to 80, and the intake centerline is still at 104), we have a cam with a lobe centerline angle of 110 in at 8 advance.

So how much more exhaust duration do we need for a nitrous cam used in conjunction with, say, a 150hp nitrous kit? That's almost like asking how long a ball of string is. In practice, it all depends on how lacking the exhaust port is in terms of flow. For a typical set of street heads and nitrous system, you can figure that about 8 degrees of extra exhaust timing will get the job done. For 200 to 300 hp, 12 to 14 degrees extra usually suffices.

Using a nitrous cam on the street has some advantages, even when the nitrous is not actually in use. Since a lot of overlap is unnecessary, the cam has civilized street manners and good vacuum. The downside is that the early opening of the exhaust valve costs some low- and mid-range torque, as well as a little fuel mileage; however, when the nitrous comes into action, the scales swing hard the other way. A nitrous cam in a 350 small-block with a 150hp kit will typically lose about 10 to 15 ft-lb low down (compared to its non-nitrous equivalent), but will get close, or possibly break even, at peak power rpm. When the nitrous comes into action, that 150hp kit can deliver up to 45 hp more, and commonly 35. If you are injecting 300-plus horsepower worth of nitrous, our own dyno testing has shown a well-spec'd nitrous cam can be worth as much as 75 hp more than its non-nitrous equivalent.

Exhaust Systems
Rule number one when it comes to exhaust systems for nitrous-equipped cars is that the system should not look like a cork. When the nitrous comes into play, it creates a lot more exhaust volume. If the system has a small amount of unwanted backpressure (and all backpressure in an exhaust system is unwanted), it can more than double when even a moderate nitrous system is activated. One example is a test on a stock 5.0 Mustang where a 125hp nitrous system, which increased rear-wheel horsepower by 102, pushed the backpressure up by 280 percent. With a baseline output of 221 rear-wheel horsepower off-the-bottle and 323 on, the stock cat and muffler were replaced with high-flow items. This pushed the output to 227 rear-wheel horsepower without nitrous, and 341 with it. With the nitrous on, the higher-flowing cat and mufflers were worth 19 more horsepower at the wheels.

Now let's talk about headers. First, header size is critical to the performance of a street motor when the nitrous is not in action.

If the headers are too big, performance suffers measurably at the bottom end (not good for an automatic with a tight street-style converter), and may not redeem itself by the time peak power rpm is reached. But when the nitrous is introduced, the previously optimal exhaust is now too small. A typical 350 Chevy making between 350 and 425 hp is best on a 1 5/8-inch primary header. At around 475 hp, a 1 3/4-inch header is ideal. Injecting 125 hp into even a 350-horse engine is going to move the header requirement to 1.75-inch diameter for best results. The big question is, at what point should the header size be stepped up, and when? First ask yourself what is most important: power with the nitrous on, or off? If it is nitrous-on, then the exhaust pipe needs to get larger by about .125 inch more than the primary diameter for every 150 hp of nitrous optimal without the nitrous.