There are two types of turbo engines that regularly associate themselves with nitrous oxide: those that lay in broken pieces tucked away in the dark corners of some machine shop, and those that help make ridiculous horsepower - the kind of horsepower that makes even tough guys in passenger seats grab onto things. Machine shops need not be so cluttered though; all that's needed is a thoughtful kit, good tuning and a not so overzealous person given the task of jet selection.
Nitrous oxide lends itself well to turbocharged engines, especially to those that call the dragstrip home. In many cases, chemical injection is exactly what some cars need to squeeze those extra couple of tenths while still using that maxed out turbocharger struggling for top-end breaths. Nitrous oxide has been known on more than one occasion to turn a 10-second car into a nine-second car.
ZEX's Turbo Nitrous System takes a different approach to this careful mix of chemistry and the car. Instead of triggering nitrous flow based on throttle angle - and leaving it activated until throttle values drop below 100 percent - the ZEX system incorporates a boost-sensitive "off" switch. Typical nitrous oxide kits (including those from ZEX) supply the chemical compound at full throttle and keep it coming until the pedal's lifted. You may be wondering why in the world you would even want to interrupt nitrous flow at any point other than when you'd otherwise shift. The answer is in turbo selection; compressor housings, turbine housings, A/R ratios and wheel trims found on most sub-3.0L engines today make dwarves out of those used just a few years ago. The consequences are longer spool-up periods, but the power potential realized at upper echelon engine speeds almost makes up for the late entrance. Almost. There are a number of tricks to fool an oversized turbine into an earlier spool: more efficient exhaust manifold designs, static compression ratio changes, and more aggressive fuel and ignition maps are a few. The idea behind all of these methods is the same: to increase exhaust gas velocity and volume in order to move the turbine wheel sooner and faster.
Injecting nitrous oxide will do the same thing and is arguably an easier way of getting it done. Conventional wisdom explains the nitrous oxide injecting phenomena as such: the chemical compound - which itself isn't flammable - is injected into the intake stream, delivering oxygen content greater than that of atmospheric air resulting in the potential to burn more air and more fuel. This, of course, creates added cylinder pressure, which translates into more torque. Exhaust gas energy is released as a byproduct of the combustion process, and exhaust gas energy is what drives turbine wheels. If said energy can be harnessed and increased, the potential for altering turbine spooling characteristics can be tailored. What this means is that for the nitrous oxide kit that might otherwise supply an additional 25-75 hp for example, typically, the power realized on a turbocharged engine will be much higher since turbine speeds will increase, and sooner.
There are a number of methods by which nitrous oxide flow can be regulated. Besides jets, which determine spray patterns and the volume released, there are also volume-based controllers that regulate flow based on values such as throttle position and progressive controllers that step up nitrous oxide content in accordance with gear changes or vehicle speed. The ZEX Turbo Nitrous System regulates nitrous content based off of throttle position as well as manifold pressure, or boost. The ZEX system is activated, as you'd expect, by an electronic mechanism that senses wide-open throttle and delivers the gas accordingly. It isn't progressive, but rather an on or off situation. When dealing with smaller, single fogger systems, this is perfectly acceptable. Where the ZEX system differs is how chemical flow is interrupted, or stopped. A small pressure-sensing micro switch puts a stop to nitrous flow based on user-defined, pre-set boost levels. Why might anybody want to stop nitrous oxide flow at any point other than when the foot's released from the gas pedal, you ask. Good question. Conventional wisdom assumes otherwise and doesn't require much to figure out: when nitrous flow ceases, power production drops and things slow down - generally not the type of scenario most race car drivers are looking for. But traction battles often defy conventional wisdom. What if top-end power production isn't an issue; in fact, what if the turbo you've selected is already delivering gobs of tire-squealing, high-rpm power, but the problem is rather what to do about bringing that beast of a turbo to life at an earlier rpm and not looking silly bogging that 700hp monster out of the hole. But shifting torque curves in the leftward direction when concerning dyno plots is often easier said than done. The ZEX kit disputes this logic, however, and makes doing so just about as easy as saying it.
We once again looked to Honda's B18C to gather our test data - not that it's necessarily the best candidate for the ZEX kit, but small-displacement Honda engines destined for sizeable turbos and big horsepower figures are often met with poor spool-up characteristics. Such is the case with the 1.8L engine when paired with Garrett's impressive GT35R turbo. Match that to a driver that's still developing his shifting skills and you've got a recipe for dramatic power drops each time that clutch pedal's depressed. To be sure, the time slips prove this isn't such a good mix.
ZEX's Turbo Nitrous System is a wet design, which means it adds both air and fuel into the intake stream once activated. It's also a single fogger setup, meaning nitrous distribution must occur inside the intake charge piping to allow for proper distribution inside the manifold. The kit includes jets ranging in increases from 25-75 hp. So how did we end up with 110 additional hp to the wheels all before 6,500 rpm and well over 75 hp just before 4,000 rpm onward? It all goes back to the added exhaust gas energy created by adding nitrous oxide to the air/fuel mixture. As a byproduct of the combustion process, additional exhaust gas pulses are created, which helped get the GT35R's turbine wheel going at an earlier engine speed. At only one bar of boost pressure the ZEX kit supplied an extra 51-peak hp between 7,000-7,500 rpm. We expected as much since that's what we jetted the system for. What surprised us were the torque and horsepower figures realized below the peak threshold. From the moment the pedal was stabbed on the dyno, we found gains of at least 30 hp. From 4,000-8,000 rpm gains of up to 110 hp were made. Keep in mind, the kit was still only jetted for about 50 hp. Again, exhaust gas pulsations are key when considering larger frame turbines such as the GT35R. Post nitrous, the B18C's 14 or so psi was established completely by 6,500 rpm; without nitrous assistance we twiddled our thumbs until the 6,500 rpm range was reached for still only partial spool-up. This early spool-up is evident especially by looking at the torque curve. Like we've mentioned in the past, you can tell the most about an engine by examining the amount of space below the torque curve when viewing dyno data. Of course, more is better.
As expected, all of this translated to the dragstrip in a positive way and reaffirmed what we knew from the dyno about an earlier spool-up. The 60-foot, 330-foot and eighth-mile estimated times were all bettered. At equal boost levels eighth-mile times were reduced by a factor of 0.5 seconds; and keep in mind, our driver hadn't exactly attended any class on how to shift better since our previous trip to the strip.
Would we have found similar results installing one of ZEX's more conventional nitrous oxide kits onto our test engine? Sure; and likely better toward the top end of the track. Had we ditched the boost-sensitive off switch we could've expected power and torque figures to keep climbing, resulting in a quicker E.T. But like many other small-displacement, big turbo powerplants, generating huge, top-end power figures isn't difficult. Shifting the power curve to a more usable zone often is. Boost-sensitive nitrous oxide injection is just one more way to make that happen.