Richard Holdener
October 1, 2008

What's the first thing that comes to mind when we tell you it's possible to add an easy 100 hp to your modified 5.0 motor (or other Ford) with one simple bolt-on? Because of the impressive power gain, you'd be right on target thinking the single bolt-on was some type of forced induction, given the boost craze lately. But in this case, you'd be wrong.

If boost isn't the bolt-on of choice, then the extra 100 hp must come from the old standby nitrous oxide, right? Actually, you're only half right, as the installation of a simple plate nitrous kit like the one we tested from NOS will easily add the requisite 100 hp, but so too will the right set of cylinder heads. What's that you say, 100 hp from a simple head swap?

Which one is worth 125 extra horsepower-nitrous or the right set of cylinder heads?

How on earth can a set of cylinder heads offer as much extra power as a nitrous system? While it's true that you'd be hard pressed to get these kind of gains from just any set of performance heads on any 5.0 motor, installing the heads on the right combination can yield impressive dividends that rival the power offered by the ol' bottle.

To understand the comparison test, we should take a closer look at how the two bolt-ons go about adding power, starting with the bottle. One of the most common misconceptions about nitrous oxide, one fueled by Hollywood's entertaining but misguided attempt at covering the undercover world of street racing, is that it's a fuel. While nitrous oxide will indeed enhance the power output of your Ford, no 5.0, 4.6 or 5.8 Windsor could run on nitrous oxide alone. The reason is that (again, unlike in the movies) nitrous oxide does not burn. While this statement may seem contradictory to its unique ability to greatly increase the power output of an internal combustion engine, there's much more to the equation than simple flammability. As we will discover, nitrous oxide has a number of unique characteristics that make it ideal for power enhancement.

The test motor came from the good folks at Coast High Performance. The 331 started out as a late-model 5.0 block. The CHP stroker kit included a cast 3.25-inch stroker crank and forged connecting rods.

Though certainly not a fuel, nitrous oxide does greatly enhance the combustion process. Technically speaking, nitrous oxide is an oxidizing agent. The additional power produced by the injection of nitrous oxide comes from the release of free oxygen molecules. The two ingredients in the basic recipe for power are air and fuel, of course assuming the two are combined in the correct ratio. In this formula, air is a rather generic term. The air we breathe is made of a number of chemicals, the two most common of which are oxygen and nitrogen (incidentally, just like the composition of nitrous oxide). It's actually the oxygen present in our generic "air" that burns to produce the expansion necessary to push pistons and rotate our crankshafts. Performance enhancements, such as turbocharging, supercharging, and most forms of normally aspirated modifications (ported heads, cams, and intake manifolds) all seek to supply the motor with additional airflow. The additional airflow increases the amount of oxygen available to burn, thus increasing the "potential" power output.

Unlike these other forms of power enhancement, nitrous oxide supplies the additional oxygen molecules in chemical form. In addition to other important benefits, this chemical supercharging eliminates the heat generated by a typical turbo system and the power losses associated with driving a mechanical supercharger.

The flat-top pistons featured dual valve reliefs to accept either inline-valve or TFS heads.

As the name suggests, the compound nitrous oxide, or dinitrogen monoxide, is composed of two nitrogen molecules and a single oxygen molecule. When heated to approximately 572 degrees F, the compound is broken into its component parts, thus releasing free oxygen molecules in the process. It's the release of these oxygen molecules that support (or more accurately, enhance) the burning of the flammable fuel (gasoline) already present in the system. As a side benefit, the released nitrogen acts as a buffer or antidetonate to allow dramatic increases in power while maintaining a given detonation threshold. In short, it's possible to add 100-150 hp to even a stock motor without fear of detonation. Of course, this assumes the correct ignition timing and air/fuel ratios are part of the equation. Pure oxygen, while offering plenty of power-producing oxygen molecules, does not possess the necessary detonation suppression quality to allow for use in performance applications. Uncontrolled burning, elevated heat levels, and a dramatic change in the detonation threshold all but eliminate the use of pure oxygen as a form of chemical supercharging.

We mentioned earlier that the power gains offered by nitrous oxide come from the release of the free oxygen molecule. Though accurate, nitrous oxide enhances the power output of an internal combustion engine in other ways as well. Nitrous oxide used for automotive applications is stored in a pressurized container (bottle) and supplied to the motor in liquid form. Once delivered to the inlet tract, the liquid turns into a gas, a process called vaporization. This transformation from a liquid to a gas requires an input of energy; in this case, the energy is heat. The vaporization of the liquid nitrous absorbs heat from the inlet air, a desirable characteristic on any motor, especially one equipped with forced induction. Once vaporized, the temperature of the nitrous oxide is still at or near -129 degrees (the boiling point of nitrous oxide). Naturally with such a low boiling point, the -129-degree nitrous still provides a dramatic cooling effect on the inlet air. This double cooling not only reduces the chance of detonation, but also increases the density of the inlet air. The denser air provides more oxygen molecules, which in turn creates more power. With so much going for it, it's easy to understand why so many street racers employ the use of nitrous.

It should be obvious from the discussion on nitrous oxide that the power output of a motor is dependent on the number of oxygen molecules it can ingest. Increasing the power output of a motor is as simple as increasing the airflow (or oxygen molecules) into the motor. Unlike nitrous oxide, cylinder heads don't possess free oxygen molecules, and therefore must provide the oxygen in the form of additional airflow. Where the nitrous oxide is forced into the induction system, the power gains offered by a set of performance cylinder heads is dependant on other factors. What this means is that adding a set of fully ported race heads to your otherwise stock motor will not offer much, or any, additional power, despite the fact that the race heads offer twice the airflow of the stockers. More airflow from the heads (as tested on a flow bench) does not necessarily equate to more power. For the additional airflow to be converted into power, the airflow must be increased through the motor-more airflow in equals more power out.

Exhaust chores were handled by a set of Hooker Super Comp headers feeding 3-inch dyno mufflers.

The problem-or challenge-associated with improving the power output of a motor with a set of performance cylinder heads is creating the proper combination. Given the dynamic equation that is an internal combustion engine, it's necessary to have not just sufficient head flow, but optimized head flow. When the optimized head flow is combined with the proper induction system, cam timing, and compression, you have the makings of one efficient powerplant. Eliminate any one of the idealized components from the combination, and you no longer have a harmonious symphony. Instead, you're stuck with a bunch of top-quality instruments simply clanging together to make noise.

The same scenario is true of an otherwise- stock motor. Adding a set of cylinder heads to a stock motor may result in little extra power, since the heads were not a major restriction to the total airflow of the motor with a stock intake, cam, and exhaust. Despite the exemp-lary head flow, the stock induction system, cam timing, and even compression will ultimately limit the power potential. This is why you see head tests where even the best flowing set of heads unearthed little power. Please resist the temptation to select cylinder heads by advertised airflow alone. Big flow numbers may or may not correspond to big power numbers. Remember, the combination is the key, and all that flow can be detrimental if the combination is not up to the task of utilizing it.

We ran a CSI electric water pump to help keep things cool. The electric water pump makes dyno testing much easier since we don't have to hook up all the accessories.

To illustrate the two different routes of enhancing power production, we took a mild 331-inch stroker assembly and sub-jected it to the dyno. The stroker was sup-plied by Coast High Performance. The 331 featured a late-model (5.0) 302 block stuffed to the gills with a cast-steel 3.25-inch stroker crank, forged connecting rods, and matching forged pistons. Tucked inside the CHP stroker assembly was a Comp Xtreme Energy hydrau-lic roller cam. The XE274HR cam offered a 0.555/0.565 lift split, a 224/232 duration split, and a 112-degree lobe-separation angle. The cam profile is a popular choice of street Mustang owners, offering excellent power through the entire rev range, especially on the larger 331 stroker (compared to a standard-displacement 302). Additional go-fast goodies on the test motor included a Barry Grant 750 carburetor, an Edelbrock Performer RPM Air Gap intake, and a set of Hooker Super Comp headers. Also present was a CSI water pump, a complete MSD ignition system, and a TCI Rattler balancer. For this test, the 331-stroker motor was equipped with a set of stock 5.0 iron cylinder heads. The only mods to the heads were valvetrain-related to allow use of the high-lift cam.

Run with the stock 5.0L (E7TE) iron heads, the 331 produced 347 hp and 398 lb-ft of torque. Now it was time to add an extra 100 hp.

The 331 stroker motor was first run in with 5W-30 Lucas conventional oil and then tested with the good stuff (full synthetic). Only minor jetting was required, and the stock-headed motor responded best to 35 degrees of ignition timing. Equipped with the stock heads, the CHP 331 stroker produced peak numbers of 347 hp and 398 lb-ft of torque. By no means a wild combination, the 331 stroker would be right at home in the engine bay of any Mustang, offering plenty of torque and over 1 hp per cubic inch. While respectable, what the motor could really use was another 100 hp.

To provide just that, we installed a Super Powershot plate nitrous system from the nitrous experts at NOS. Once installed, we had an extra 100 hp (the jetting was adjustable up to 150 hp) literally at our fingertips. After heating the bottle to ensure an optimum 900 psi, we were rewarded with almost exactly 100 extra horsepower. The peak power numbers jumped from 347 hp and 398 lb-ft, to 451 hp and a stump-pulling 526 lb-ft of torque. Nitrous never fails to impress on the dyno. Want to transform your small-block into a big block? Just push the button.

Our first bolt-on came in a big blue bottle. Nothing is as easy or efficient at adding extra power as a simple nitrous kit. This Super Powershot carbureted plate system from the nitrous experts at NOS featured everything we needed to add anywhere from 100 to as much as 150 extra horsepower.

After the sweeping success of the Super Powershot system, we were anxious to get to the cylinder-head swap. Waiting in the wings were AFR 185 heads, which have always impressed us with both their flow numbers and how well those numbers translate directly into power numbers. The AFR 185s topped the list in our "Ultimate Guide to Cylinder Heads," and here was another example of a stroker motor just begging for some additional head flow.

The question now was, how would the power offered by the AFR heads compare to the tremendous gains offered by the NOS system? The head swap required using different pushrods and a fresh set of Fel-Pro 1011-2 head gaskets, but once everything was in place, we were ready to make some noise. As usual, the AFR 185s did not disappoint us, as the power output of the CHP 331 stroker jumped from 347 hp and 398 lb-ft, to 450 hp and 446 lb-ft. Not only did the AFR heads increase the peak horsepower output by more than 100 hp, but the peak torque numbers were up as well (from 398 lb-ft to 446 lb-ft). In fact, the AFR 185 heads improved the power output throughout the rev range, from 2,600 rpm all the way to 6,100 rpm. The AFR heads shifted where the motor made peak power, from 5,600 rpm with the stock heads to 6,100 rpm, a sure indication that the stock heads were a restriction on this application. Compared at 6,000 rpm, the AFR heads improved the power output by an amazing 125 hp.

Next up was a head swap to a pair of 185 heads from Airflow Research. While just about any set of aftermarket aluminum cylinder heads would offer gains compared to the stock heads, the AFR 185s have always proven to be impressive performers.

While it may seem beyond belief that a set of cylinder heads can be worth more than 100 hp, know that in our testing for the "Ultimate Guide to Cylinder Heads," a head swap was worth over 200 hp. Replacing the stock E7TE heads on a wild 392 stroker with a set of AFR 205s improved the power output by an amazing 243 hp at 6,500 rpm. Does this mean a set of heads is always worth 243 hp? Heck, no. Of course it would be nice, but as indicated previously, it's all about the combination.

Now the question is, what can be expected of the extra 100 hp? In a heads-up shootout in identical cars, the 450hp nitrous car would certainly win, thanks to the tremendous average power production. Where the AFR heads offered peak gains of 100 hp, the NOS system improved the power output by 100 hp at every rpm range, from roughly 4,000 rpm to 6,000 rpm. The extra 100 hp at 4,000 rpm translates to an extra 128 lb-ft of torque. This compares to the just 40 extra lb-ft offered by the head swap at the same engine speed, thus the NOS car would motor off into the distance. Obviously the ideal combination would be the 331 stroker with the AFR heads and the NOS 100hp shot, but that's a test for another day-albeit one with predictable 100hp results.

Power Numbers: Stock Heads VS. NOS
Power Numbers: Stock Heads vs. AFR 185 Heads