It’s impossible to walk through a car show or even a Wednesday night cruise and not witness several different approaches to what has been dubbed “power adders” by those in the know. This arena has grown radically in the last decade or so with the development of better and more efficient superchargers like centrifugal blowers, screw superchargers, and of course turbos. The nitrous circus also continues to evolve with both smaller and easy-to-install street systems and those outrageous multi-stage nitrous oxide systems that seem to get more complex by the week.
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With the expansion of not only large cubic inch small-blocks as well as these power adders, horsepower has never been easier to make. The old standard for smallblocks used to be 400 hp. That has now gradually grown to 500 hp as the mark you need to achieve just to keep up with the other Camaros on the boulevard. But now with superchargers and better cylinder heads, 800 hp is not unusual for a big-inch nitroused small-block. For a 400–ci smallblock, 800 hp is 2 hp per ci (hp/ci). Not long ago, that number would have made the cover of several magazines and earned the engine builder entry a first floor spot in the Small-Block Haul of Fame. But today, 800 hp is almost standard fare. If you want stupid-big numbers, let’s talk about a 287–ci turbocharged small-block Chevy that powers Ken Duttweiler’s Comp Eliminator car that with a single, monstrous tur bocharger is capable a jaw-dropping 1,600 hp that equates to a stupefying 5.5 hp/ci.
So now that we’ve established that power numbers are easy to make, let’s look into how the camshaft relates to each of the big three power adders – nitrous, superchargers, and turbos. We give you a quick overview of each and then address how cam timing relates to all three.
Nitrous oxide is an odorless, colorless gas consisting of two parts nitrogen and one part oxygen atom to create a compound that appears as N2O. Nitrous oxide is compressed into a liquid so that it can be easily stored in a small container under extremely high pressure, usually around 900 psi. When released in your engine, the first thing that occurs is that the energy used to compress the gas into a liquid is released during the change from a liquid to a gaseous state, reducing the inlet air temperature in the intake manifold by roughly 65 degrees F. This temperature reduction alone is worth roughly 6 percent power, even if the nitrous didn’t contribute to the combustion process—but it does!
Next, nitrous oxide is a fairly stable organic compound, which means it doesn’t release its combined oxygen element easily. In this particular case, nitrous requires a combustion temperature of 575 degrees F before the oxygen is available to contribute to the combustion process. But once this temperature occurs, it certainly lives up to its nickname as supercharger in a bottle. Since the idea of any normally aspirated engine is to stuff as much air into the engine as possible, injecting nitrous into the engine at a finely tuned rate can do wonders for power. Of course, this means you must also inject a given ratio of fuel along with the nitrous to prevent the combustion process from running excessively lean and melting all those pretty pistons you invested so much money to buy.
The beauty of the nitrous oxide compound is that the two parts nitrogen also play a part by contributing to slowing the burn rate of combustion from the additional oxygen. This is why no one injects pure oxygen into an engine, since the burn rate would basically turn into a true explosion with devastating consequences to an engine.
One of the reasons that nitrous is so popular is because a basic 150-hp nitrous system is an external bolt-on kit that can be installed on almost any car in a few hours and requires no special internal or external components to make it work. As long as your engine can produce at least 4 to 5 psi of fuel pressure under load with the nitrous engaged, these nitrous kits are exceptionally easy and fun to use. The down side to nitrous is that even a 15pound bottle doesn’t last long, which means you become close friends with your nitrous supplier since he sees you often, and at $3 per pound nitrous is anything but free. But it sure is fun.
This power-adder takes a little more space to cover since we now have several variations of what used to be a onehorse town. The two basic styles of supercharging are positive displacement and non-positive displacement superchargers. Positive displacement superchargers encompass Roots-, screw-, and piston-style compressors. The most popular is clearly the Roots blower, using a pair of either two- or three-lobe rotors that turn inside a case to push air into the cavity above the intake ports. A Roots blower really isn’t a true compressor, but rather an air mover. It just moves air faster than the engine can use it, creating pressure in the intake manifold.
A screw-type compressor such as the Lysholm style units sold by Whipple Industries is a true compressor that squeezes air between twin screws. Because the screw compressor is a true compressor, its adiabatic efficiency (see sidebar) is better than a Roots. While the screw compressor is more efficient, it hasn’t been highly successful for street performance use, mainly because the Swedish company building the compressors has yet to build a large enough compressor to move enough air to make serious horsepower. But for smaller displacement applications, these are excellent superchargers.
Centrifugal superchargers are much more similar in design to what you could call a crank-driven turbocharger. The compressor wheel is driven by a two-part step-up ratio in order to generate the high compressor wheel speeds necessary to make these radial-flow superchargers work. The first step-up ratio is between the crankshaft drive pulley and the supercharger driven pulley. Then, a gear-drive step-up ratio occurs inside the supercharger to pump the speed up again to roughly an overall ratio of around 7:1. This means that if the engine is spinning 6,000 rpm, the compressor wheel is actually spinning at roughly 42,000 rpm. It is this immense speed that centrifugally compresses the air as it travels through the snail shell of the supercharger.
One downside to engine-driven superchargers is that each blower requires a significant crankshaft horsepower to drive. Even for a mild street engine, we’re talking about 35 to 50 hp required merely to drive a supercharger that may only then deliver an increase of 200 hp to the crank. If there was some alternative way to drive these blowers, a supercharger could gain an additional 25 percent power increase with no other changes.
Many enthusiasts consider the exhaust-driven turbocharger to be “free” horsepower since it has no direct connection to the crankshaft. While it is true that “waste” exhaust energy in the form of heat and pressure are used to turn the turbine wheel, this is not exactly a free lunch. Generally, some pumping losses are involved with piston effort required to push the exhaust gas past the turbine wheel, but these loses, while real, are substantially less than those required at the crankshaft to spin a Roots or centrifugal supercharger.
The advantage of turbocharging is that it offers tremendous power advantages without necessarily overstressing the engine. The disadvantage to turbocharging in the past had to do with the poor combination of carburetors and turbos, and few ever really came up with a simplistic accomplishment of that task. But with the advent of electronic fuel injection (EFI), now the engine can be fed the proper ratio of air and fuel that can be finitely controlled to ensure the engine never suffers from poor mixture distribution, which creates lean conditions in certain cylinders, usually followed by a burned piston!
Recently, turbo technology has improved to the point where the big problem of “turbo lag” has also been eliminated. Even very large diameter single turbo applications can sit at the starting line of a drag strip and create boost based strictly on engine speed and not load. This means that you can now use a manual transmission with a turbo, sit there at the starting line with the clutch in, and make as much boost as necessary to launch the car. This new generation of turbos eliminates the need to run an automatic transmission and brake stall for the engine to make boost.
The bottom line is that turbocharger systems are becoming increasingly popular although their price is still well above those more traditional supercharger kit prices. A typical small-block EFI-packaged smallblock also requires more custom fabrication than supercharger systems, but this may change as turbos enjoy increasing popularity.
According to the engine builders that we’ve interviewed, you can basically treat turbocharged, supercharged, and nitroused camshafts all the same way. Cam selection for these applications deals with taking into account the basic “funnel theory” of power-adder engine tuning. The funnel theory suggests that any power adder is like a funnel, in that it enhances the efficiency of the inlet side, but the outlet side of the funnel remains somewhat restricted. For example, if the cylinder heads remain the same but we pressurize the inlet side, it’s like adding a funnel to the inlet side of the engine. Since the outlet size (the exhaust port) does not change, we need to come up with a way to enhance the exhaust side in order to make maximum power.
The reasoning for this is simple. If you cannot purge the cylinder of the remaining exhaust gas at the completion of the exhaust stroke, either because of a restriction or insufficient time (in this case, degrees of exhaust duration), then the next inlet stroke still contains the remnants of the previous exhaust stroke. As the intake valve opens, the exhaust gas pressure remaining in the cylinder may be slightly higher than the intake manifold pressure, so the exhaust could (and will) easily travel up the intake tract. At the very least, it is eventually pulled back into the same cylinder. At worst, this exhaust gas, commonly referred to as reversion, mixes in the intake manifold plenum with the incoming gas for other cylinders. Since this exhaust gas does not burn a second time, reversion has hurt the volumetric efficiency and killed power because we’ve created a built-in exhaust gas recirculation (EGR) device. That’s hardly the way to make good power.
This process is easily seen when you look at an engine that first runs normally aspirated and then is quickly converted over to a supercharger with no other changes. Let’s say our normally aspirated engine achieved 400 peak horsepower at 6,500 rpm. Adding a centrifugal supercharger to this engine and running it again with no other changes creates a big peak power improvement by 40 percent—now we’re making 560 hp. But a funny thing has also happened, peak horsepower normally aspirated was at 6,500 but now the engine makes peak power at 5,900, which is 600 rpm lower than the normally aspirated peak.
Several things are happening here. First off, we’re probably paying roughly a 40 to 50 hp penalty to spin that centrifugal blower at 6,000 rpm. That’s a big reason why peak horsepower didn’t occur at a higher engine speed. But it’s also entirely possible that the exhaust side of our engine is less efficient now at evacuating the entire exhaust load since the cylinder is now operating at a much higher pressure and with more mass to purge. In this particular example, the crossover point is probably around 6,100 or 6,200 rpm where the engine no longer makes more power because the exhaust side can no longer efficiently evacuate the cylinder.
But let’s look at our engine a little more closely. Let’s say that our smallblock is running a single pattern cam where the intake and exhaust lobes are the same duration and lift. One quick way to crutch our engine and fool this air pump into thinking that it has bigger heads is to increase the exhaust duration by six or eight degrees in order to give the engine more time (in degrees of duration) to evacuate the cylinder and allow the engine to make more peak power.
Timing of these events is important since we don’t want to rob the engine of the potential to make power by opening the exhaust valve too soon. This merely hurts peak power since pressure is still pushing the piston down, contributing to power. But we also don’t want to open the exhaust valve too late, or we run the risk of running out of time on the closing side. On the other side of the lobe, we also don’t want to close the exhaust valve too late, since that adds overlap. But the early closing exhaust valve is probably why our engine example couldn’t make more power above 6,000 rpm. The key here is to extend the exhaust event duration by just the right number of degrees of duration while also opening and closing the valves at the precise moments.
The overlap portion of the cam timing-curve, between intake and exhaust, is the most critical element for a supercharged or turbocharged engine. The overlap portion is where the exhaust valve is just closing while the intake valve is just opening. Since we’re using pressure to force fresh air and fuel into the cylinder, the overlap cycle (while still important) becomes less critical. In normally aspirated engines, we rely on that negative pressure pulse reflected from the end of the header collector to create a greater differential in the cylinder, which adds more fresh air and fuel from the induction side of the engine. But with a turbocharged engine as an example, a turbine wheel is in the way of this pressure excursion before it can get to the cylinder. This is not the case with a supercharged engine, but again, the pressure on the intake side is generally so great that the effect of this negative pressure pulse is not nearly as critical.
The critical concept about overlap is to limit the duration of the overlap function to help prevent pushing fresh air and fuel directly out the exhaust valve during the overlap period. Generally, an earlier closing exhaust valve followed by a slightly later opening intake valve reduces the effect of the intake charge shooting right out the exhaust. This basically creates a lobe separation angle for supercharged and turbocharged engines of around 112 to 114 degrees of lobe separation angle.
This is somewhat simplistic since longer duration cams may actually require even wider lobe separation angles, but if you look in the Comp or Crane catalogs, the supercharged and turbocharger cams most often fall into this area.
So if we put the two concepts of longer exhaust duration along with a wider lobe separation angle, we have the makings of a decent supercharger or turbocharger engine camshaft. Since we already established an intake closing point (and therefore have a good idea of the intake duration), we have most of what we need to create a good blower cam. While a custom cam has never been easier to come by, wait before you run off in search of your local cam grinder’s home number. It’s possible that the major cam companies already have what you’re looking for.
Let’s take a typical street engine and bolt on a big 8-71 supercharger. It could also be a large centrifugal, but let’s stick that blower right through the hood. We have a 383-ci small-block with a good bottom end, a 4340 steel crank with high-quality aftermarket rods, and very good cylinder heads, like a set of Air Flow Research 210-cc aluminum heads. Now all we have to do is pick a cam. Let’s take a look at what would work.
Since this is a street engine, we don’t want to spin it much past 6,500 rpm. So let’s stick with a cam with duration at 0.050 of less than 240 degrees. We looked through several cam catalogs since we decided we wanted to stick with an off-the-shelf camshaft. The selections were limited compared to the normally aspirated cams, but we came up with a Crane mechanical flat tappet cam that looked very promising. The Crane F-278-2 lists in the cam spec box with 238 degrees of intake duration and a nice 10 degrees of additional exhaust duration to help blow down the cylinder at higher engine speeds. Plus, the cam also offers decent lift of 0.500 inches on the exhaust side with an excellent 114-degree lobe separation angle.
Once we’ve chosen the camshaft, we are not limited to just this configuration. Let’s say we have a chance to run our engine on the dyno and we start with the standard lash of 0.022 inches. For our first experiment, let’s say that we tighten the exhaust lash to 0.018 inches and run the engine again. We discover that the top end power improves slightly, but we lose midrange torque as a result. By tightening the lash, we added exhaust duration and overlap, which helped the top end power, but the increased overlap cost us more than we wanted to give up in the midrange.
We then decide to return the exhaust lash to the stock spec and instead add a 1.6:1 roller rocker to the exhaust side and try that. This time, the power improves slightly throughout the entire RPM curve. This change added 0.033 inches of valve lift throughout the entire RPM curve as well as a slight amount of duration, which helped improve power throughout the entire curve. These are ways to improve the power without having to go through the expense and effort of swapping cams when the results may only be minor changes in power. If we had added duration by tightening the lash on the exhaust and seen a major power increase throughout the entire RPM curve, then it would have been obvious that we would need a new cam with more exhaust duration.
Up until now, we haven’t mentioned nitrous cams specifically since in many ways they operate much like a supercharged or turbocharged application. However, with nitrous the inlet tract is not pressurized like it is in either of the other two applications. As such, we could get off on a wild tangent and begin the discussion by suggesting that nitrous cams could still benefit from additional overlap and that the slight amount of residual cylinder pressure still present when the intake valve opens is not nearly as critical as with a supercharged engine. The big determiner in this situation is probably how efficient the exhaust ports are on your nitrous engine.
Many nitrous engine builders we’ve spoken to end up tuning their engines more like a normally aspirated engine than a supercharged engine. This could be because they are already using heads with excellent exhaust ports, or because the overlap period is the critical function of the combination. You can choose your favorite theory, but it appears that treating the engine as if it were a normally aspirated engine seems to be winning at least at the present time. So basi cally, you would choose a cam with an intake duration and intake closing point that would support your peak horsepower RPM point, and then spec the cam based on how well you think your cylinder heads will flow when hit with the additional power.
It’s also worth mentioning that the amount of nitrous you hit the engine with plays a big part in determining how well the camshaft plays in this power game. Those small starter nitrous kits that pump in between 125 and 150 hp respond the least to major camshaft tuning changes. It’s really only as you get into the big-load 250- to 400hp kits that nitrous cam tuning becomes critical. The point here is that the returns on nitrous tuning with cam timing on a mild 125 hp kit is probably minimal while the larger kits respond more favorably.
Tuning for supercharger, turbos, or nitrous really isn’t that difficult. Paying close attention to the engine’s basic requirements and then giving the powerplant what it needs always pays off in terms of more power. That’s where the concept of believing your testing comes into play. If you’ve got a favorite theory, but your testing doesn’t support it, believe your testing and invent a new theory.
Written by Graham Hansen and Posted with Permission of CarTechBooks