Picking camshaft specs has never been an easy task for the layperson. Camshaft companies have done their best to make camshafts as easy to understand as possible, and by and large, the average enthusiast does have a far more solid grasp on camshaft selection than in years past. However, once you add in the variable of forced induction, not only does camshaft selection become more critical, it also becomes much more confusing.
To help enthusiasts understand the special requirements and challenges involved with spec’ing a camshaft for a turbocharged combination, Comp Cams put together this video to help explain the basics. However, since we’re not really a “just the basics” kind of publication, we decided to dive deeper into the issue directly, by enlisting Comp Cams’ Engineering Group Manager Billy Godbold to help explain the challenges the user faces when selecting the proper camshaft for a turbocharged application.
To understand the challenges associated with a turbo camshaft, first, we need to take a look at the basic theory of camshafts and how they operate when no artificial pressurization is present in the system.
“Naturally aspirated engines have one atmosphere of pressure at the inlet, and one atmosphere of pressure in the exhaust,” Godbold says. “We create all the motion of flow either through piston movement or wave tuning.”
Here, we see that a naturally-aspirated engine sees the same 1 atmosphere - or 14.7 psi absolute - on both the intake and exhaust side of the engine. The balanced 1:1 pressure ratio allows for wave-tuning to help create charge motion.
“Wave-tuning” is an extremely complex subject, but can yield great results in a naturally-aspirated engine.
“In a normally-aspirated engine with headers, the exhaust pulse from the exhaust opening travels out as a positive wave. You set the header length such that an overtone is reflected back as a negative wave arriving back at the chamber at intake opening,” Godbold says.
“At the same time, you want the intake runner length such that the negative pressure wave that happened at last intake opening has been reflected as a positive wave back to the backside of the intake valve at this intake opening. The combination of the negative pressure wave in the chamber and a positive wave in the runner can result in a net pressure drop across the valve at intake opening, even though the piston is still going up towards TDC.”
Wave tuning is a critical part of a naturally-aspirated engine’s efficiency, since it is the only way to create charge motion other than—and sometimes in spite of—piston motion. “Wave-tuning is what made the huge jump from engines that only had effective filling in the 40- to 70-percent volumetric efficiency range to the common 110- to 130-percent numbers seen in today’s race engines,” Godbold says.
So while you may be wondering what that has to do with turbo camshaft design, we promise, we’ll be coming back to this point later.
Once you add artificial pressurization of the intake system, like you do when adding a supercharger, you have a source of charge motion other than piston movement or wave tuning.
“With a supercharger, you have a higher pressure on the inlet side, and lower pressure on the exhaust,” says Godbold. “So we can use that to initiate airflow, even before the piston starts moving away from top dead center.”
In a supercharged application you pressurize the intake side of the engine—to 3 atmospheres of pressure in this example—but the exhaust side remains at 1 atmosphere, creating a 3:1 pressure ratio on the intake side.
While there are some similarities between supercharging and turbocharging on the inlet side—mainly the pressurization of the intake tract to assist in cylinder filling—there are significant differences on the exhaust side between the two combinations, which lead to an entirely different set of camshaft specs needed between the two applications. When it comes to camshafts there is no such thing as “boost is boost.”
A Kink in the Exhaust
Now, getting into the complex world of turbocharged camshaft specifications, we need to address the biggest challenge faced by engineers, and that happens to be where the turbocharger’s magical “free horsepower” comes from – the exhaust turbines.
In older, less efficient turbochargers, even when running at two or three atmospheres of boost, the exhaust restriction caused by the turbine wheel can create enough backpressure to achieve a pressure ratio of 2:1, with the exhaust side being the more pressurized of the two. When you are dealing with higher pressures on the exhaust side than the intake side, there are special considerations that need to be understood.
Notice the pressure difference between the older, inefficient turbocharger’s exhaust side and the intake side. The 2:1 pressure ratio is opposite that of the supercharged engine’s in this example, with 4 atmospheres of pressure on the exhaust side and two atmospheres on the intake side. That pressure differential creates a unique environment in which the turbo camshaft has to operate.
“In [turbocharged] applications, we have to be very careful with valve overlap. If we open the intake too soon, while the exhaust is still open, that pressure differential can cause a backflow,” Godbold says. “Sometimes you’ll see someone use a naturally-aspirated camshaft in a high-backpressure turbo setup, and there is such an abundance of blowback, you’ll actually see black or brown deposits in the intake manifold.”
“An inefficient turbocharger, and a moderately efficient turbo, remotely mounted both have similar profiles. We give both applications more intake than exhaust duration and wider lobe separations,” Godbold says. “That works with higher boost than backpressure numbers, where you need to make all the low-end torque without relying on boost, and then making sure you have enough stability and duration to allow the engine to run up into the RPM range where the turbo can really help.”
As turbochargers have evolved, not only have their compressor sides gotten more efficient, but so have the turbine sides. Modern high-end turbochargers still cause restriction in the exhaust tract, but will bring backpressure down to a level equal to the pressurization of the intake tract.
“Now that we’re dealing with similar pressures on both sides, with high-density charges on both sides, we’re back into a system that is more similar to a naturally-aspirated model,” says Godbold. “We can use piston motion and some wave tuning to get the charge motion at a lower velocity and with less valve timing.”
Remember when we talked about wave-tuning in naturally-aspirated combinations and said we’d be coming back to that? Now, not only are there power increases from the more efficient turbocharger design, but camshaft designers can now take advantage of wave-tuning as well, to even further increase the power output of a turbocharged combination.
With the more efficient turbine sections of modern turbochargers, the intake and exhaust pressures are back to a 1:1 ratio. Although elevated from a naturally-aspirated application, the equalized pressures allow camshaft designers to apply some of the theories behind N/A camshafts to turbo cams.
With the more efficient turbochargers on the market today, there is still an exhaust restriction - there’s no getting around that fact - but they bring the restriction level down to that of the amount of boost they are providing to the intake tract. That 1:1 pressure ratio allows wave-tuning to come into play again. However, the more efficient turbochargers have the byproduct of being able to efficiently turn more RPM than their predecessors, which is another variable that needs accounting for in camshaft design.
“The biggest change I see today is not only the pressure differential dropping but also the increase in RPM. If you have a rather large turbo on a smaller engine, you need RPM to move enough air to get the most out of the turbo,” says Godbold. “For the combination of these reasons, many turbo applications will have custom grinds with the smoother HLO lobe family to allow engine speeds approaching 8,000 rpm in a hydraulic roller application.”
Shelf or Custom
So, with the complexities of a turbocharged camshaft now apparent, it begs the question, is there a decent off-the-shelf turbo camshaft? The answer is yes, for both older and newer turbocharged combinations. “We honestly have more shelf cams for the older turbo packages than the newer race type systems,” Godbold says.
If an off-the-shelf cam doesn’t suit you, the custom-spec camshaft route is always an option. However, a custom camshaft isn’t a magic wand that will somehow fix mismatched components, or an incorrect combination.
There are quite a few options for turbo combinations on the shelf at Comp Cams, and unless you have a unique combination or set of operating parameters, chances are, a shelf cam option exists that would work well for you.
Custom camshafts really need more than just a cam swap to see big gains.
“The only people seeing a 50-plus horsepower gain from a custom cam over a shelf cam would be those operating well above 6,800 rpm.” says Godbold. “Some applications will see a bigger change, though. For someone running a shelf cam on smaller LS engine, they could easily pick up 100 or so horsepower by choosing a cam that would allow safe operation to 7,500-plus rpm with a larger, low-backpressure turbo. For most typical street/strip applications at lower RPM, there would be almost no difference in performance with a COMP custom cam versus a shelf turbo grind.”
While we’ve tried to expand upon the science that Comp Cams’ video illustrates, we’ve still only scratched the surface of the high-level brainpower that goes into creating the perfect camshaft grind for a turbocharged application. Luckily for enthusiasts, Comp Cams’ engineering team is on-call and able to assist customers down what can seem like a complicated path.