Camshaft design is one of those dark arts which is spoken about in very general terms in the mainstream, while the really heavy mental lifting is often done out of sight of the general public. Part of the reason for that is that the actual physics of camshaft and cylinder flow dynamics is an extremely complex science, and part of it is because each individual combination of parts and specs has a different “ideal” camshaft. Even similar combinations can vary in what their ideal camshaft specs are.
Luckily, Howards Cams and Racing Components has come out with a new line of shelf-stock camshafts for a wide variety of forced induction applications. Known as Howards’ “Boost!” line of camshafts, the engineers at Howards Cams have aimed to take a lot of the guess-work out of forced-induction camshaft design, and reduce the need for a custom-spec’d bumpstick for a good chunk of the forced induction crowd. We sat down with Ben Herheim of Howards Cams, and picked his brain about some of the more detailed aspects of forced induction camshaft design.
What the Numbers on the Card Mean
First, we need to define the terms we are going to be using. When you look at a camshaft specification card, there are some key numbers which are always paid attention to which cams are compared by. Peak lift is probably the primary way most people define a “big cam” and is, as the name implies, the distance that the valve is “lifted” off of the valve seat. It can be notated as the camshaft’s physical highest amount of lobe lift, or “at the valve” with a given rocker ratio. The latter can be altered – intentionally or not – through the use of different ratio rocker arms and is easily calculated by multiplying the actual lobe lift by the rocker ratio.
Then, there is duration, which, in its most absolute definition, is the amount of time – in degrees – that the follower is at a point higher than the base circle. However, to complicate things, there are a number of different types of “duration” numbers. The first and least accurate reference measure is the “advertised duration.” It is least accurate, because as the name implies, it was written by people trying to advertise, and in the past could be measured at anything from .004-inch to .022-inch of lift, depending on manufacturer and type of camshaft. To combat these discrepancies, most manufacturers list a “duration at .050-inch” spec, which is the duration of the cam with the lifter at fifty-thousandths of an inch or greater.
Taking it All In
Forced induction engines don’t rely on the cylinder to be at a lower pressure than atmospheric to fill, only lower than manifold pressure. – Ben Herheim, Howards Cams
Regardless of aspiration method, the goals of the camshaft design are to optimize cylinder filling and exhaust removal via the precise timing of valve events. The difference between naturally-aspirated and forced-induction intake events differs largely due to the difference in pressure on the back side of the valve in each application.
A naturally-aspirated engine relies on the vacuum created on the piston’s downstroke to create lower-than-atmospheric pressure within the cylinder to draw in the intake charge. A forced-induction application, as the name implies, starts the cycle with greater-than-atmospheric pressure on the back of the valve.
“Forced induction engines don’t rely on the cylinder to be at a lower pressure than atmospheric to fill, only lower than manifold pressure,” Herheim says. “Inlet valve opening can occur later due to manifold pressure being higher than atmospheric. That pressure differential also allows the inlet valve closing to be later than naturally-aspirated and still make a decent amount of low-RPM cylinder pressure.”
Like the intake side, the exhaust valve timing theory is quite a bit different for a forced-induction application. While a naturally-aspirated engine relies on exhaust scavenging to help effectively get the gasses to move in the desired manner, that’s not as much the case in a forced-induction application. To further complicate matters, exhaust profile design differs between types of forced induction, as well.
“Forced-induction engines require less overlap area than a naturally-aspirated engine for exhaust scavenging and to fill the cylinder,” says Herheim. “Lobe separation angle affects the valve overlap area, and in most cases forced-induction cams are designed with a 112-to-118-degree lobe separation angle. Naturally-aspirated cams are typically in the 106-to-114-degree lobe separation range.” Quick to remind us that we’re speaking in generalities, Herheim adds: “There are outliers in both cases when you look at extremes.”
Blowers Versus Turbos – The Everlasting Debate
When it comes to the exhaust side, there are also differences between the types of power-adders used. “As you refine and develop an engine for a specific application and type of forced induction, the cam requirements change,” Herheim says. “Typically, it’s the exhaust design that is different on turbocharged applications as compared to a supercharged application.”
The general theory behind the two applications is that on a supercharged application, you want to get the exhaust out as quickly and efficiently as possible. The goal is to reduce any kind of pumping losses posed by the additional volume of spent gasses in the cylinder trying to exit the cylinder. By adding duration on the exhaust side, you are essentially holding the door open for those pesky gasses to exit through, a little longer.
In a turbocharged application, you have some different concerns that factor in. The first being the inherent restriction of exhaust flow by the exhaust turbine. With that – for lack of a better term – “blockage” in the exhaust path, exhaust scavenging becomes more critical in the design of the exhaust lobe, to ensure efficient operation of both the turbocharger and the airflow within the cylinder.
Ramp It Up
Hydraulic Flat Tappet:
Gen III/IV LS-series 3-bolt
Small-block Ford (5.0/302 and 351W)
One of the reasons we defined the differences in duration measurements at the beginning of the article, is to be able to help you understand how the numbers of two camshafts might differ, as well as helping you to understand how the different duration measurements relate to one another. If you’ve ever looked closely at different cam lobes, you might notice that there can be similar advertised duration numbers on the card, with quite differently-shaped cam lobes. There is a way to extrapolate which cam will have a more aggressive lobe shape by looking at two different duration measurements.
“Ramp rate is a term tossed around by many people with no clear definition,” Herheim says. “The way we can compare [camshaft lobe] designs more evenly, is to use the old Harvey Crane method of ‘Major Intensity,’ which is defined as the duration measurement at .020-inch, minus the duration at .050-inch.” The lower the major intensity, the faster the valve will be moved from valve seat to peak lift (which is also referred to as a cam lobe being more aggressive).
Interestingly enough, the lobe shape of a naturally-aspirated camshaft lobe and a forced-induction camshaft lobe are not inherently different. “In most street applications, [naturally-aspirated and forced-induction] profiles have the same ramp, or major intensity. Race applications for both will generally be more aggressive than their street counterparts,” says Herheim.
Picking the Right Forced-Induction Camshaft
After reading all the above, you might be wondering when you need to switch to a forced-induction camshaft. “You need an application-specific cam when you plan on optimizing the engine for a specific task. Typically, anything more than a 20-percent increase in power, from naturally-aspirated, will benefit from a cam change,” says Herheim. He was also quick to note that you still need to have a system of properly-matched components; that a forced-induction camshaft won’t change a combination’s inherent attributes or act as a band-aid for mismatched components.
While we’ve tried to explain the basics of forced-induction camshafts, we’ve barely scratched the surface of the engineering voodoo that goes into optimizing a forced-induction camshaft profile. Fortunately, there are off-the-shelf options, like Howards Cams’ Boost! line, that should make camshaft selection a little easier without the need to go through the process of ordering a custom camshaft.
However, if you have an oddball or radical combination, Howards Cams does offer custom grinds and has the engineering staff to help work up a grind that will be perfectly matched to your combination. Whether off-the-shelf or custom, a forced-induction-specific camshaft will help you elevate your boosted engine’s performance to the next level.