From turbo-equipped OEM vehicles straight off the showroom floor, to hot rods and muscle cars featuring huge aftermarket blowers, boost is one of the most popular ways to make added horsepower. Using information provided by Billy Godbold, Valve Train Engineering Group Leader at COMP Cams, we will explain the variety of forced induction systems currently available and the types of camshafts best suited to each type of boosted application. COMP has part-numbered forced induction cams in stock for most engines, along with a wide variety of custom grind options.
The goal of all boost systems is to get as much air-fuel charge as possible into the chamber by the time the intake valve closes, thus avoiding huge exhaust pumping losses. Engines are all basically air pumps. With turbo systems, it is important to make sure the air is going in the correct direction at intake opening. The current availability of a combination of better exhaust housings, wheels, ball bearing designs and super-efficient turbines has made that far easier than even a few years ago.
On turbo as well as blower systems, it is important to consider the strengths and weaknesses of each forced induction system. Then the cam may be chosen based on that system’s performance level and features. A variety of cams are available for a wide range of applications, but here for simplicity we will compare the various features of LS-style cams.
These are the five most common subgroups of forced induction cams:
Race Turbo Systems (backpressure very close to boost in mid- to upper-RPM range):
In these applications the cams look very much like those found in normally aspirated (N/A) engines, except they are generally slightly smaller since they are moving much denser air, thereby greatly improving mass flow at similar or slower velocities than N/A.
In all cases of forced induction, it is not necessary to rely on piston movement to start drawing the intake charge. In good turbo systems, if the boost is greater than the backpressure at the exhaust valve, the intake charge will move in as soon as the intake opens. Overlap may waste significant air going through the chamber, but if the turbo compressor is oversized compared to power requirement, then trading efficiency for power can be an advantage. It is common to see turbo race cams in the mid 260s or larger duration at 0.050″ lift on rather tight lobe separation angles (such as 114), with even larger exhaust lobes if the exhaust turbine side is very good. The same N/A engine would probably have about 10-15 degrees larger duration on the intake lobe for the same RPM, and the exhaust split on a race turbo is most closely related to backpressure, whereas in N/A engines it will depend mostly on exhaust/intake flow percentage.
Due to these variances, and the type of racing these cams are being used for, racers will often specially order a custom grind cam for these systems.
Street Turbo Systems (backpressure above boost in mid- to upper-RPM range):
In good street turbo systems, the cam durations are trending larger while the lobe separation angles (LSA) are trending narrower. It is a good idea to go a bit small on cam – at low RPM there will be both low boost and low backpressure. If the engine can be cammed to be very responsive at low to mid-RPM if it were N/A, boost at higher RPM can be used to extend the power range and make the huge power numbers often achieved with turbo engines. Most street turbo systems are partial to hydraulic roller cams with intake profiles in the low 220s to high 230s at 0.050″ lift – depending upon RPM. Again, higher backpressure systems like smaller exhaust lobes and wider LSA than low backpressure systems. Basically, if the backpressure is ever higher than the boost amount, the overlap needs to be limited significantly.
A good example of a Street Remote or Rear Mount Turbo camshaft is the Remote Mount LSR™ Turbo. This is a street/strip hydraulic roller that features good responsiveness and excellent power when used with a remote mount turbo system.
Large Positive Displacement Race Blower Systems (great airflow capacity relative to engine displacement):
When NHRA Fuel cars run on Friday night, a large volume of air and nitro are pumped straight through the engine and out of the pipes before the next exhaust opening ignites the mixture. It is quite a dramatic effect for spectators – both visually and acoustically. This type of cam is not just applicable for Fuel cars, but really for any positive displacement application where mileage is not a concern and the blower size is not the major limiting factor on power. In NHRA Stock and Super Stock classes, the 4V Ford Mod may be run with two different positive displacement blowers. With the small blower, the best cams are roughly 16 degrees smaller than those used with the large blower. However, the large blower cams are typically installed on much tighter centerlines. The two applications have the same block and heads and run roughly the same RPM. But when the blower size is limited, the racers are forced to be much more conservative with the air, and hence they run almost identical exhaust opening and intake closing, but FAR less overlap.
Here is another case where rules and applications are the main variables to contend with, making a custom grind the best option for racers.
Smaller Positive Displacement Street Blower Systems (blower airflow is the main limit to power):
On most positive displacement blowers, the manufacturer can tell the user about how much air a blower will flow at a certain boost and how much power that it can support. If the blower will support 1500 HP but one only wants to make 750 HP (to save bottom end or for other reasons), it is possible to be pretty wild with the cam and have almost zero negative performance repercussions (aside from mileage). Positive displacement blower engines designed for the street tend to make a great deal of low-end torque due to the nature of the boost curve. However, they can fall off at high RPM if exhaust and intake pumping losses are allowed to build with a late exhaust open and early intake close. Therefore, the bigger cams tend to make great power, but it is necessary to pay attention to overlap if the blower flow is limited.
The LSR™ Roots Blower cam is a perfect choice for these types of street systems, as it takes into account all of the aforementioned engine characteristics. A hydraulic roller, it is the best option for street/strip applications equipped with a Roots-type supercharger.
Centrifugal Blower Systems (make greatest amount of boost at mid- to upper-RPM range):
These systems are incredibly efficient at high RPM, but provide little boost in the mid- to low-RPM range. More like the street turbo system, the cam should be selected to provide good power, even when used in a normally-aspirated application in the low engine speed or launch RPM category. Often, wider LSA and large exhaust lobes are seen on centrifugal systems. This is because exhaust pumping losses can limit performance at high RPM if the engine is fighting too hard to push out the exhaust once the centrifugal blower has reached its ideal high-RPM window. Overlap effect vs. airflow is almost identical to a positive displacement system. The main change seen on the centrifugal cams is an earlier intake closing to make the low-RPM torque.
Aptly-named LSR™ Centrifugal Blower cams are designed to make strong power when used with one of these systems, and are available for both street and race applications.
These are the most common types of forced induction systems—and the applicable cams. However, due to the number of variables between applications, it is always best to consult a tech before hitting the track or the street with any boosted hot rod.