Modern cams are typically ground with much faster rates of lift than traditional older gri
Traditionally, we have been exclusively dealing with two-valve pushrod engines for decades; however, this is changing. With Ford leading the pack, first with the Cobra four-valve and now the new Coyote 5.0 liter, we are certain that multivalve engines will establish a bigger presence in the future. For some insight in cam considerations in multivalve engines, we spoke to engine builder Kenny Duttweiler. “On the four-valve twin-cam heads, the interesting thing is how much smaller the cam is to do the same job,” Duttweiler says. “To get a certain amount of horsepower per cubic inch, the camshafts will generally be 20-30 degrees smaller.
“Depending upon the design of the head,” Duttweiler adds, “you are often limited on lift, but in lift you’ll find it does not need anything as aggressive as a pushrod engine. If you look at the area of the intake valve compared to the bore as contrasted to a two-valve, there is quite a bit more breathing area. You step up the duration as the rpm increases; for example, we built a Toyota 4.5L engine destroked to 3.5 liters, and the cams were 262 degrees at .050, and it made peak power at 10,200 rpm—and put out 1,800 hp. That is about the biggest we have run; the big ones are usually 250 to 255 degrees at .050, and you’ll often be in the 230-at-.050 range with less radical combinations.” Kaase’s observations seemed to agree. “In the race engines with the four-valve, in general, it seems they need about 20 degrees less camshaft,” he says. “The heads flow substantially more, especially at the lower lifts.”
EFI cam requirements can vary from those of a carbureted application, especially if the en
EFI vs. Carb
There are considerations that are particular to EFI applications when selecting a cam, especially if modifying an existing EFI-equipped car with a factory engine management system. Chase Knight from Crane Cams says: “A lot depends upon whether the ECU is going to be reprogrammed or replaced. These things are generally very vacuum sensitive. It is important to know whether the system will have the tunability to make the camshaft work with the control system. It also depends upon what the individual is interested in. If there is a concern about idle quality, then it will require a wider lobe separation or shorter duration to achieve better idle vacuum.”
Of course, the requirements will vary as you transition from a modified OEM application to a hot street or race setup with an aftermarket stand-alone engine management system. Jeff Says from Lunati tells us: “With an OEM-type throttle-body injection system, they can be untunable with long-duration, high-overlap cams. As you grow into full race EFI systems, the injection becomes irrelevant as far as the camshaft, because you can tune it around anything.”
A key factor that is often overlooked is the intake manifold configuration. Long-runner OEM-style intakes such as the LS engine pieces promote strong midrange torque. These manifolds tend to respond to a slightly wider lobe separation angle in comparison to a conventional four-barrel intake manifold.
Blowers, turbos, and nitrous can have a dramatic affect on the required camshaft. Generall
Special Considerations: Turbos, Blowers, Nitrous
Power adders will alter the cam requirements, sometimes in similar ways. All three of these power adders will force in a larger intake charge, creating the potential for a large power increase. Duttweiler relates his experience: “I deal with both blowers and turbos and, believe it or not, there are real similarities. In particular, you don’t need as much duration, or as aggressive a lobe profile, or as much valve lift. Generally speaking, you are oftentimes going to run 20-30 degrees less duration than with a normally aspirated engine of a comparable rpm range. When you look at the range of durations we run on turbo camshafts, it is almost like the camshaft isn’t the main player. You have so many other things going for you with good cylinder heads and the pressurization on the intake tract; that takes care of the cylinder fill. Obviously, if you want to fill the engine with more air, you need to do it for a longer period of time, so you need a bigger camshaft.
“In the lower power stuff, under 1,000 hp, you can be fairly mild with the camshaft and do a really good job. If you want to make, say, 2,500 hp with a small-block or 3,000 with a big-block, you have to cam it accordingly. On the big horsepower stuff you stay in the area of 270-280 at .050, while from 1,500-1,000 hp you’re going to be in the 250-260 range at .050. If you are in the 700-900hp range, those cams should be in the 220-240 at .050, depending upon the cylinder head. We run the lobe separation closer on the blower motors than we do on the turbo motors. You are not dealing with the backpressure with a beltdriven blower, you are just blowing it out the pipe.
“With turbos you are going to find backpressure, and when you are on the overlap cycle at higher boost levels, you are going to get an EGR effect in the intake runner. It will be a brief one, but it will be counterproductive to power production. So, the turbo engine will have a wider lobe separation. Say a typical big-cam normally aspirated engine will have a lobe separation of 112-113 degrees, while on the turbocharged engine it’s probably going to be 116-118 degrees of lobe separation. At the same time it will be a smaller camshaft in duration, so the overlap period will be substantially less. I prefer an earlier opening on the exhaust valve to get the valve far enough off the seat when the piston is moving up. With the turbo you are pushing a lot more exhaust out. Effectively you are doubling the displacement at 15 psi of boost, so you need to get a head start on the exhaust valve to simply blow down the cylinder sooner.
“Nitrous also has an effect on the cam timing for best power. Generally, the challenge here is providing for effective exhaust scavenging with the additional mixture burnt with nitrous. We typically see wider lobe separation angles in conjunction with longer exhaust duration. The cam phasing can be critical with nitrous, with more advance initiating the exhaust opening sooner to help blow-down the cylinder.”