In this second part of our three-part series on camshafts, I'm unraveling the inner workings of camshafts. If you haven't read the first part, I highly suggest you get a copy or at least have some basic knowledge about camshafts. In this month's issue I'll talk about the basic power concepts of camshafts and the detailed terminology used to describe them. You'll see that even though two camshafts might both be advertised at 290 duration, there could be huge differences between them.
Most of you are familiar with the terms duration and lift on camshafts. The more duration and the larger the lift the more power, right? A lot more goes into cam design than you think and sometimes a well-designed cam can make power with smaller advertised measurements. Here's a quick engine primer to get you thinking about how an engine makes power. On a four stroke engine four events happen to make one power event: intake, compression, power, and exhaust.
The crank makes two revolutions in this process. For each revolution, the piston starts at the top, travels down, and then goes back to the top. The very top position is called top dead center (TDC), and the very bottom position is called bottom dead center (BDC). The intake stroke consists of the piston starting at TDC and the intake valves opening, allowing fresh air and fuel to enter the engine while the piston is traveling down toward BDC. Next is the compression cycle where the intake valves are closing and the piston travels from BDC back up toward TDC. With the intake charge being compressed, the ignition sets off the charge near TDC and begins the power stroke. The combustion pressure pushes the piston down from TDC back to BDC. The final cycle is the exhaust cycle where the exhaust valves are open and the piston is pushing the exhaust gases out as it travels up to TDC. The cycle repeats itself with the intake valves opening and the piston traveling back down, drawing in fresh air.
Camshafts control when the intake and exhaust valves open and close. Camshafts also control how far the valves open. Seems pretty simple: Open the intake valve when the piston is at the top of the intake stroke and close it when the piston reaches the bottom. Same for the exhaust valve: Open after the combustion stroke and close it back at the top. However, if we did this we'd have an engine that wouldn't make good power and it would all be at a very low rpm. Realistically the intake valve opens early (before TDC) and close later, well after the piston reaches BDC and it's already on the compression cycle. The same with the exhaust, it opens early before the power stroke is fully completed (before BDC) and closes after the piston reaches TDC of the exhaust stroke. The precise timing of these events determines how the engine will run through the rpm range. Certain timing events will benefit high rpm running and make more peak horsepower, while others will make more power down low at the cost of upper end horsepower. If your brain hurts already, take a rest, re-read, and get ready for some more cranial punishment. Ready? Most of you know what your compression ratio is, but what if I told you that your compression ratio changes when you change your camshaft? All automotive manufacturers and aftermarket companies rate engines and piston specifications as a static compression ratio. Static compression ratio is the volume of the cylinder when the piston is all the way down at BDC, compared to the volume when it's all the way up at TDC.
We now know that the intake valve doesn't close at the bottom of the intake stroke, but stays open partly into the compression stroke. This means that as the piston is traveling upward on its compression stroke the intake valve is still open! The later we close the intake valve the less compression we'll have because we're letting air back out. Dynamic compression ratio is the compression ratio taking into account the closing of the intake valve event. We know that a higher compression ratio yields more power due to higher combustion pressure on the power stroke. So now if we close the intake valve later, we reduce our dynamic compression ratio and also hurt power. That doesn't sound good, especially since most performance camshafts close the intake valve later than stock cams. There's some truth to that. Since the dynamic compression ratio goes down most of the time you'll see a loss of power at lower rpm with aggressive camshafts. At the same time though, the power at higher rpm will increase. Since the dynamic compression is lower, why isn't it losing power everywhere? Grab some caffeine and do some mental stretching because here comes the big one.
Air is a compressible fluid and as it travels at different velocities and pressures through an engine it has momentum. This concept is why a camshaft that holds the intake valve open longer can make more power at higher rpm. At low engine speeds the air has less momentum and the cylinder fills in with all the air it can get and some of it might even be pushed back out as the piston starts going up on its compression stroke. At high engine speeds where the piston and air is moving much faster, we can actually have air still cramming in after the piston is going back up on its compression stroke. A good analogy would be to imagine a train crashing into a solid wall with the back of the train keeping momentum and piling up on the wreckage. I mentioned the intake valve is opening before the piston is all the way at TDC and that the exhaust valve doesn't close until after TDC, that means that both valves are open at the same time! This is called overlap, and the same concept of momentum applies here. The idea is that the air going out the exhaust port will help pull in fresh intake charge as the intake valve is opening. Too much overlap can hurt idle and low speed running. Not enough overlap and it can hurt top end horsepower. It's a complex interaction that is determined by various factors throughout the engine.
The intake manifold design, head port design, valve size, valve lift profile, rod-to-stroke ratio, exhaust port design, and exhaust manifold design are all key components in determining the power output of an engine and how it produces power across the rpm range. This subject is very complex and auto manufacturers and top-level race programs have teams of engineers working on designing better engines. To keep it simple, let's just stick to the concept that holding the intake valve open longer will generally reduce low-rpm power and increase high-rpm power. This leads into the cam duration and valve timing events.
Cam duration refers to how many degrees of crankshaft rotation that the valve is open. Looking at a lift curve, which shows valve lift in reference to crankshaft degrees, you can see that it resembles a bell curve.
The idea is to gently lift the valve off the seat, open it quickly, hold it open, then close it quickly, and set it gently back down. At very low lifts there's very little airflow going on so most manufacturers rate their cam's duration at a certain lift. For example, the same cam can be listed as having 300 degrees of crankshaft duration at 0.004 inch of lift or it can have 240 degrees of duration at 0.050 inch of lift. Most of the cams that I'll be testing have advertised duration ranging from 270 to 280 degrees of duration. This advertised duration really must be taken with caution as it depends on what lift the manufacturer is specifying, some specify 0.004-inch lift while others use zero lift. Let's say we have two different cams rated at 240 degrees of duration at 0.050 inch, from the graph you can see how the same duration cams can have very different lift profiles.
If one cam is more aggressive on opening and closing the valve, it can have the valve open further for a longer period, theoretically supporting more horsepower. This type of aggressive profile will be hard on the valvetrain, requiring stiff valvesprings and lightweight valves that might only last 5,000 miles before wearing something out.
To show you how duration is calculated, let's look at this example. Taking an advertised duration of 0.004 inch lift, the intake valve would open roughly 20 degrees before TDC, stay open throughout the intake cycle, and then close 60 degrees after BDC during the compression stroke. The advertised duration of this cam would be 20 degrees, plus 180 (the intake stroke), and 60 degrees, for a total of 280 degrees.
Cam Lobe Data Collection
As I mentioned in the previous article I'm using the Performance Trends Cam Analyzer to record all of the cam data. The system consists of a test fixture that holds the camshaft and allows it to spin concentrically. A rotary encoder attaches to the end of the cam to measure the rotation and a linear encoder rides on the lobe to record the lobe displacement. All this data is fed into the Cam Analyzer software through a data acquisition box.
Not to complicate things further, but the valvetrain in the Evo head is complex. It's referred to as Cam On Rocker Arm (CORA). The cam lobe doesn't act directly on the valve, it transfers the motion through a rocker arm that pivots on one end and pushes on the valve stem on the other. On the opposite and easy end of the spectrum, the new Evo X has a cam-on-bucket design in which the lobe rides on a bucket lifter that acts directly on the valve stem. In the case of the Evo VIII, the trick is getting the cam lobe motion to translate correctly to valve motion. This procedure requires modeling the dimensions of the valvetrain in the Cam Analyzer software so it can compute one valve motion to the other. The tricky part is getting these exact dimensions; luckily, I had access to a coordinate measuring machine and was able to reverse engineer the head and valvetrain to get exact dimensions. Now I'm able to compare valve motion from one camshaft to another.
Using the data recorded by the Cam Analyzer we can look at the valve lift curves. Let's follow a lift curve and see what's going on. The valve lift curve shows crank rotation in degrees along the bottom (x-axis) and valve lift along the side (y-axis). Starting from left to right, we see the exhaust valve starting to open. This is the opening ramp, where it gently starts to lift the valve off the seat and into the main opening ramp. Near the peak lift of the cam the valve slows down and starts to head down the main closing ramp. As the valve gets closer to fully closing on the seat, the motion slows down and the valve gets gently set down. If this gently closing ramp wasn't there, the valve would get slammed onto its seat, possibly bouncing it off, causing damage and losing power. You can see that even though the exhaust valve isn't fully closed, the intake valve is starting to open. This is the period of overlap that I talked about earlier. I'll discuss overlap in more detail in the next article. Ideally we'd want to open the valve as fast as possible, hold it open, and then close it as fast as possible. Physical limitations of the valvetrain simply don't allow for this, and careful consideration must be taken to how fast valves open and close. Aggressive cam profiles require strong valvesprings to control valve motion and can even cause valvetrain components to wear out quickly. The design of camshafts is complex and great consideration must be taken into the power and longevity requirements of the engine. An endurance motor that has to run 500 miles of high-rpm running needs different cam profiles than an all-out drag race motor that only sees very limited run time. The Performance Trends Cam Analyzer can show the important data of a cam profile that determines many of the critical components needed for cam applications. For the average street tuner car this means choosing a cam that's not overly aggressive on the valve motions so that the engine lives worry free for thousands of miles. The Cam Analyzer helps me see the valve motion, and correlate valve opening and closing events with power gains on the dyno. The next and final article will cover valve events for the intake and exhaust, and help bring to light why some cams might idle better and some just make tons of power at high rpm.
Results: Stock Cams
What can I say, they're stock. They idle like butter at 900 rpm and have zero low-speed driveability issues. At 22 psi they pushed 418 whp, quite a bit lower than all the other cams and almost 100 whp down from last month's test of the Tomei 280 cams. Turning the boost up to 30 psi, the power climbed to 505 whp.
Forced Performance 4R cams
Idle quality is decent, while not as good as the mild HKS 272s, it's much better than the aggressive Tomei and Crane offerings. A few pulls later to dial the fuel in and the car made 465 whp at 22 psi, whereas at 30 psi I got 536 whp. After a few timing changes with the cams gears, the best power curve is found as per the recommended installed specs.
BC Brian Crower Stage 3:
The BC Brian Crower Stage 3 cams have a very similar idle quality and low-speed driveability to the FP4Rs. On power pulls, the power came in at 451 whp at 22 psi, and 527 whp at 30 psi. The BC Brian Crower Stage 3's did require some adjustment on the cam gears to get them to the factory specifications. While most cams were within 1 degree when checking out the specs on the stand, these cams had to be retarded 3 degrees on the intake and advanced 2 degrees on the exhaust side to check out. For an experiment I put them at zero, as if someone installed them without degreeing them, and the idle and power output suffered. This was a prime example why checking cam timing is important.
Comparing all three horsepower curves at 22 and 30 psi.
Next month's issue will bring a few more new cams, and I'll go into each particular valve timing event and why it affects the way the engine runs. I'll also look at specific cams and compare lift curves and valve events to see why they do or don't work.