The camshaft and valvetrain directly determine not only an engine’s personality, but also how reliably an engine will perform throughout its service life. When it comes to camshafts, there are probably more misconceptions than there are facts. We’re here to dissolve most of the myths and get you headed in the right direction on your small-block project.
To understand how to pick a camshaft and valvetrain, we must first understand how it all works. Choosing a camshaft profile is rooted in how we want an engine to perform. Are we building a street engine where low- and midrange torque are important, or are we building a high-revving racing engine that makes peak torque in the high revs?
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A camshaft manufacturer’s catalog lists dozens of camshaft types for the same type of engine. This is where it gets mighty confusing for the first-time engine builder. We see words like lift, duration, lobe separation, base circle, lobe centerline angle, and valve overlap. What does this information mean and how will it affect your engine’s performance?
One rule we tend to forget about in engine building is you can go with a stock camshaft grind, which all of the aftermarket manufacturers have available. You can go with a dead stock camshaft, get a smooth idle and good low-end torque. Crane, for example, has a flat-tappet 289 High Performance grind available that will give your 289/302 build that genuine Hi-Po sound. There are mild and aggressive grinds available for 5.0L High Output engines, not to mention the GT-40 1993-95 5.0L Cobra V-8. The nice thing about all of it is packaging. Comp Cams and Crane, for example, offer complete camshaft and valvetrain kits that include compatible springs and cam profiles. All you have to do is order the kit and do the installation. A kit provides a no brainer solution that ensures you’ll have components that work well together.
There seems to be a greater variety of roller-tappet camshafts available out there that allow you to dial in performance. The nice thing about roller cams is the aggressiveness they offer without sacrificing idle quality and drivability. Roller cams are better because they greatly reduce internal friction and allow for more aggressive ramp speeds and lifts, without sacrificing drivability. The roller cam is clearly a better choice for these reasons.
Let’s talk more about cam selection and choices. In the old days, we had two basic choices – mechanical and hydraulic flat-tappet camshafts. We went with mechanical camshafts whenever we were going to spin the engine at high RPM. A mechanical cam gave us the precision needed for high-RPM operation, because mechanical lifters don’t pump up at high revs. We adjust the valve lash and watch what the engine does at high RPM. Mechanical lifters are nothing more than the middle man between the cam lobe and valve.
Hydraulic lifters were conceived to take maintenance out of the equation because they automatically adjust themselves. The hydraulic lifter’s internal piston rides on a cushion of oil pressure that keeps everything snug against the pushrod, rocker arm, and valvestem. As the cam lobes and other valvetrain parts wear, the lifter takes up the slack, keeping the entire valvetrain nice and snug. Hydraulic lifters tend to “pump up” at high revs, making them undesirable for extreme racing applications.
Roller tappets are nothing new, even when they were introduced in Ford factory production engines in 1985. They reduce internal friction and give the valvetrain greater levels of precision. Roller tappets also allow us to run more aggressive cam lobe profiles that give us more power without adversely affecting drivability. Like flat tappets, roller tappets are available both in mechanical (solid) and hydraulic.
Camshafts – The Inside Dope
Always bear in mind what you’re going to have for induction, heads, and exhaust. The sharp engine builder understands that in order to work effectively, an engine must have matched components. Cam, valvetrain, heads, intake manifold, and an exhaust system must all work as a team or you’re just wasting time and money. If you’re going to use stock heads, which we expect with a stock or near-stock engine, your cam profile need not be too aggressive. Opt for a cam profile that will give you good lowand mid-range torque. Power doesn’t do you much good on the street when it happens at 6,500 rpm.
The best street performance cams are ground with a lobe separation between 108 and 114 degrees. When we keep lobe separation above 112 degrees, we improve drivability because the engine idles smoother and makes better low-end torque. This is what we want from a street engine. Anytime lobe separation is below 108 degrees, idle quality and streetability suffer. But there’s more to it than just lobe separation.
Compression and cam timing must be considered together because one always affects the other. Valve timing events directly affect cylinder pressure. Long intake valve duration reduces cylinder pressure. Shorter duration increases cylinder pressure. Too much cylinder pressure can cause detonation (pinging), but have too little and you lose torque. You can count on cam manufacturers to figure stock compression ratios into their camshaft selection tables, which makes choosing a camshaft easier than it’s ever been. Search manufacturers’ websites for their recommendations based on your application and you will be pleased with the result most of the time.
The greatest advice we can offer the beginner is to be conservative with your cam specs if you want reliability and long life. Keep with a conservative lift profile with under .500 inch of lift. High-lift camshafts beat the daylights out of a valvetrain, andtheyputvalve-to-piston clearances at risk. Instead of opening the valve farther (lift), we want to open it longer (duration) and in better efficiency with piston timing (overlap or lobe separation).
Valve overlap is the period between exhaust stroke and intake stroke where both valves are slightly open. This improves exhaust scavenging by allowing the incoming intake charge to push remaining exhaust gasses out via the closing exhaust valve. Were the exhaust valve completely closed, we wouldn’t get any scavenging. The greater the overlap in a street engine, the less torque the engine will make down low where we need it most. This is why we want less valve overlap in a street engine and more in a racing engine, which will make its torque at high RPM. Street engines need 10 to 55 degrees of valve overlap to make good torque. When valve overlap starts getting above 55 degrees, torque on the low end begins to go away. A really hot street engine might need more than 55 degrees of valve overlap, but not much more. To give you an idea of what we’re talking about, racing engines need 70 to 115 degrees of valve overlap. For a street engine, we want valve overlap to maximize torque, which means taking a conservative approach.
You should also consider lobe separation angle when choosing your cam. Choose a lobe separation angle based on displacement, valve sizing, and how the engine will be used. The smaller the valves, the tighter (fewer degrees) lobe separation should be. However, tighter lobe separation does adversely affect idle quality. This is why most camshaft manufacturers spec their cams with wider lobe separations than the custom grinders.
Duration is likely the most important dynamic to consider for a street engine. We can increase duration whenever we want to run less lift. Why? Because we get airflow into the cylinder bore two ways – lift and duration. We can open the valve farther (more lift) for less time (duration), or we can open the valve less and keep it open longer to get the same amount of airflow. Each way will have a different effect on performance. Excessive duration hurts lowend torque, which is what we need on the street, so we have to achieve a balance between lift and duration.
Valve lift is an issue we must think about as it pertains to an engine’s needs. Small-blocks generally need more valve lift than big-blocks. As we increase lift, generally we increase torque. This is especially important at low- and midrpm ranges where it counts on the street. Low-end torque is harder to achieve with a small-block because these engines generally sport short strokes and large bores. Your objective needs to be more torque with less RPM if you want your engine to live longer. High revs are what drain the life out of an engine quickly.
So, to make good low-end torque with a small-block, we need a camshaft that will offer a combination of effective lift and duration. As a rule, we want to run a longer intake duration to make the most of valve lift.
Written by George Reid and Republished with Permission of CarTech Inc
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