In my experience, the best streetperformance cams are ground with a lobe separation between 108 and 114 degrees. Keeping lobe separation above 112 degrees improves drivability because the engine idles smoother and makes better low-end torque. There’s also more vacuum at idle for accessories and power brakes. This is what you 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 they always affect each other. Valve timing events directly affect cylinder pressure and ultimately working/ dynamic compression. Long intakevalve duration reduces cylinder pressure. Shorter intake duration increases cylinder pressure. Too much cylinder pressure can cause detonation (pinging). Too little and torque is lost. Cam manufacturers figure stock compression ratios into their camshaft selection tables, which makes choosing a camshaft easier than it’s ever been. Follow the cam manufacturer recommendations and you will be pleased with the result most of the time.
This Tech Tip is From the Full Book, FORD 351 CLEVELAND ENGINES: HOW TO BUILD FOR MAX PERFORMANCE. For a comprehensive guide on this entire subject you can visit this link:
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Lift and Compression
Be conservative with your cam specs if you want reliability and an engine that lives a long time. Choose a conservative lift profile (.500-inch lift). High-lift camshafts beat the daylights out of a valvetrain. And they put valve-to-piston clearances at risk. Watch duration and lobe separation closely, which help you be more effective in camshaft selection. Instead of opening the valve more (lift), you want to open it longer (duration) and in better efficiency with piston timing (overlap or lobe separation).
Always bear in mind your induction, heads, and exhaust. The savvy engine builder understands that in order to work effectively, an engine must have matched components. Cam, valvetrain, heads, intake manifold, and exhaust system must all work as a team or you’re just wasting time and money. If you’re going to use stock cylinder heads, your cam profile need not be too aggressive. Opt for a cam profile that gives you good low- to mid-range torque. Torque doesn’t do you any good on the street when it happens at 6,000 rpm. Choose a cam profile that makes good torque between 2,500 and 4,500 rpm. Otherwise, you’re just wasting engine.
One thing to remember with camshaft selection is how the cam works with your engine’s cylinder heads. Take a close look at valve lift with a particular cylinder head and determine its effect. Some camshafts actually lose power with a given head because there’s too much lift or duration. This is why you want to understand a cylinder head before choosing a camshaft. You want to seek optimum conditions with any cylinder head/camshaft combination. This means doing your homework before making a decision.
What type of fuel do you intend to run in your engine? This also affects camshaft selection. You can actually raise compression if you’re running a mild camshaft profile or using a higher octane fuel. Camshaft timing events must be directly tied to compression ratio. The longer the duration, the lower cylinder pressure and working compression. The shorter the duration, the less air brought into the cylinder, which also affects compression.
Your objective needs to be the highest compression possible without detonation, which harms the engine. With this in mind, you want the most duration possible without compression extremes. Duration is what gives you torque, as long as compression is sufficient.
Valve overlap is the period between exhaust stroke and intake stroke when both valves are slightly off their seats. This occurs to improve exhaust scavenging and cylinder filling. It improves incoming intake charge to push remaining exhaust gases out by the closing exhaust valve. With the exhaust valve completely closed, you wouldn’t get scavenging. The greater the overlap in a street engine, the less torque the engine makes down low where you need it most.
This is why you want less valve overlap in a street engine and more in a racing engine, which makes its torque at high RPM. Increased valve overlap works best at high RPM.
Street engines need 10 to 55 degrees of valve overlap to be effective torque powerhouses. When valve overlap starts traveling above 55 degrees, torque on the low end falters. A really hot street engine needs more than 55 degrees of valve overlap, but not much more. For example, racing engines need 70 to 115 degrees of valve overlap because that is what you need at high RPM.
For a street engine, valve overlap should maximize torque, which means taking a conservative approach in the first place. Push overlap as far as you can without compromising torque. You also have to figure in lift and duration with valve overlap to see the complete power picture.
Lobe Separation Angle
Lobe separation angle is another area of consideration in street cam selection. This camshaft dynamic is chosen based on displacement and how the engine will be used. Rule of thumb is this: Consider lobe separation based on how much displacement and valving you’re going to be using. The smaller the valves, the tighter (fewer degrees) the 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 in a street engine is likely the most important dynamic to consider in the selection process. Why increase duration whenever less lift is desired? Because your airflow gets into the cylinder bore two ways: lift and duration. You can open the valve more and for less time to get more airflow. Or you can open the valve less and keep it open longer via duration to get more airflow. Each has a different influence on performance. Duration is determined by the size of the cylinder head and amount of displacement, and how the engine will be used. Excessive duration hurts lowend torque, which is what you need on the street. So balance must be achived by maximizing duration without losing low-end torque. This is done by using the right heads with proper valve sizing. Large valves and ports don’t work well at all for street use. Mix in too much duration and you have a real slug at the traffic light.
So what does this tell you about duration? You want greater duration whenever displacement and valve sizing go up. Increasing duration falls directly in line with torque peak and RPM range. And this does not mean you necessarily gain any torque as RPM increases. It means your peak torque simply comes in at a higher RPM range. For example, if your engine is making 350 ft-lbs of torque at 4,500 rpm and you increase duration, you may well be making that same amount of torque at 5,200 rpm. In short, increased duration does not always mean increased torque.
Compression has a direct effect on what duration should be. When you’re running greater compression, you have to watch duration closely because it can drive cylinder pressures too high. Sometimes you curb compression and run greater duration depending on how you want to make power. When you have greater duration, the engine makes more power on the high end and less on the low end. This is why you must carefully consider duration when ordering a camshaft. Higher compression with a shorter duration helps the engine make torque down low where you need it most in a street engine. The thing to watch for with compression is detonation and overheating. Maximum street compression is around 10.0:1.
Think about valve lift as it pertains to an engine’s needs. Small-blocks generally need more valve lift than bigblocks. As you increase lift, you generally increase torque. This is especially important at low- and mid-RPM 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. Revs are what drain the life out of an engine more quickly.
To make good, low-end torque with a small-block, you need a camshaft that offers a combination of effective lift and duration. As a rule, you want to run longer intake duration to make the most of valve lift. You get valve lift via the camshaft to be sure. But, rocker arm ratio is the other half of the equation. The most common rocker arm ratio is 1.6:1, which means the rocker arm gives the valve 1.6 times the lift as at the cam lobe. When you step up to a 1.7:1 ratio rocker arm, valve lift becomes 1.7 times, which you find at the lobe.
It is best to spec a cam on the conservative side, especially if you’re building an engine for daily driving and weekend race use. Whenever you opt for an aggressive camshaft with a lot of lift, you’re putting more stress on the valvestem, guide, and spring. The constant hammering of daily use with excessive lift is what kills engines without warning.
Taking this excessive wear logic a step further, it is vital that you ascertain proper centering of the rocker arm tip on the valvestem tip when you’re doing valvetrain setup. This is performed by using the correct-length pushrod for your application. Use a pushrod checker to properly configure your Cleveland’s rocker-to-stem geometry.
A pushrod checker is little more than an adjustable pushrod used to determine rocker arm geometry. If the pushrod is too long, the tip is undercentered on the valvestem, causing excessive side loads toward the outside of the cylinder head. If the pushrod is too short, the rocker arm tip is over-centered, causing excessive side loading toward the inside of the head. In either case, side loads on the valvestem and guide cause excessive wear and early failure. This is why you want the rocker arm tip to be properly centered on the valvestem for smooth operation.
One accessory that reduces valvestem tip wear and side loading is the roller-tip rocker arm. Roller-tip rocker arms roll smoothly across the valvestem tip, virtually eliminating wear. Stampedsteel, roller-tip rocker arms are available at budget prices without the high cost of extruded or forged pieces.
A dual-pattern camshaft runs different profiles on the intake and exhaust side to meet individual need. You run dualpattern profiles whenever you’re pushing up the revs. Typically, a dual-pattern camshaft runs a shorter exhaust-valve duration due to less time required to scavenge the exhaust gases at high RPM. It is also beneficial whenever you’re running nitrous or supercharging/turbocharging where exhaust scavenging is rapid and furious where you need more duration.
Another reason for running dualpatterns is Ford’s traditionally restrictive exhaust ports. Though the 351C-4V is blessed with generous intake port sizing, it is cursed with restrictive exhaust ports, which call for a dual-pattern cam profile. Intake needs to behave differently than exhaust. This is where you need more duration on the exhaust side in an effort to improve scavenging.
Written by George Reid and Republished with Permission of CarTech Inc