camshafts

The cam is a very critical part of an engines valvetrain. The cam is what lifts the valves into the combustion chamber allowing air to enter the cylinder. Cam profile selection is very critical on how an engine will run, and how log the cam will live. The cam along with the cylinder heads control how much air can enter the engine. The more air in the engine, the more power that can be made (within reason). This does not mean the most aggressive cam is the best one for you. You can get away with a fairly agressive cam on a carbureted vehicle. Be careful on a fuel injected engine as too much overlap will drop vacuum and will not allow the fuel injection to work correctly.

 

You should be safe with a cam that mainly adds lift and maybe a little overlap. The cam ALWAYS needs to be selected to work with a combination of other engine parts, or you will lose power.If you have low vacuum especially on a fuel injected car you may need a vacuum canister, this can help to mask the low vacuum problem but will not completely solve it. It is the optimal combination of an engine’s parts that will make power. That is why mis-matched parts will never work well. Camshafts are a very complex and misunderstood engine part. In pushrod engines it pushes a lifter up, lifting a pushrod, which pushes on a rocker arm. As the one side is lifted the opposite side is pushing down the valve to open it. This arrangement allows the cam to be located down in the cylinder block. This is instead of the cam being located on the cylinder head. This arrangement is not ideal because it limits the amount of rpm’s. Although lightweight parts and carefully designed profiles can still turn reasonable rpm’s such as nascar engines do. Normally pushrods add weight and make the spring have a hard time keeping the lifters riding on the cam. It is very crucial that the lifter follows or stays on the cam lobe. If the lifter leaves the camshaft it will return with the force of the spring and bounce. This will eventually tear and hammer the cam lobe until it can no longer work. It also sends metal through the engine and can damage other engine parts. Another way a lifter can leave the cam is a too aggressive lobe ramp. The ramp is the side the lifter is riding up on as it rotates toward the peak of the lobe. If it accelerates too aggressively it can jump over the “nose” or top of the cam. When it comes back down it crashes hard on the back side of the lobe. To minimize the bounce, the valvespring can be made more stiff and the pushrods and spring keepers made lighter. This will help keep the lifter on the cam lobe. The lifter rides on a thin hydrodynamic film of oil. This keeps the metal parts from touching but still creates friction. This type of cam/lifter combination is called a flat tappet camshaft. There is another popular cam/lifter combination called a roller cam. Although it is actually the lifters that have the rollers on the bottom of them. These create much less friction and can free up as much as 15 horsepower dependant of the engine and rpm. These cams also will live longer because of the lack of friction. SOHC stands for single overhead cam. This type of cam sits over the head and presses down on followers. These compress the springs and press open the valve. The lack of valvetrain weight means that the engine can turn much higher rpms than a pushrod engine. The cam uses rocker arms to actuate the valves. The rocker arms are used because the valves are not on center with the cam. The SOHC setup is usually used on two or three valve cylinder heads. DOHC means dual overhead cam and is simply two cams positioned opposite the center of the cylinder. The cams are located over the valves. This is usually used with a four valve per cylinder configuration. The reason two cams are needed is that there isn’t enough room on a cam for that many lobes. The rocker arms would also get very complex with four valves per cylinder. The profile of the lobe should have alot of overlap and lift for performance. Variable valve timing is being used on many new engines. They can use mild cam lobe profiles for emissions, low fuel consumption and good torque. They can also use agressive profiles for midrange to high rpm performance. Single cam engines are limited to a compromise.