What size cam should i use

The camshaft is the brain that actuates lifters, which in turn moves pushrods and rockers to open the valves. Once the lifter crosses the nose of the lobe, valve spring pressure forces the valves to shut. As you can imagine, the relationship between the lifter and the cam lobe is pretty important. Ultimately, all racers and performance enthusiasts will want a cam or cams that offer the most power and reliability.

This all leads to advanced methods for cam selection. Experts rely on experience and computer software programs that can predict camshaft performance. Computer modeling is really neat, as you can change cams virtually with all your engine specs in place and see different predicted results. In more aggressive applications, you should consider testing with a Spintron.

Spintron machines are used to physically run the rotating assembly and valvetrain components while allowing the operator to collect invaluable data. The Spintron machine uses an electric motor to spin the engine, so you can witness and record valve movement at real engine speeds.

These days, racers want a custom cam for a specific application. Common obstacles, however, are class rules and budget. For instance, Stock eliminator requires factory valve lift to be maintained, but racers can go with any duration or overlap. While this sounds simple, it opens the door to try endless combinations of duration and overlap — at least as many as budget or time will allow. Stock eliminator rules require factory induction, stock valve sizes, and factory lift on the cam, so extending duration is the only way to improve VE.

But, as I found out years ago, getting aggressive with ramp rates and selecting a huge duration cam can lead to disaster. The engine experienced massive float that led to a dropped valve after just a dozen runs. Increasing spring pressure solved the problem, but it was an expensive lesson. When selecting a performance cam, you need to be sure the fuel system and intake are capable of supplying an adequate air-fuel charge and that the exhaust has the capability to get rid of the exhaust gasses.

Too much gear ratio may run the cam out of its power range too soon, while too low of a gear ratio may bring the cam into the power range. A camshaft with. Next would probably be the budget. The above cam should have no problem making good power up to around 7, rpm but will probably be pretty sluggish under 3, rpm, so installing this camshaft into an engine with a stock rotating assembly that will only handle, say, 5, rpm or an engine with stock cylinder heads, intake, and exhaust manifolds that will not allow enough flow into or out of the combustion chambers would not make a lot of sense.

Lift is the least important of all the numbers. Cylinder heads are forgiving; should you miss the optimum lift number, the engine will probably still run pretty well.

If you miss the timing event numbers, you can easily turn the engine into a creampuff. Advertisement 3. If you are removing a good used flat tappet cam and lifters and are planning to use them again in the same or another engine, you must keep the lifters in the order they were removed from the cam they were on. If the used lifters get mixed up, discard them, install a new set of lifters, and break in the cam in again. You can use new lifters on a good used cam, but never use used lifters on a new cam.

Advertisement 4. Incorrect Valve Spring Pressure. Recommended valve spring seat pressure for most street-type flat tappet cams is between lbs. More radical street and race applications may use valve spring seat pressure between lbs. For street hydraulic roller cams, seat pressure should range from lbs. Mechanical street roller cams should not exceed lbs. Race roller cams with high valve lift and spring pressure are not recommended for street use, because of a lack of oil splash onto the cam at low speed running.

Never install springs without verifying the correct assembled height and pressures. Spring coil bind. This is when all of the coils of the spring contact each other before the valve fully lifts. Valve springs should be capable of traveling at least. Advertisement B. At least. Valve to piston interference. This occurs when a change in cam specs lift, duration, or centerline is enough to cause the valve and piston to contact.

Minimum recommended clearances are. Rocker arm slot to stud interference. As you increase valve lift, the rocker arm swings farther on its axis. Therefore, the slot in the bottom of the rocker arm may run out of travel and the end of the slot will contact the stud and stop movement.

The slot in the rocker arm must be able to travel at least. Advertisement 6. Distributor Gear Wear. The main cause for distributor gear wear is the use of high volume or high pressure oil pumps. If these types of oil pumps are used, reduced cam and distributor gear life will result.

However, you can increase the gear life by adding more oil flow over the gear area to help cool off the point of contact.

Some engines use a thrust plate to control the forward and backward movement of the camshaft in the block. The recommended amount of end play on these types of engines is between. Many factors can cause end play to change. When installing a new cam, timing gears, or thrust plates, be sure to verify end play after the cam bolts are torqued to factory specs.

If the end play is excessive, it will cause the cam to move back in the block, causing the side of the lobe to contact an adjacent lifter. Advertisement 8. Broken Dowel Pins or Keys. The dowel pin or Woodruff key does not drive the cam; the torque of the timing gear bolts against the front of the cam does. Reasons for the dowel pin or key failing are: Bolts not being torqued to correct specs, incorrect bolts of a lower grade stretching and losing torque, not using the correct hardened washer which may distort and cause torque of the bolt to change, LocTite not being used, or some interference with the cam, lifters, or connecting rods causing the cam to stop rotation.

A broken camshaft is usually caused by a connecting rod or other rotating part coming loose and striking it. Sometimes the cam will break after a short time of use because of a crack or fracture in the cam due to rough handling during shipping or improper handling prior to installation.

Features: Justina Reusch, Reusch Diesel. Magazine Current Issue Past Issues. Connect with us. Advertise Subscribe Contact Us. Compound Turbo 5. By Engine Builder. Are you building an engine for everyday driving? Most street driven vehicles seldom see the high side of 5, rpm, and most cruise at 1, to 2, rpm on the highway depending on how they are geared.

According to one cam supplier we interviewed, the best approach is to get more lift with the rocker arms and less with the cam.

Sometimes the rpm drop between shifts is rpm. Cams with durations in the to degree range measured at. If you follow these instructions, in most cases, you will choose the correct camshaft. Compression Ratio: The true "measured" compression ratio is a very important fact in determining which camshaft is best for your combination. Using the pistons advertised compression ratio can be very misleading too. The compression ratio is a function of cylinder head volume in c.

In situations where you can't check these dimensions we suggest you check your cylinder cranking pressure. In order to get this number, remove all 8 spark plugs. Do not squirt oil into the cylinders. Then crank it over and take cranking pressure readings on at least 2 or 3 cylinders.

Why do we ask for all of this knit picky information? Why don't we just pick a cam like everyone else does and just sell it to you? Because we don't want to be like everyone else. We really care that you get what you need and we are not happy unless you are making a lot of power. Cylinder pressure note:. The cylinder pressure is a result of 5 things, the static compression ratio, cylinder head material iron or aluminum , the closing point of the intake lobe, the altitude of operation of the engine and the final displacement of the engine.

When using 93 octane gasoline, build your engine for psi cylinder pressure with iron heads. Whenusing aluminum heads aim for psi cylinder pressure.

This is generally the maximum safe pressure and power without detonation or odd spark curves. As the camshaft gets larger duration at. The Power Curve The graph below describes what you should expect when you build an engine for a specific power range. If the engine breathing combination cam, carb, intake, heads, headers and compression ratio are properly matched, the duration at. This way you can compare the potential results with cams from various sources.

Note that as the engine power increases, the power curve will become narrower and steeper. It will also move up in the RPM scale. If your combination develops power in the "A" curve and you choose a smaller duration. Notice that the "B" curve is wider, but has a lower peak power. If you choose a larger duration. This curve is narrower, but has higher peak power. With a Hughes Engines camshaft curve "D" , the power will start sooner and pull longer, resulting in more peak power.

The RPM location of the peak power will remain about the same as long as the duration at. It will also increase the cylinder pressure. Retarding the camshaft a higher intake lobe centerline number vs. However, these rules are not absolute. Depending on the engine combination, the opposite results can occur. Call for details with your specific combination. We are happy to supply these figures. However, we will only supply them at.

Advertised duration is not checked at the same point with every manufacturer. A lot of lower priced computer dyno programs will not accept duration at. Most of these simulation programs are not designed with Mopar engine features, such as quench heights, longer rods, lifter diameter, etc. Therefore, the results from these types of programs are misleading. The better programs and there are very good ones naturally cost considerably more.