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Camshaft Definitions - Ramp, Flank, and Nose

CAMSHAFT DEFINITIONS

When discussing camshafts, enthusiasts often get confused with the terminology used to describe the various parts of the camshaft. We hope the diagram on the left and the definitions below will help enthusiasts better understand camshafts and the related terminology.

• RAMP: The textbook definition of ramp is the section of the cam from the base circle to where the valve physically begins to open, or finishes closing. It is also commonly referred to as a clearance ramp; or in other words the part of the cam lobe where the camshaft will close up the initial tappet clearance (lash) and the tappet/follower will make initial contact (on the opening side) or end its contact with the camshaft (on the closing side). Skunk2 defines ramp as the portion of the profile from the base circle to the point of maximum valve acceleration. Skunk2 Fast Ramp Technology helps the valve go from zero to maximum acceleration as quickly as possible and still maintain superior valvetrain stability.

• FLANK: is defined as the end of the ramp section to the point where the valve reaches maximum velocity. We frequently hear people talk about “aggressive ramps” when they are actually trying to describe the flank and how quickly the valve is opening. It is important to find the balance between opening the valve too quickly and not opening the valve quick enough. If the valve is not opened quick enough, “area under the lift curve,” the airflow is not optimized. If the valve is opened too quickly the camshaft may run off the tappet, and it will become difficult to slow the valve down enough as it goes over the nose.

• NOSE: is defined as the section between the maximum velocity on the opening side and maximum
velocity on the closed side, or rather the section of the cam where the valve spring forces are keeping the valvetrain from separating from the cam surface. Controlling valve accelerations over the nose is critical to preventing valve float and high-rpm valvetrain stability. Skunk2 Amax Technology allows us to design the flank and nose section of the cam to maximize area under the curve and still maintain valvetrain stability.


HOW TO DEGREE CAMSHAFTS OVERVIEW

One of the keys to making power is to properly set camshaft timing; in other words, when valves open and close in relationship to the position of the piston and crankshaft is critical to the performance of the engine. The process of properly setting the camshaft position is referred to as “Degreeing the Cam”. Many beginner tuners mistakenly believe that to degree cams means setting the cam gears at a certain position such as “+1 intake & -2 exhaust”. Though this information may be useful at times, these settings may not be accurate on all motors. For example when the deck of a head or block
is machined, it will retard the cam timing. So the cam gear setting method may only apply to engines using the same type of cam gears with exact same head and block heights; and this also assumes that the given cam gear settings are the correct location for the cams. The most accurate way to set camshaft position is to properly “degree the cams”; this way you can be sure the cams are in the right position regardless of engine variations, deck heights, and cam gear marks. The method we are
introducing is a simple method for setting cam positions using peak lift measurements. Cam degreeing can also be used to check valve opening and closing positions, durations at various lifts, and peak lift measurements.

Step 1: Install a Degree Wheel onto the end of the
crankshaft, and bolt a pointer onto the block. The pointer
can be a sharpened piece of welding rod or coat hanger
that can be bent to change the position of the pointer.
Rotate the crankshaft to TDC, you can use a dial indicator
inserted down the spark plug hole or the piston stop
method; the piston stop method is more accurate. When
the crankshaft is at TDC, move the pointer so it points to
TDC / 0 degree on the degree wheel.
Step 2: Set-Up dial indicator with the tip on the retainer,
not the rocker arm. To get an accurate reading, It is
important to make sure that the axis of the indicator is
parallel with the axis of the valve. Make sure the rocker is
on the base circle of the camshaft, in other words, make
sure the valve is completely closed, and zero out the dial
indicator. We recommend that you degree the cam with
the lash set at 0.000.”
Step 3: Rotate the crankshaft. When the cam starts to
open the valve, the dial indicator will show the amount
of valve lift. Rotate the crankshaft and stop when the
pointer is pointing at the specified peak lift/center line
position. Loosen the cam gear bolts and rotate the
camshaft until the indicator is showing that the cam is at
peak lift. Tighten the cam gear bolts. Rotate the engine
two more rotations, stopping when the dial indicator
reaches peak lift, look down at the degree wheel to
make sure the position of the crankshaft is in the correct
location. If not, repeat step 3.
  Step 4: Move the dial indicator to the other side of the
head, and repeat steps 2 and 3. When peak lift positions
of both the intake and exhaust cams are set in the proper
locations, the cams are considered to be degreed in.
  Helpful Tip 1: When degreeing a camshaft, make sure
that you rotate the crankshaft in the direction the
engine normally runs. If you over shoot the position the
crankshaft is supposed to be in, do not rotate the engine
backwards, it will throw off your numbers because the
tensioner only works properly in one direction.
  Helpful Tip 2: If you are having a hard time finding
the centerline because the cam dwells at peak lift, you
can take a reading of the degree wheel when the cam
reaches max lift less 0.003” before and after peak lift.
The middle of those two positions will be the centerline.


Cam Diagram Cycle

 

CAM TIMING EVENTS AND 4-STROKE ENGINE OPERATION


Cam Timing, or rather when valves open and close in relation to piston and crank position, is critical to making power. The graph above and the explanation below are an attempt to explain what goes on inside a four-stroke engine, define key terms used when describing cam set-up, and help you better understand the importance of proper cam timing.

1: Piston is at the top of the bore or Top Dead Center (TDC) and both valves are closed. Ignition occurred about 20o-40o before. The piston is being pushed down by the combustion pressure.

2: By 90o after top dead center (ATDC) the cylinder pressure is already starting to decrease and the exhaust valve can begin to open safely before the piston reaches its lowest point or Bottom Dead Center (BDC). The combustion cylinder pressure pushes the burnt fuel mixture/exhaust gases
out the exhaust port.

3: The piston then changes direction after it reaches BDC and begins to help push out the remaining exhaust gases. It is important for the valve to open early enough so the exhaust valve is nearly wide open when the exhaust stroke begins. This reduces the resistance, known as pumping losses, caused by the piston trying to push against the exhaust pressure. Opening the valve earlier will give the engine more time to blow down the exhaust pressure.

4: The exhaust valve is at its maximum opening or peak lift. This is the exhaust centerline position, or rather how many degrees peak lift occurs before top dead center (BTDC). It is important that the peak exhaust lift occurs when the piston is near its maximum velocity on the exhaust stroke to reduce pumping losses.

5: Before the exhaust stroke is complete and the piston reaches TDC, the intake valve begins to open as the exhaust valve continues to close. The exhaust gases traveling out the exhaust port create a suction that helps to draw in the intake charge. This phenomenon is commonly referred to as “scavenging”. When to open the valve is critical because it will determine how much the valve is open when the piston is at maximum velocity on the intake stroke; thus increasing volumetric efficiency (VE).

6: As the piston reaches TDC, both the intake and the exhaust valves are open. The period of time between #5 and #7 is commonly referred to as the overlap period. On low rpm engines the overlap period lasts around 20o-30o. On high rpm race engines overlap may be as long a 50o - 100o.
This much overlap causes the engine to run rough, and the intake charge to go right out the exhaust ports at low speeds.

7: As the piston is moving downward, the exhaust valve closes shut. The later the valve is closed may help with high rpm performance, but will result in poor low rpm operation and emissions.

8: The intake valve reaches its maximum opening or peak lift. This is the intake centerline position, or rather how many degrees peak lift occurs after top dead center (ATDC). It is important for the centerline to be near peak piston velocity on the intake stroke in order to optimize cylinder filling.

9: The piston reaches BDC and begins to travel upward. Notice that the intake valve is still open. Even though the piston is pushing upwards, the inertia generated by the speed and mass of the air/fuel causes the mixture to continue to rush in and fill the cylinder. This phenomenon is called
a “supercharging” effect and is the reason why some naturally aspirated engines can even fill the cylinder up to 130% of its volume.

10: The intake valve closes shut before the piston reaches maximum velocity on the compression stroke. When the intake valve is closed ultimately determines the optimum operating rpm range and also the dynamic compression ratio of the engine. Closing the valve early results in good low
rpm operation, but limits power output and rpm. Early valve closing also results in higher cylinder pressures and increased pumping losses during the compression stroke.

11: Before the piston reaches TDC the spark plug ignites the compressed charge. The higher the rpm, the earlier the ignition must begin. More efficient engines do not require as much timing advance. After the piston reaches TDC, the combustion pressure pushes down on the piston beginning the power stroke once again. Back to 1.