Engine and cam numbers

[LittleBear]

Just to help explain a little about cam numbers and piston location related to these numbers. I am not going into how this affects performance; there are far better experts on that subject than me.

Lets take a Andrews TW-37G.
Here are the numbers
Intake: 18/38 236 .510
Exhaust: 46/14 240 .510

Warning this may make your head hurt.

First some basics: (I am dealing with Doofs after all)
TDC = TOP Dead Center, the point at which the piston reaches its highest point in the cylinder.
BDC = Bottom Dead Center, the point at which the piston is at its lowest point in the cylinder.
The crank shaft rotates 360 degrees in one complete stroke TDC to TDC.
There is 180 degrees between TDC and BDC.
The cams rotate once for every 2 revolutions of the crank on a big twin motor.


The Cam numbers for this cam mean:

Intake: The valve is open 0.053 inches at 18 degrees before TDC and is 0.053 inches from closing at 38 degrees after BDC. Total duration is 236 degrees. Lift is .510 inches at the valve.

Exhaust: The valve is open 0.053 inches at 46 degrees before BDC and is 0.053 inches from closing at 14 degrees after TDC. Total duration is 240 degrees. Lift is .510 inches at the valve.

Duration: Measures how many degrees of crank shaft rotation the valve stays open. These duration numbers are measured from the time the valve is 0.053 inches open until it is back to 0.053 inches from closing completely.
To calculate the intake duration; add the two intake numbers plus 180 degrees, in this case, 18+38+180=236.
To calculate the exhaust duration; add the two exhaust numbers plus 180 degrees, in this case, 46+14+180=240.

In the picture below, for the 37G cam, the actual measured duration angle at the lobe would measure 120 degrees since the cam rotates at 1/2 the speed of the crankshaft.


Lift: This is the total distance the valve is away from the valve seat when it is fully open, at the peak of the cam lobe. It takes into account the standard Rocker Arm ratio of 1.6. Total lifer movement is the lift, in this case .510, divided by the rocker arm ratio of 1.6 or .319 inches. That is the actual height the cam lobe raises the lifter. Increasing the rocker arm ratio increases the lift. If your rocker arm ratio was 1.65, this cam would give you a valve lift of .526 inches.

Not shown but an often calculated value is the Overlap. Overlap is the number of degrees of crank shaft rotation when both valves are open at the conclusion of the exhaust stroke and the beginning of the intake stroke. For this cam, the intake opens at 18 degrees before TDC while the exhaust closes at 14 degrees after TDC. The Overlap for this cam is 18+14=32 degrees.

Now for some more math to make the head hurt even more. You Doofs can add right.

Let’s start at TDC on the Compression or Power Stroke.

Both valves are closed, the spark plug fired several degrees ago and all hell is breaking loose in the cylinder as the air fuel mixture burns.

The piston is forced down by the violent explosion you caused and after 134 degrees of rotation (46 degrees before BDC, 180-46=134) the exhaust valve is beginning to open.

The piston hits BDC at 180 degrees of crankshaft rotation, end of stoke 1, and begins it upward travel to force more of the burn fuel out the exhaust valve.

The piston travels up and the crankshaft rotates another 162 degrees (18 degrees before TDC, 180-18=162) and the intake valve begins to open. 342 degrees of total rotation

The Piston hits the apex of its upward travel at 360 degrees of crankshaft rotation and stroke 2 is over. At this point, both valves are open.

The crankshaft rotates another 14 degrees and the exhaust valve is almost closed. This is an easy one; the exhaust valve is almost closed at 14 degrees after TDC.
Total duration of the exhaust valve was 46 degrees before BDC, plus 180 degrees from BDC to TDC plus 14 degrees after TDC which equals 240 degrees.
This also was the valve Overlap time. The intake opened 18 degrees before TDC and the Exhaust closed 14 degrees after TDC, for a total of 32 degrees of crankshaft rotation while both valves were open.

You still with me, we are half way there, 2 strokes down 2 to go.

We are now on the intake stroke. The intake valve is opening all the way as the piston travels down into its hole. After another 166 degrees of crankshaft rotation, (180 minus the 14 degrees we already traveled down) the Piston hits BDC. End of Stroke 3. Total crankshaft rotation up to this point 540 degrees.

The Crankshaft Rotates another 38 degrees, the piston moves up in the cylinder and the intake valve is almost closed. (38 degrees after BDC). Total rotation, 578 degrees.

The piston continues up in the cylinder, the crankshaft rotates another 142 degrees and the Piston reaches the top of the Compression stoke again. End of stroke 4 with a total crankshaft rotation 720 degrees.

Thus the term 4 stroke motor, and what cam numbers mean and where the valves open and close in the process. That is also why the crank shaft gear, the pinion gear as it is called, is 1/2 the size of the cam gears. The cams rotate once for every 2 revolutions of the crank on a big twin motor.

Hope this was not too remedial for everyone.