More Cam info from: Piper Cams - Technical terminology
"Cam Timing: The position of the camshaft relative to the crankshaft. This is expressed as the number of degrees that full lift occurs after top dead centre (tdc) in the case of the inlet, and before tdc for the exhaust. This figure is included in the
catalogue pages, but to calculate this, take the duration figure and divide by 2. EXAMPLE: With an inlet cam of 23/76, the duration is the addition of these two numbers, plus 180, equals 270. Then divide by 2 resulting in 135. Deduct the number of degrees before tdc that the valve started to open, ie 23 degrees - the result 112. The valve is correctly timed with full lift 112 degrees after tdc.
Valve Timing: The opening and closing position of inlet and exhaust valves relative to the crankshaft as figures before and after TDC and BDC
Lobe Angle: The angle between the inlet and exhaust lobe, measure in degrees.
Ramp: The ramp is the part of the profile that takes up the valve clearance and slack in the valve train gradually, before the valve is actually lifted from the seat. It also rests the valve gently back to the seat after the closing flank. Mechanical profiles use a much larger ramp than hydraulic ones, as the hydraulic cam follower should be in contact with the lobe at all times. The height of the ramp dictates what measurement the valve clearances should be set to.
Flank: This is the part of the profile between the ramp and nose. It is the most important part of the whole design. The flank controls the velocity and acceleration of the valve train. The acceleration / deceleration rate must be within the working limits of the valve spring, too much and valve float with occur. Generally high acceleration & velocity figures are beneficial to engine performance.
Nose radius: The larger the nose radius the better. Our profiles are designed to utilise the biggest nose radius possible to keep the stresses to a minimum.
Dwell: As the valve reaches full lift it will stop moving for a few degrees before starting to drop back towards the seat, this period is known as the dwell. When checking the cam timing using the full lift figure method the mid-point of the dwell should be taken as exact full lift.
Rocker Ratio:The ratio between valve motion vs cam follower motion. Push rod engines typically use a ratio of between 1.1:1 & 2.0:1. Over head cam, direct operating engines obviously have no rocker ratio as the cam follower motion is exactly the same as the valve motion.
Overall height: The measurement from the nose of the lobe to the bottom of the base circle, in a straight line through the centre of the lobe.
Base circle diameter: The measurement across the lobe, calculated by measuring the overall height and subtracting the cam lift."
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Piper Cams frames page
Applying the Formulae
Remember, the only good way to judge a cam's duration or timing is at .050 lift - b/c thats where it really starts to matter the rest is ramp-up.
So, now that we understand the guidelines, lets look at a couple examples of the math in action:
STOCK CAM
Cam Lift (Intake): .210"
Cam Lift (Exhaust): .197"
*TR-Mx6 found .230"*: Stock cam lift #'s/rocker ratio/valve lift - MX6.com
*based on .230" exhaust cam lift*
Intake valve lift = 0.374
Exhaust valve Lift = 0.410
So divide one with the other and you get the ratio of the rockers at max lift.
Rocker arm ratio = 1.78
Duration at .050''
Lobe duration (Intake): 197º
Lobe duration (Exhaust): 208º
Duration at .000''
Lobe duration (Intake): 239º
Lobe duration (Exhaust): 247º
Valve timing stock: (.000'' lift)
IN: Open BTDC 10º
IN: Close ABDC 49º
EX: Open BBDC 55º
EX: Close ATDC 12º
Source:
Camshaft Calculations - Lobe Center / Duration
"Your F2 has an Overlap of 22.00 degrees and has in Intake Duration of 239.00 degrees. The Exhaust Duration is 247.00 degrees. The Inlet Cam has an Installed Centerline of 109.50 degrees ATDC. The exhaust cam has an Installed Centerline of 111.50 degrees BTDC" - Crower Cams (thanks to Sick)
Ok, good stuff. Now lets start crunching numbers with the right formula(s).
To make sure that the number are right we will calculate in to ways.
#1 Obvious, look at the pic method.
comes out to 22* overlap at .000 lift.
#2
to find duration at .000 lift - In= 10+180+49=
239* Ex= 55+180+12=
247*
instead of just guessing the lobe seperation we'll find the real thing.
{[(in. dur / 2) - dist BTDC] + [(ex. dur / 2) - dist ATDC]} / 2 = Lobe Separation
{[(239/2) - 10] + [(247/2) - 12} /2 =
110.5* Lobe Separation
ok, now we can find overlap:
239 + 247
---------- - 110.5 = 11º x 2 =
22º of overlap at .000 of lift
4
Now remember that we are supposed to look at cam specs at .050 lift? But we don't have timing numbers for that lift so we need to do it with some math...
Ok so now that we know the math method works, we can find out what our overlap at .050 is. Using:
Duration at .050''
Lobe duration (Intake): 197º
Lobe duration (Exhaust): 208º
197+208
--------- - 110.5 = -9.25 x 2 =
-18.5 overlap at .050 of lift
4
Typically a overlap spread of -8 degrees to +2 is a safe bet. Of course this will differ with whatever combination header, turbo and exhaust is used, so those #'s could be higher or lower.
