Mystery CAM - what can you tell me about this lift and duration Options

[Aaron Cox]

friend here is buying another friends motor...

we went over and measure lift and duration with a dial indicator and a degree wheel...

cam had overlap (visible - did not measure) which makes me positive its a carb cam...

anyway... the specs (lift is +/- 3 tho)
___________ EXHAUST___INTAKE
LIFT (valve) .410" ___ .422"
Duration 165 deg 175 deg

duration... we measured when it opened, and measured when it closed.... we got big numbers so we divided by 2 to get what we have above... not sure those are correct... but those numbers also coincide to half lift....


so raby/brett/clayton others... what can you tell me about this cam? does it fit a WEB/engle profile?

motor was rebuilt many years ago... its a 1911.. and the cam definitely had visible overlap (both valves open for a few degrees, one closing, one opening)

[SLITS]

1.) Was valve lift measured at the valve or lifter?

2.) Generally, duration is reported at 0.050" lifter rise and numbers are given in degrees before/after top dead center or bottom dead center.

3.) The other parameter that is missing is the lobe centers.

So, there really isn't a whole lot that can be said.

[Aaron Cox]

yes. measured lift at the valve.... (no ratio calcs to do that way)

[Brett W]

Something is not right with your numbers. When you checked the duration with a degree wheel, how did you do it?

You need to make a timing pointer that is attached to the block. You also need a piston stop or you need a dial indicator that will reach down in the sparkplug hole and tell you when you have exactly reached TDC, so you can set your pointer right. ONce you do that measure from .050 lift. When the valve reaches .050 lift tell me the reading on the degree wheel. Continue to rotate the engine over to where the valve stops .050 from the seat. Then read this number.

Based on that info I can give you a better idea of what you have. Might be a Web 91 or 86 series, although it might be a Elgin, Isky, Fat, LogiCam, etc. If Had some more numbers I could look through my records.

[Aaron Cox]

ahh.. i KNEW we screwed up the duration...

we measured with a dial indicator... the degree the valve opened... and the degree the valve closed..... had a pointer and all...

so its the degrees from opening at .050 till it reaches .050 when it closes?

[swood]

You guys won't be able to figure it out without the chopstick method.

[Brett W]

You need to set the engine to TDC with your DI. Once you have that. Set your degree wheel to 0. Now rotate the engine over and measure the lift from the valve retainer. Add both of the numbers together and add to 180. Let me know what you have.

[KaptKaos]

You guys won't be able to figure it out without the chopstick method.

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[Aaron Cox]

cool!

HW assignment for next weekend...

The camshaft which opens and closes the valves makes one complete revolution (360 degrees) while the crankshaft rotates TWICE (720 degrees). Camshaft timing is usually expressed in terms of crankshaft degrees relative to the piston location in the cylinder; Top Dead Center (TDC) & Bottom Dead Center (BDC).

Valve Overlap

The number of degrees in cam shaft rotation that the intake & exhaust valve are open at the same time (The point in which the intake valve opening point BTDC and the Exhaust Cam's closing point ATDC). Increasing the number of degrees of overlap tends to move the power band up the RPM band but you will sacrifice low end power. Increasing the overlap can increase peak power, but only if the exhaust system is efficient enough to properly scavenge the cylinder. Decreasing the overlap tends to boost lower rpm performance.

Lobe Centers

This is the angle between the intake and exhaust camshaft lobe peaks described in camshaft degrees. This angle dictates two important events: the valve overlap around TDC, and intake or exhaust valve closure delay there is in the relevant stroke (inlet/exhaust). Tightening the lobe center angle produces more overlap around TDC and wider angles mean less overlap.

Valve Lift

Valve lift is the distance that the valve travels from the seat of the head to its farthest point. As the engine speed increases the need to increase valve lift is necessary to keep the inlet speeds from exceeding the Mach Index value of .6. This is the point which volumetric efficiency falls off.


Cam Duration

Duration is the period of time, measured in degrees of crankshaft rotation, that a valve is open. Duration (at .050-inch lifter rise) is the deciding factor [in] what the engine's basic rpm range will be. Lower duration cams produce the power in the lower rpm range & larger-duration cams operate at higher rpm, but you will lose low end power to gain top-end power as the duration is increased. For each 10-degree change in the duration at .050 inch, the powerband moves up or down in rpm range by approximately 500 rpm."