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John L

Cam Lift and Valve lift

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Would some kind fellow please explain the above please, so I fully understand.

I understood that camshaft lift is the point that the follower just starts to rise. probably using a DTI.

I understood that valve lift is the point that the valve actually starts to open, with the std vale clearance applied, again with a DTI on the valve spring cap.

My camshaft provider says they are same, but surely this cannot be correct, unless there are different crankshaft degrees for the two surely, please put me out of my confusion, or have I got it wrong?

So if I have a camshaft that says inlet opens at 35 degrees before TDC, do I take it to be cam lift or valve lift, surely it must be stated with the cam spec?

Please put me straight...….

John

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I have always assumed that inlet opening at say 35 deg BTDC would mean when the follower starts to move and yes strictly speaking you have to pick up the valve clearance before the valve actually starts to open.

You set the camshaft timing from the max lilft position so the valve clearance doesn't matter.

The crank completes two revolutions for every one that the camshaft makes.

Rgds Ian

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The difference between cam lift and valve lift is the rocker ratio, the distance from rocker shaft centre to valve tip, divided by distance from rocker shaft centre to push rod.

Most Triumph rockers have a ratio of 1.15:1 if memory serves, so a cam lift of 0.300" for example, results in valve lift of 0.300 x 1.15 = 0.345".

Nigel

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Don't forget valve clearance !!

Camshaft suppliers provide 4 numbers:

Camlift

Valvelift

Valveclearance

Rocker ratio

Example Kent Cam 280°:

Camlift: .293", Valvelift .423", Clearance .022" @ RockerRatio 1.52

.293 * 1.52 = .445" - .022" = .423"

People often forget to check their Rocker Ratio. Original Rocker Ratio is 1.46 and 1.5. Kent specifies their datasheet with 1.52 and Newman 1.5.

Same goes with timing. Reducing or increasing Valve clearance, you can change your timing. Changing VC from .022 to .020 you increase your duration from 280 to 288°.

Jochem

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On 11/7/2019 at 4:12 AM, JochemsTR said:

Don't forget valve clearance !!

Camshaft suppliers provide 4 numbers:

Camlift

Valvelift

Valveclearance

Rocker ratio

Example Kent Cam 280°:

Camlift: .293", Valvelift .423", Clearance .022" @ RockerRatio 1.52

.293 * 1.52 = .445" - .022" = .423"

People often forget to check their Rocker Ratio. Original Rocker Ratio is 1.46 and 1.5. Kent specifies their datasheet with 1.52 and Newman 1.5.

Same goes with timing. Reducing or increasing Valve clearance, you can change your timing. Changing VC from .022 to .020 you increase your duration from 280 to 288°.

Jochem

Very clear explanation, thank you

I was told cam grinders/ designers arrive at a valve clearance because they add a quietening ramp to the cam profile equal to the vc. This ensures the slower rise to take up the vc before proper and faster lift starts.  
Peter W

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This was the subject of a longbthread on Sideways recently.   http://sideways-technologies.co.uk/forums/index.php?/topic/8106-how-to-measure-a-cam-in-the-hand/&tab=comments#comment-108100.  

There the added factor was introduced in that it is a convention (not always used, I fear) that the onset of lift, and fall,  is measured at 0.05" above the base circle.

John

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