Cam to Crank Timing

[Dadandson]

I set the cam to crank timing last night and I would like to double check my method before I button my 1275 up. Bought the cam from Winner's circle and they provided the timing data. Using a dial indicator I set the first piston at TDC. Set the timing wheel at zero with a pointer. I then took the crank all the way around to 9 degrees before TDC. Then I moved the indicator to the top of the push rod of the #1 intake. I rotated the cam, going clockwise for both cam and crank, until the it rose exactly .050". I have a vernier timing chain gear so locking it down was easy. I then triple checked my numbers to allow for the slight chain slack until it was perfect. I would appreciate any input before I button this up as it will be difficult to adjust once the motor is in the car. Thanks in advance.

 

[Trevor Jessie]

Maybe I do not understand what you are saying. Can you explain again how you are finding the top of the cam lobe?

 

[Roger]

If the cam maker says you need 50 thou' lift at 7deg BTDC, you've done it right.

 

[Trevor Jessie]

Oh, I see what you are saying. Most cams that I have done have specified to time on full lift of the cam lobe. So, I find it similar to finding TDC on the piston. .020 over/under and then average.

 

[Dadandson]

I inserted the push rod into the block so it rides on the cam lobe, #1 intake. I set the 1" travel Dial indicator, with about .50" of pressure, into the "bowl" of the push rod and rotated the cam, without rotating the crank, until I measured .050" of lift.

 

[Trevor Jessie]

That is probably close enough for what you are doing, but it really depends on the lobe shape.

 

[Dadandson]

Made a mistake in original post. The WCC 100 cam settings are NINE degrees BTDC, not 7.

 

[dklawson]

The important thing about setting the cam angle is measuring precisely where the max lift AND where TDC are. You cannot simply turn the crank until you see the piston stop moving and call that TDC. The piston will dwell at TDC for a few degrees.

You need to get the piston where it's dwelling at TDC and zero the indicator. Then turn the crank BACK and then slowly toward TDC. Stop at about 0.020" before TDC and record the crank angle using the degree wheel. Keep turning until the the piston is 0.020" below TDC and record that degree wheel angle. Average the two values and then turn the crank until the degree wheel pointer is pointing at that average value. Without moving the crank, turn the degree wheel until the pointer is aimed at zero degrees on the degree wheel.

Likewise, when determining the max lift of the cam, you have to find an approximate MAX lift point and zero the indicator there. Then you move the crank BACK and stop a few thousandths before (and after) the indicator reaches zero... and average the two angles you measure.

I'm sure that's clear as mud.

Finally, the cam grinder will have told you the optimum cam angle. I assume you're fitting a new timing chain. It will stretch a bit during use. It's not uncommon to "compensate" for this stretch by setting the cam 1 to 2 degrees "early" so it runs close to optimum once the chain stretches.

 

[Trevor Jessie]

Good explanation Doug. But realistically how much HP loss would you expect if you were off by a degree or two? I doubt it would be measurable in most of the HP range. However, if you can gain even a fraction of a HP by taking a few extra minutes to get it spot on, then why not (it is free).

On SBC we would often move the cam timing to move the HP peak up or down the rpm range depending on where we needed it.

 

[Dadandson]

Thanks for the reply Doug but, using the method you suggested for the cam would mean that the cam must be Asymetrical. I know the crank is but I am not sure about the cam.

 

[dklawson]

Regardless of whether the cam is symmetrical or not, there will be a period of a few degrees at max lift. During that period your indicator will not move much if at all. If you don't take angular readings rising and falling and average them you won't have any idea where the middle is. If the idea of 0.020" seems to wide (particularly if the cam isn't symmetrical) then use a much smaller number, even if it's only 0.002"

As for the importance of setting the cam angle accurately, I would take the time to get accurate readings for TDC and max lift, and possibly use the additional couple of degrees to compensate for chain stretch. After all you have invested in the vernier timing gears and you expressed concern about meeting cam grinder's spec. If you're not going to accurately measure the angles and set the vernier, you could have used the existing, stock, dot-to-dot alignment.

 

[2091351]

Regardless of whether the cam is symmetrical or not, there will be a period of a few degrees at max lift. During that period your indicator will not move much if at all. If you don't take angular readings rising and falling and average them you won't have any idea where the middle is. If the idea of 0.020" seems to wide (particularly if the cam isn't symmetrical) then use a much smaller number, even if it's only 0.002"

As for the importance of setting the cam angle accurately, I would take the time to get accurate readings for TDC and max lift, and possibly use the additional couple of degrees to compensate for chain stretch. After all you have invested in the vernier timing gears and you expressed concern about meeting cam grinder's spec. If you're not going to accurately measure the angles and set the vernier, you could have used the existing, stock, dot-to-dot alignment.

I agree, this is a VERY important part of the rebuild process. +/- a few degrees here and there is not what you want.

Take care, Trust but Verify!

Steve

 

[Dadandson]

Doug is right. Thanks