I've characterized the dynamic properties of the early BB SHP mechanical lifter cam from the lift data on drawing no. 3863144. The camshaft service part number is 3863143 because the camshaft is an "assembly" consisting of the finished camshaft and locating dowel pin. Other Chevrolet cams follow this same pattern, which is why the number on the cams is different than the part numbers listed in the catalog.
This cam is the "early" version with the groove in the rear journal. I believe it was used through 1966, then replaced with a different part number beginning in 1967 that had no groove, but I need confirmation on the model year change. To the best of my knowledge, the later cam without the rear journal groove has the same lobe.
Lobe dimension data and dynamic behavior data indicate that the top of the clearance ramp is .012" above the base circle, so this times rocker ratio would indicate "ideal" running clearance. Chevrolet appears to specify clearance based on the specified "theoretical" rocker ratio, without taking actual ratio behavior into account.
.012" x 1.7 = .0204"
Actual ideal running clearance should be tighter if real rocker ratio behavior is taken into account. Based on SB rocker ratio behavior - 1.37:1 at the lash point and 1.44:1 at peak lift with an approximate 0.3" lobe height, I estimate BB rocker behavior as 1.55:1 at the lash point and 1.63:1 at peak lift. A 1.55:1 lash point ratio implies an ideal running clearance of 1.55 x .012" = .0186".
Clearance for L-78 is specified as .020"/.024", but was opened up to .024"/.028" for L-72. The Federal Mogul clone of this cam recommends .020"/.024". Since both the inlet and exhaust lobes are identical including the opening and closing clearance ramps, equal clearance is implied as is the case with the 30-30 SB cam. I can only speculate that larger exhaust valve clearance was specified to allow for valve stem growth during hard running, which was probably an engineering judgement call, since measuring valve stem growth on a hard running engine is not easy to do.
As I mentioned before, this same lobe is used on the inlet side of the LT-1 cam, indexed two degrees later and on a nominal .599" base circle versus 0.649" for the 143 cam.
I still need to understand BB rocker ratio behavior from the lash point to peak lift, but since I don't have a BB to measure I request assistance from anyone who can perform the task. You need two dial indicators. Set one on the rocker arm in line with the pushrod and the other on the retainer parallel to the valve stem. Set the lash at about .015" then rotate the engine through the lift cycle and record the pairs of indicator readings. The ratio of the two indicator readings is the actual rocker ratio at that point in the cycle, and it's most important to get good data near the lash point. Make several passes and look for consistency, which will ensure that the readings are accurate. There's no need to set up a degree wheel and record crank angle as part of this test.
Duke
This cam is the "early" version with the groove in the rear journal. I believe it was used through 1966, then replaced with a different part number beginning in 1967 that had no groove, but I need confirmation on the model year change. To the best of my knowledge, the later cam without the rear journal groove has the same lobe.
Lobe dimension data and dynamic behavior data indicate that the top of the clearance ramp is .012" above the base circle, so this times rocker ratio would indicate "ideal" running clearance. Chevrolet appears to specify clearance based on the specified "theoretical" rocker ratio, without taking actual ratio behavior into account.
.012" x 1.7 = .0204"
Actual ideal running clearance should be tighter if real rocker ratio behavior is taken into account. Based on SB rocker ratio behavior - 1.37:1 at the lash point and 1.44:1 at peak lift with an approximate 0.3" lobe height, I estimate BB rocker behavior as 1.55:1 at the lash point and 1.63:1 at peak lift. A 1.55:1 lash point ratio implies an ideal running clearance of 1.55 x .012" = .0186".
Clearance for L-78 is specified as .020"/.024", but was opened up to .024"/.028" for L-72. The Federal Mogul clone of this cam recommends .020"/.024". Since both the inlet and exhaust lobes are identical including the opening and closing clearance ramps, equal clearance is implied as is the case with the 30-30 SB cam. I can only speculate that larger exhaust valve clearance was specified to allow for valve stem growth during hard running, which was probably an engineering judgement call, since measuring valve stem growth on a hard running engine is not easy to do.
As I mentioned before, this same lobe is used on the inlet side of the LT-1 cam, indexed two degrees later and on a nominal .599" base circle versus 0.649" for the 143 cam.
I still need to understand BB rocker ratio behavior from the lash point to peak lift, but since I don't have a BB to measure I request assistance from anyone who can perform the task. You need two dial indicators. Set one on the rocker arm in line with the pushrod and the other on the retainer parallel to the valve stem. Set the lash at about .015" then rotate the engine through the lift cycle and record the pairs of indicator readings. The ratio of the two indicator readings is the actual rocker ratio at that point in the cycle, and it's most important to get good data near the lash point. Make several passes and look for consistency, which will ensure that the readings are accurate. There's no need to set up a degree wheel and record crank angle as part of this test.
Duke