The engine in my L 72 was rebuilt about 10 years ago and has about 6,000 miles on it. The replacement cam is the 143 clone made Speed-Pro, part no. CS-165R. I plan to have the valve lash checked/adjusted next week and the Speed-Pro cam card calls for an intake setting of .020 and exhaust setting of .024. My question is should I tell my mechanic to adhere to these settings or is there any performance advantage to deviate from these suggested settings?
-
-
Re: Valve Adjustment for L 72
Both sides use the same lobe and the clearance ramp height above the base circle is .012". This is from my analysis of the lobe data from the engineering drawing. As a point of interest, this lobe is also used on the inlet side of the LT-1 SB cam, but the base circle is slightly smaller.
The correct maximum running clearance is .012" times rocker ratio, so it comes down to the rocker ratio. Like the SB 1.5:1, the BB 1.7:1 is a "theoretical number", and it is not constant throughout the lift event. The SB ratio starts at 1.37 at the lash point and achieves 1.44 at max lift with about a 0.3" lobe.
I have a couple of data sets for BB rocker ratio, but they have too much scatter to be considered conclusive. If BB rockers are proportionally similar to SB rockers the lash point ratio should be 1.55 and 1.63 at max lift, however, the data I do have indicates the numbers could be somewhat higher.
The initial clearance specs for this cam on '65 L-78 were specified as .020/.024", but this was increased to .024/.028" for L-72 in '66. (You sent me the Speed Pro data, John. Doesn't it list .024/.028"?) Now, .012 times 1.7 is .0204, which I expect is how the .020" setting clearance was arrived at, but it uses the "theoretical" rocker ratio without taking into account a probably lesser lash point ratio. I don't know why the clearance specs were increased for '66.
It's noteworthy that despite both lobes being identical, greater exhaust clearance was specified, yet this was not done for the 30-30 SB cam, which also uses the same, identical lobe on both sides.
The larger clearance on the exhaust side could be for the additional exhaust valve expansion when running hard, but that's why mechanical lifter cams have clearance ramps; .020" is the MAXIMUM running clearance that either valve should see in order that the valves are gently unseated on opening and seated on closing at clearance ramp velocity. Greater clearance means the the valve is both unseated and seated at higher than clearance ramp velocity, which adds additional loading to the valve train and valve seat. Clerance ramps should be high enough so that maximum expected running valve clearance still maintains lash, and .012" high clearance ramps should be more than adequate for valve stem length and expected expansion at sustained maximum power.
Perhaps you could have your mechanic measure the rocker ratio behavior.
The valves should be set cold, engine not running, using the methodology I developed for SB cams - inlets at 90 ATC and exhausts at 90 BTC. Adjusting both valves on a cylinder at TDC will result in inaccurate clearances because the lobes are slightly on the clearance ramps at TDC. Using my indexing scheme guarantees that ANY mechanical lifter cam will be on the base circle. The L-72 total lobe duration is about 502 crankshaft degrees including both opening and closing clearance ramps, so out of 720 degrees of crankshaft rotation the lifter is only on the base circle for about 218 degrees. The corresponding numbers for the 30-30 cam are 554/166. You have to have a scheme to ensure that the lifter is on the base circle while setting lash or you will just screw the pooch.
It's a relatively easy procedure once you get the hang of it and can easily done by the DIYer with basic hand tools.
If anyone needs the procedure, e-mail me, and I will sent you the latest revision from late last year. All SB clearance numbers are refined, but I have not yet committed to recommending BB clearances until I can get a reliable set of rocker ratio behavior data.
In the meantime my recommendation is to go with .020/.024, or if you want to take what I consider is a mild risk, .020/.020.
I'm going to take another look at the rocker ratio behavior data sets and put in another post later today.
Duke -
Re: Valve Adjustment for L 72
I reviewed the lash data again and it's still inconclusive, but here are some observations. One data set shows a peak average ratio at maximum lobe lift of 1.62 and the other is 1.68. They should be the same. Both data sets were fairly consistent near peak lift, so there could have been a set up error on one.
Setup is very important as the valve must be lashed to zero, no more - not even a thousanth - or even slight negative lash - or the low lift data, which is critical to picking the lash point ratio will have significant errors.
Base circle and lobe runout of about +/- .001" is allowable, so a max runout lobe will also foul the data at low lifts along the clearance ramps, but most base circles probably only have half the maximum allowable runnout.
My hunch is than if I can ever get consistent data set the rocker ratio behaviour will be proportional to SB rockers, so the inlet lash should be set at 1.55 x .012" = .0186". Use a .018 gage and your lash should be somewhere between .018 and .019. The exhaust side requires some discussion.
The area availalbe for exhaust gas to transfer heat to the valve is a function of valve head area, which increases with the square of valve diameter. On average 80 percent of valve cooling is through the seat and seat area only increases linearly with diameter. Bottom line, a BB exhaust valve on average will run hotter than a SB exhaust valve, which is why Chevrolet may have added extra lash on the exhaust side even though the lobe is identical.
At idle hot and cold lash is about the same - at least according to SB measurements I have done, but under high load the exhaust valve stem will expand and reduce the lash. The stem is cooler than the head, but the exhaust stem gets hotter than the inlet stem and expands more. I don't know why they didn't do what Duntov did with the 097 - just add a longer ramp for more clearance ramp height on the exhaust side. Other than the longer, taller exhaust clearance ramp the Duntov lobes are the same on both sides. At any point above the top of the clearance ramp as measured from the point of max. lift, the Duntov exhaust lobe drawing data is exactly .004" more on the exhaust side than the inlet side.
Also, all should understand that lash varies with operating conditions, and we want it to be more than zero, but not large enough that the valves are opened and closed on the lobe flanks rather than the clearance ramps, which is why the cold setting clearance is the MAXIMUM running lash we want to see, nothwithstanding the four thou. of "Kentucky windage" that the engineers appear to have added on the exhaust side. and sufficiently generous clearance ramps should prevent the exhaust lash from closing up and hanging the valve open. That will burn a valve in nothing flat!
So if I had a SHP BB and wanted to set the lash now in the absense of necessary data to "nail" the answer, I would go with .018" on the inlet side and .020" on the exhaust side.
For aluminum heads, tighten the cold lash settings by .002". The aluminum will expand more than the steel pushrod, so the normal operating temperature clearance at idle and light load will expand about .002" from cold.
DukeComment
-
I almost forgot your question
The short answer is no.
When you adjust your valves you'll likely have .001-.002 variation between valves. Why worry if it runs okay.
A performance DIFFERENCE can be obtained by changing the lash to change the torque curve of the engine and that can be a performance advantage for your particular needs.Comment
Comment