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Is there a general rule-of-thumb as to what the hot lash clearance will be, based on measuring only the cold ? Generally is cold measure less than the hot value ? Or will some be less, some more, based on location in the block ?
Engine in this case is '65 L76 with stock 30-30 camshaft. [0.030-0.030" ?]
On a cast iron engine (block and heads)it doesn't change significantly, except when the engine is run very hard. The exhaust will close up, but you can't measure this at idle because the exhaust valve runs cooler at idle than at WOT. This is the reason most mechanical lifter cams have greater specified lash for the exhaust.
I recommend 30-30 clearance of .025"/.025" cold, and it must be set using a specific procedure I developed to ensure the cam is on the base circle for each valve.
E-mail me if you want the Word document that has the procedure and all the whys and wherefores.
It's interesting that all (or at least most) of the service literature lists valve clearances of .030"/.030", but the GM drawing for the 30-30 cam (which I now have) lists .025"/.025".
I know, Clem, you have stated previously that some GM publications, early on, listed .025"/.025".
I am in the process of "reverse engineering" the Duntov, 30-30, LT-1, and L-72 cams. In particular, having accurate lift, velocity, accleration, and jerk data allows one to pick off the top of the clearance ramps very accurately. Then clearance is determined by applying the actual rocker ratio at the lash point (1.37:1 for SBs, unknown for BBs) I just completed the analysis table using Excel for the LT-1 cam last night, and should have the rest done this week.
The GM drawings for the 30-30, LT-1, and L-72 cams include tables that specify tappet lift (to five decimal places) every cam degree from the point of maximum lift to the base circle for both flanks (all but the Duntov have asymmetrical lobes). The data can be laid out on a spread sheet to determine velocity, acceleration, and jerk.
The Duntov cam data is presented as actual manufacturing data with cutter advance specified rather than tappet lift. This makes the Duntov cam data somewhat different because cutter advance does not necessarily represent actual tappet lift depending on cutter geometry.
The one insight I've already gained is that the 30-30, LT-1 and L-72 cams have lobes in common. The 30-30 and L-72 both use a specific and identical lobe on both inlet and exhaust. The LT-1 inlet lobe is common to the L-72, and its exhaust lobe is common to the 30-30, but all three cams have unique lobe phasing.
The Duntov inlet and exhaust lobes are identical above the clearance ramps. The exhaust lobe just has a .004" higher clearance ramp to allow for more exhaust valve stem expansion during hard running.
I've also tried mixing and matching the lobes and phasing with the Engine Analyser simulation program in an attempt to find better valve timing for a SHP SB with massaged heads and have come up empty. Those Chevy engineeers back in the late sixties knew their stuff. They didn't have the analysis and simulation tools I have on my PC, but they sure had excellent intuitive insight into valve train dynamics and timing for optimum torque bandwidth!
Duke ..
Could at least some of those Chevy engineers have cut their teeth with respect to camshaft design, valve train dynamics, etc. during World War 2 - the height of piston engine development with respect to aircraft powerplants. .. ? .. Seems that was one incredible period for technological leaps and bounds! ..
John
i think that GM may have increased the lash in production to get more bottom end out of the engine for street driving. if my memory does not fail me the 097 cam also had 2 different valve lash listings.
Yes, insight was gained into valvetrain dynamics in the WWII era when the MIT Sloan labs first used high speed motion picture photography to view valve train behavior.
GM developed the Optron system in the early sixties, which used an optical sensor on the valve to create a signal that was displayed on an oscilloscope. The first anamoly to show up was valve bounce on closing, and I believe this is why the 30-30 cam has an asymmetrical lobe. The L-72 lobe has greater asymmetry.
Nowadays the OEMs have very sophisticated valve train dynamics analysis programs, so they are able to push acceleration and jerk profiles to the limit throughout the valve event without going over the edge at any point.
It's interesting that most of the commonly known "hot rod tricks" were developed during the thirtes and forties as a great deal of research, much of it government funded, went into aircraft engine development.
Items such as "high performance" valve head and valve bowl geometry and rotated H-beam connecting rods were all developments of that period. Eventually the hot rod industry caught on, espeially those who read textbooks and technical papers.
The Duntov cam ramps heights above the base circle are .008" on the inlet side and .012" on the exhaust side. Other than the ramps, lobe eccentricity is the same on both sides.
It's commom for cams to be designed with taller exhaust ramps on the exhaust side to give more room for stem growth when the engine is run hard, so there is no chance that lash will be completely taken up and hang the valve open, which would cause the valve to burn in a matter of seconds.
i was not referring to the difference between intake and exhaust but .008 and .012 was one listing for that cam and there was also a different listing of .012 and .018 i believe.
I remember that also. I think it was printed in the 62 shop/service manual?? It listed the valve lash adj for each 57-62 engine and it seemed to bounce between .008/.018 and .012/.018 from one year to the next. I've always been curious about the different specs for the same cam. I've also seen both of these settings listed for 63.
Compensating for a Cold Engine when Adjusting Valve Lash
When installing a new cam, the engine will be cold but the lash specifications are for a hot engine. What are you to do? There is a correction factor that can be used to get close. We mentioned that the alloy of the engine parts can be affected by thermal expansion in different ways, therefore the amount of correction factor to the lash setting depends on whether the cylinder heads and block are made out of cast iron or aluminum.
You can take the "hot" setting given to you in the catalog or cam specification card and alter it by the following amount to get a "cold" lash setting.
With iron block and iron heads, add .002"
With iron block and aluminum heads, subtract .006".
With both aluminum block and heads, subtract .012".
Remember this correction adjustment is approximate and is only meant to get you close for the initial start up of the engine. After the engine is warmed up to its proper operating temperature range, you must go back and reset all the valves to the proper "hot" valve lash settings.
I agree with your statement. I've always added roughly .002 int and .003 exh to cold settings to compensate for heat expansion. (iron/iron) If I'm using a new combination or in a different car/engine, I usually run the engine until normal op temp is reached, then set at least one or two cylinder pairs of valves to specs. Then when the engine is back to cold a few hours later, I measure the difference in lash which tells me roughly the difference between hot and cold setting and then I can set all of the valves cold using the recalculated spec.
Another problem that a lot of guys have is that they don't understand the meaning of HOT engine compared to COLD. Running the engine for five or even ten minutes until the coolant temp is 180 is not near long enough. It's the OIL temp that tells us when an engine is at operating temp. During this warm up period, the valve lash will actually go up and down as different parts of the engine expand faster than others. Once the oil temp stabilizes, usually more like 15 to 20 minutes, the engine is considered to be at operating temp and everything inside has reached it's normal operating temp. The .002 and .003 works best for my big block while .001 and .002 seems to be accurate for a small block.
I've set 30-30 cams at less than .030/.030 hot and cold and they don't seem to like those numbers at all. The best was always at specs., .030 hot and running.
The inlet lash for '63 was tightened to .008" in order to get a little more effective inlet duration since the Duntov cam is relatively milder in the longer stroke 327 than the 283. I don't know why they waited until '63 to change the spec.
Both the '63 owner's manual and '63 Corvette Shop Manual specify the inlet valve clearance at .008".
I have also seen the .008" inlet lash listed in a fifties vintage Corvette News as an optional setting for "weekend performance events".
My recommended "rocker ratio adjusted" lash settings for the Duntov cam are .010"/.016", with .008" inlet recommended for 327s.
In several attempts to measure a difference between hot and cold lash on my
Duntov-cammed 327, I could never find a discernable difference, and this makes sense since the thermal expansion rates of the steel pushrod and cast iron block and head are very close to the same.
Actual running clearance will vary depending on engine operating condtions. The exhaust clearance, in particular, will likely close up in hard running due to growth of the valve stem because the exhaust valve head runs considerably hotter at sustained WOT than at idle (though the stem temperature remains considerably lower than the valve head). but it's rather difficult to measure valve clearance at 6000 revs WOT.
Mechanical lifter cam clearance ramps are designed to allow a range of running lash, and exhaust ramps are typically higher to allow for valve stem expansion without risk of taking up all the lash and hanging the valve open.
At the other end of the spectrum, if the clearance is too great the valve will be lifted off and set down on the seat at higher than ramp velocity, which shock loads the valve train, increases the rate of valve seat recession, and can cause the valve to bounce off the seat on closing at high revs.
For normal street driving you are better off being a couple of thou tight than loose, and the clearance will tend to increase with service.
When you start looking at the microscopic details of clearance ramp design and actual rocker ratio variation and range, it becomes crystal clear!
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