This week's assignment: SHP Big Block Valve Lash

[Duke W.]

The following is the L-78/72/71 lobe data at and near the top of the clearance ramp - opening, first group of data; closing, second group of data. The angle is cam degrees measured from the point of maximum lift. Inlet and exhaust lobes are IDENTICAL, including clearance ramps! Based on this data, what does everybody think the lash should be set at?

Angle..Lift..Velocity...Acc.x 10..Jerk x 100

67 0.02527 0.002480 0.003200 0.000125
68 0.02295 0.002160 0.003175 0.000625
69 0.02095 0.001845 0.003075 0.001500
70 0.01926 0.001545 0.002875 0.002625
71 0.01786 0.001270 0.002550 0.003875
72 0.01672 0.001035 0.002100 0.004625
73 0.01579 0.000850 0.001625 0.004375
74 0.01502 0.000710 0.001225 0.003625
75 0.01437 0.000605 0.000900 0.003000
76 0.01381 0.000530 0.000625 0.002375
77 0.01331 0.000480 0.000425 0.001625
78 0.01285 0.000445 0.000300 0.001125
79 0.01242 0.000420 0.000200 0.001000
80 0.01201 0.000405 0.000100 0.000875
81 0.01161 0.000400 0.000025 0.000500
82 0.01121 0.000400 0.000000 0.000125
83 0.01081 0.000400 0.000000 0.000000
84 0.01041 0.000400 0.000000 0.000000
85 0.01001 0.000400 0.000000 0.000000

70 0.02440 -0.001720 0.002200 -0.001875
71 0.02279 -0.001510 0.002000 -0.002000
72 0.02138 -0.001320 0.001800 -0.002000
73 0.02015 -0.001150 0.001600 -0.002125
74 0.01908 -0.001000 0.001375 -0.002375
75 0.01815 -0.000875 0.001125 -0.002375
76 0.01733 -0.000775 0.000900 -0.002125
77 0.01660 -0.000695 0.000700 -0.001875
78 0.01594 -0.000635 0.000525 -0.001375
79 0.01533 -0.000590 0.000425 -0.000750
80 0.01476 -0.000550 0.000375 -0.000500
81 0.01423 -0.000515 0.000325 -0.000375
82 0.01373 -0.000485 0.000300 -0.000250
83 0.01326 -0.000455 0.000275 -0.000375
84 0.01282 -0.000430 0.000225 -0.000375
85 0.01240 -0.000410 0.000200 -0.000250
86 0.01200 -0.000390 0.000175 -0.000375
87 0.01162 -0.000375 0.000125 -0.000375
88 0.01125 -0.000365 0.000100 -0.000125
89 0.01089 -0.000355 0.000100 -0.000125
90 0.01054 -0.000345 0.000075 -0.000375
91 0.01020 -0.000340 0.000025 -0.000375
92 0.00986 -0.000340 0.000000 -0.000125
93 0.00952 -0.000340 0.000000 0.000000
94 0.00918 -0.000340 0.000000 0.000000
95 0.00884 -0.000340 0.000000 0.000000

Duke

[Donald O.]

Re: This week's assignment: SHP Big Block Valve La

My best guess is 0.020/0.024" cold.

Don

[Michael H.]

Re: This week's assignment: SHP Big Block Valve La

I have a question about the print that you are using but first my wild guess at the answer to your question. I see a dramatic change in lift/degree at roughly 68-69 deg on opening and roughly 71-72 deg on closing so I would have to guess that this is the point of zero lash and initial valve movement at .020 and .024 lash.

One of the 1965 Corvette News magazines had an article on the new 396 engine and gave all the info, including cam specs. I don't remember the exact numbers but I do remember that intake and exhause were identical, as you mentioned. However, in one of the 1966 Corvette News issues and many other GM documents that came out in 1966, the numbers were quite different and the intake/exhaust no longer shared the same lift/duration. Again, I don't remember the exact numbers but it was a substantial difference so I have to wonder if the information you have may have been the original for a cam that was never used. I'll look for the info but I don't think it's here in FL. If anyone has these issues of Corvette News or possibly a late 80's copy of the Chevrolet Power Manual, the specs for the 143 cam would be in it. I've always wondered why these numbers changed between these years.

Michael

[Joe R.]

Re: This week's assignment: SHP Big Block Valve La

Duke & Michael,

Yep, I have the copy of Chevy Power and the 143 is listed as 0.024 on the Intake and 0.028 on the Exhaust.

Nuf Said!

JR

[Michael H.]

Re: This week's assignment: SHP Big Block Valve La

Thanks Joe. Does it also list the intake/exh figures for lift/duration and give the numbers for open/close ect?

[Joe R.]

Re: This week's assignment: SHP Big Block Valve La

Michael,

Yep, You want me to do some typing don't you?

Intake

Open 44 BTC
Close 92 ABC
Duration 316
Lift .500

Exhaust

Open 86 BBC
Close 36 ATC
Duration 302
Life .500

Overlap 80

That's all folks,

JR

[Michael H.]

Re: Enhanced typing skills

Thanks again Joe. Think how much your typing skills have improved just by listing those cam specs. Hey, I've been using TWO fingers for several years now, up from one that I started with.

Looks like quite a difference between intake/exhaust. I'm pretty sure the original cam design was the one that used equal int/exh figures but I also wonder if it could have had something to do with the difference in lash. Using the .024/.028 instead of .020/.024 would shorten up the duration but I don't see how it could do it unequally. Sure hope someone has those Corvette News issues so we can compare.

[Clem Z.]

Re: Enhanced typing skills

my orignal GM specs for the 65 396 and the 66 427 are different. 1965 lash .012 and .018,IO 61,IC 107,lift .4970,EO 102, EC 66, lift .503. 1966 lash .024 and .028,IO 54, IC 102, lift.5197, EO 102, EC 54, lift .5197

[Michael H.]

Re: Enhanced typing skills

Clem,

Interesting numbers. The 66 numbers sound more like the numbers that were in the 65 Corvette News. Equal int/exh lift and duration, like the numbers that Duke has but I have to wonder if the checking clearance has something to do with this being different than the later 143 cam. Problem is, GM seldom mentioned the method that they used to obtaine these figures and when they did, the checking clearance was often different from one GM test to another. I noticed the total lift was almost exactly .020 higher than the later 143 cam specs listed in 67.

I have no idea what the numbers mean for the 65 that you listed. Sounds like small block lash settings and the lift/duration are very strange. I have to wonder just how many different sets of numbers are floating around for these early cams.

[Duke W.]

More background info

The lobe lift data is from the latest revision of GM drawing 3863144. Trust me, THIS IS THE LOBE as designed. The 3863143 that this cam was available under through service parts is an ASSEMBLY that consists of the finished cam and dowel pin.

Consider the purpose of the clearance ramps. Actual running lash varies somewhat with operating conditions, and we want this taken up as gently as possible on opening and then seated as gently as possible. Note that acceleration and jerk begin before actual velocity increases, which is expected, but velocity, accleration, and jerk are computed from the data one degree earlier and one degree later. so it is an average. between the actual values at the two lift data points.

Rather than getting into an argument of what Chevrolet publication said what valve clearance, I am hoping to draw on the knowledge and analytical skills of the audience to determine what the valve lash SHOULD BE. Forget what Chevrolet says. For the purpose of this discussion, it's not relevent, and would be just fine if you didn't know so there is no built in bias. I'm looking for some good engineering opinions.

Consider the highest lift that lash should be taken up to protect the valve train from excessive shock load on both opening and closing. In order to determine this you must know the actual rocker ratio at the lash point. Trust me, it's not 1.7:1, just like the SB is not 1.5:1. The SB ratio starts at about 1.37:1 at the lash point and peaks at about 1.44:1 at max lift with an approximate 0.3" lobe. BB rockers probably exhibit similar behavior, so I estimate them at 1.55:1 at the lash point and 1.63:1 at max lift.

Until someone measures the rocker ratio behavior, the best you can do is determine the lift or range of lift above the base circle where lash should be taken up. Then someday we may have a rocker ratio value to determine actual setting clearance. I measured the SB rocker ratio behavior back in the seventies and have been seeking BB data ever since. I can't believe that no one has ever done this, but...whatever!

Sorry about the data spacing. I copied and pasted from my Excel sheet that I typed in the lift data to determine the other dynamic properties, and the DB script doesn't maintain original spacing.

Duke

[Michael H.]

Re: More background info

Duke,

I think you're correct on the published numbers issue. I've been doing a little math here and I can nearly duplicate either set of published duration numbers just by using different "checking clearance" figures. GM typically used .020 but not always so we have no idea what they used in different publications. I'll do a little research on this tomorrow.

As far as efficiency, I've tried the 65 and 66 settings years ago and although the car sounded nasty, it was never fast. There was a dramatic drop in power. I don't remember ever using the .020/.024 on any dyno runs so I have no info but it will be interesting to hear from any one that does.

[Clem Z.]

duke the cam specs for the 65 396 i posted

are listed as cam # 3863144

[Everett Ogilvie]

Re: This week's assignment: SHP Big Block Valve La

If I am thinking about this correctly, I say 0.018. This assumes identical intake and exhaust lobes, but has no factor for increasing the exhaust clearance as is typically done. Am I even close?

[Gene M.]

At what pressure angle?

Duke,
I really do not follow the method of data collection. But I can offer this, the "reading" of the cam lobe profile will be at different pressure angle points if read off a dial indicator point vs the flat surface of a lifter sliding in the lifter bore. Kinda like the duration increase achieved with a mushroom lifter. Not going thru all the data this may account for small differences in ramp opening and closing rates.
All this would have nothing to do with tappet clearance as that occurs at the cam base circle, unless the cam had excessive runout.

[Duke W.]

Re: At what pressure angle?

The heading above the data table on the drawing reads: "Cam contour based on flat tappet face".

You are correct that a dial indicator directly against the lobe will yield a different data set, especially in the mid range of lift where the cam/tappet contact point moves off the center of the tappet, but this data is actual tappet rise, so it represents true motion of the tappet in the engine.

Duke