This week's assignment: SHP Big Block Valve Lash

[Duke W.]

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

Congratulations, Everett! Looks like you're the first one to come up with a number, but how did you arrive at .018"? What was your rationale and thought process? Would this be actual setting clearance? If so, what rocker ratio did you use. and at what cam angle would lash be taken up on opening and closing.

Duke

[Everett Ogilvie]

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

I simply took the lift at the point when the acceleration went to zero, and multiplied by the rocker ratio (I used 1.55, as we all know that we don't know the real mean of the rocker ratio - someday I hope to get you your dial indicator measurements). This gave 0.018, and as far as actual, I think one would indeed want to include a factor for more clearance on the exhaust valves (for safety during hard running), but I am not sure what that would be - I suppose you could use 018 for intakes and 022 for exhaust.

[Duke W.]

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

I'm in general agreement with your analysis. If you look at the opening data, the top of the constant velocity ramp appears to be about .012" above the base circle, so .012" times actual rocker ratio at the lash point would be the most lash we would want on a running engine to minimize shock loading and seating the valve at exess velocity, which can cause the seat to pound in.

It's interesting to compare the 30-30 and L-72 cam. BOTH have identical inlet and exhaust lobes, including ramps, yet only for L-72 did Chevrolet call for higher exhaust side lash. Likely this is to allow for some valve stem expansion during hard running, but during normal street driving the exhaust valve stem doesn't run much hotter than the inlet side. Research (Taylor) indicates that about 75 percent of exhaust valve heat transfer occurs through the seat with the remainder through the valve guide. Taylor shows some valve temperature data. At high load the center of the exhaust valve can reach 1400F, but drops rapidly as you move up the stem.

Lobes are typically designed with taller ramps on the exhaust side in consideration that actual running exhaust clearance varies more over the range of operating conditions. This is the case for the Duntov cam. The inlet ramp is .008" high and .012" for the exhaust side. Other than the difference in ramp height both lobes are identical. The reason for the taller exhaust lobe is to allow more room for stem expansion to insure that the stem won't expand enough to keep the valve from seating, which will rapidly burn the valve.

With the 30-30 cam the choice was to use very tall, identical ramps on both sides and specify equal setting clearances. The L-72 cam, designed about a year later used the same philosophy of identical ramps, but specified greater exhaust side clearance. There obviously was a reason for this change in philosophy, and I'd like to know if it was based on testing (such as actually measuring valve stem temps or otherwise determining how much clearance tighted up at high load) or just engineering judgment. Unfortunately we will probably never know the reason.

Duke

[Duke W.]

Re: More background info

"Checking clearances" are specified to check and verify cam indexing and timing, but what I'm trying to determine are "setting clearances", which will ensure that the actual running clearances will be in a proper range over the entire load regime.

BTW. the Speed Pro data sheet for the 143 clone (CS-165R) that John Lolli sent me calls for setting clearances at .020"/.024" (hot), which reverts back to the original GM clearance specified for the L-78 in '65, rather than the .024"/.028" that was specified for '66 and later.

The valve timing diagram on the GM drawing is based on .024"/.028" clearacne, but these do not necessarily mean that these should be setting clearances, and none are specifically specified. Since the drawing was "redrawn" on 10-15-65 as revision 8, all previous data and changes were lost, which would include any reference to previous clerances. Additionally, the clearance data is flagged by change "14" and the change record says "valve timing diagram revised". The month is illegible, but the year was 1966. Authority is listed as "76278", and I'd sure like to see that Engineering Change Order.

Duke

[Michael H.]

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

Duke,

I agree with your reasoning but there's another factor that enters the picture and it's one that no one seems to address. While the valve stem expands with temp and causes the lash to decrease, the valve head diameter expands also and causes the opposite reaction, loosening the lash. The valve head diameter increases and, because of the 45 deg face angle, raises the head and loosens lash. This would be a more immediate change in lash when comparing idle lash to hard pull lash, especially as the engine is going through it's warm up period.

All of this would result in loose settings until the entire engine is at operating temp. This is one of the reasons why drag race engines favor a tighter valve setting but road race engines or street cars that will see continuous operation at proper temp (and above) should be set to recommended specs. Years ago, GM (and Mickey Thomson) did in depth testing on valve lash change during warm up and the graphs that resulted would amaze you. The lash first decreased, then increased for about 10 minutes of operation and finally decreased again as the temp came up. The change was substantial, as much as .004 at times.

If GM changed the published valve lash on the 425 and 435 HP engines to .024/.028, I'm convinced it was either to correct a mistake in the earlier publications or because they realized the earlier settings were incorrect. Everyone that I know has been using .024/.028 for almost the last 40 years.

Michael

[Michael H.]

Re: More background info

That's interesting. Do you suppose it's possible that the GM recommended lash was one of the things that was changed on the drawing dated 10-15-65? I suppose that would answer the question about why the early manuals showed the .020/.024 settings.

For many years, I suspected that the 65-66 143 cam was somehow different than the 67 and later cam (other than the grooved rear journal) because they always sounded different but I now wonder if it was all due to the revised lash setting. 65-66 always sounded nastier than the smooth sound of 67 and later.

[Everett Ogilvie]

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

If I recall correctly, the Power book calls for 020/020 (or 022/022) on the L88 cam (I will verify this tonight). I don't know the ramp heights on this cam, but it is interesting that intake and exhuast are spec'd the same.

[Clem Z.]

the specs i have for the #535 L-88 cam

are lash.020/.020,IO 61, IC 113, lift .562, EO 108, EC 72, lift .584. GM also listed a cam #3904366 for the 375,425 and 435 HP engines. specs are, lash .024/.028,IO 44,IC 92,lift .520, EO 86, EC 36,lift.520. VS the #143 cam specs, lash .024/.028, IO 54, IC 102, lift .5197,EO 102, EC 54, lift .5197 if any one wants a copy of the 30+ cams and specs i have send a e mail

[Duke W.]

Re: More background info

Yes, that's my suspicion - the clearance was changed after the cam was designed and in service. The question remains: Why?

I don't think the '67 cam lobe is different. The only reason the part number changed was because of the elimination of the rear journal groove, which is analagous to the "change" in part number for the '57 Duntov cam versus the '56 version. Same lobe - they just eliminated the rear journal groove.

The P&A catalog lists the 3863143 for all SHP 427 and 454 from '65 to '71 and the drawing clearly shows that the 143 cam assembly has the rear groove. There's nothing in the P&A catalog about the rear journal groove issue, but it's addressed in the Chevrolet Power Manual.

Duke

[Duke W.]

Re: the specs i have for the #535 L-88 cam

Lets not confuse the issue with the L-88 cam. Power manual specs lists clearances of .024/.026, with "checking clearances" of .020/.020. Also rememeber that L-88 has aluminum heads, so that's another variable that will alter running clearances relative to setting clearances.

The drawing has a note as follows: "May be made from 3904366 casting". Isn't this the cam without the rear journal groove?

Timing listed on the drawing for the 143 is 44-92/86-36 at .024"/.028 clearance, same gross lift at 1.7:1 rocker ratio, whilch is what you listed for the 366, Clem. Power Manual lists the same timing numbers for the 143. I have to believe that the drawing is correct, and the numbers you listed for the 143 are in error or are based on some other checking clearance.

Duke

[Duke W.]

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

Very interesting! Can you provide more data? Also, I'd be interested in HOW they managed to measure running lash at high speed and load.

Were the tests done on iron block and head engines or some other materials combination?

Duke

[Michael H.]

More questions

Duke,

The more I learn about this, the more questions I have. In looking at the numbers for the 143 from the original print and specs with 336/336 deg duration, is it possible that the later numbers were calculated from the revised lash settings? In other words, could the duration shrink from 336 to 302/316 just by using the later .024/.028 lash figures instead of the .020 checking clearance that they probably originally published? If you have the rest of that chart that you posted originally, it would tell us the duration at the later lash figures if we do the math.

Also, is it at all possible that the 65-66 cam was unique and the 67 was a slightly different grind? There was a new cam released in fall of 66 for the 67 model year, part number 3904366, and it has the exact same specs that the later power books show for the 143. Was this the old 143 that was changed to become the new unit for 67 without the rear groove and slightly modified specs? I think the original application was for Chevelle but why would there be a unique part number for it when the 143 would have been the same? The ater service 143's had no rear groove so it would have been identical.

I would really like to hear from some people that have an original 425 HP cam with the rear groove. That would insure that it's a correct for the time assy line installed part.

Also, is the reproduction cam that you mentioned modeled after an original 143 with the groove? Is that why the lash specs are listed at .020/.024?

[Michael H.]

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

Duke,

Still looking for the sheets on running valve lash change but I did find some really interesting things on the way. Found a 1967 GM printing of the HD/Power Manual, probably the only one in existance on the planet today. Some pretty interesting part numbers in there, including the correct 67 only Corvette headers and GM part numbered American racing wheels etc etc. I'll keep looking for the cam sheets.

I'm reasonably sure the running tests were done using cast iron heads and blocks. I wasn't involved in these tests so I don't know how they were able to monitor lash at speed. I do remember that they described the procedure but I don't remember what it was. The engines had to be running at high output and RPM to duplicate the heat generated at the valve because, as you know, the valve head temp would immediately by reduced when the engine was stopped.

I once had a P&G valve gapper that would mount on a valve spring retainer and you could watch the actual lash on it's dial indicator as the engine ran but I never tried to use it at anything over about 800 RPM. Was interesting to watch the lash change through warm up tho...

[Duke W.]

Re: More questions

The "lash point duration" will change with lash, and even though the inlet and exhaust lobes are identical, different lash will yield different timing duration.

I believe GM timing numbers assumed a constant rocker ratio of 1.7:1, which is not correct, but we wont' know the actual rocker ratio behavior through the full range of lift until someone measures it.

In my original post there is enough data to determine duration at any reasonable lash. Just divide the lash by 1.7 to determine lifter rise and read it off the chart. You can use linear interpolation between points to get a reasonably accurate lash point timing and duration.

My take is that the 366 cam is the same as the 143 except for the lack of the rear journal groove given the note on the drawing that the 143 can be ground from the 366 blank. The drawing does not have detailed dimensions other than the lobe profile and there is a note that it is identical to 3856354 except as shown, so the 354 drawing has all the other data besides lobe profile. I don't have the 354 drawing, and I assume it is a base big block hydraulic cam - maybe the 396/325 HP application. I can't imagine that there are OE 143 cams with or WITHOUT the rear groove! They are not functionally interchangeable!

If John Lolli or someone else who has bought the CS-165R cam from Speed Pro checks in they can report on the rear journal configuration. The Speed Pro data sheet I have for the CS-165R makes not mention of it.

Duke

[Craig S.]

Re: More questions

Duke - I have an as of yet uninstalled CS-165R right here......and the rear journal is not grooved, I have to throw it in my lathe to groove it before installing it in my 66 942 block. I also have a 1989 vintage crate LS6 engine from GM, that supposedly has the 4362 solid cam, which was the last incarnation of the 3143 as far as I know. Supposedly, it was not a grooved rear journal, but mine sure was (even though in a later 454 block). I used the procedure in the power book to solder and redrill the rear cam bearing to 0.060" to control oil flow loss. I almost feel like pulling the plug on the back of the block to check the stamping on the end of the cam to see if it says 4362 or 3143......Craig