Ancient history.....

I was looking through one of my old reprint books and found a reference to a Frank Mcgurk SBC camshaft circa 1955... Had a lash of .008 and .015. Then of course the 097 with it's .012 and .018....As I looked through an old Vic Hubbard catalog circa 1961 some time ago there was not a single cam that had a lash over .020.I supose that for normal passenger car use these lash settings were sufficient to deal with the expansion and contractions and a bit of extra cushin .....But apparently under full throttle racing ...especially circle track and supercharged type drag race use they would burn up exhaust valves....Granted the cooling systems were lousy then with low pressure caps and alchol based coolant....and valve materials were not quite as good as todays stuff..

So somewhere around 1960 or so Chrysler did a bunch of testing and found that a wider lash eliminated the burned up valve problems....so all thier Hi Perf Solid lifter V8 stuff had .028/.032 cold settings..... By 1964 Chevy got into the act with the 30-30 Duntov... Ford's 427 got .025 and .018 on the 289... The BBC got .024/.028 and later the LT-1 with .024/.030 ect ect....

So by the mid 60's both the OE and most aftermarket cams guys had embraced the wide lash theory and many of these problems went away....

I'm looking through a 1968 Vic Hubbard catalog and I'm glancing at the listing of Engle Cams that they had available then....Thier flat tappets had lashs of .018/.024/.026 and .028... pretty much like everyone else did during those years...

But Engles Rollers of the day were back to the 50's type lash....with not a single cam listed here in that catalog with a lash over .020..... here are a few examples for various engines... all rollers...

SBC.....
Spec Circle track...

76-32...... 32-76 ... 288 adv .500 lift .012-.014
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Chevy 409
Long duration comp...

92-46...... 46-92 ... 318 adv .500 lift .013-.015
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Chrys 392 Hemi...
Blown Fuel....

No timimg specs...... 350 adv .525 lift .010-.010
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Chrys 426 Hemi...
Blown comp....

No timing spec....... 325 adv .560 lift .020-.020
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Certinally by the 70's these lobes were all gone and replaced by newer ones....I guess that the tight lash expiriment didnt work ??
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By the 1980's Crane introduced for a couple years thier " TLF " series of lobes that they offered in thier catalogs for only a couple years then dropped... these had lash settings of .010-.012.
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Now a days there are lots of " tight lash " lobes with lashes from .012-.018 available....


Just out of historical curiocity <SP> and from a designers point of view do you think it is the availability of better materials such as better valve materials that dont expand as much as the older material and the better cooling systems that have allowed the return to a tighter lash type cams... or is it a better understanding or different interprtation of what a clearence ramp should do.... or with more modern grinders and computor software that has allowed this return...

Granted that the ramps are can be very different in design... but...

Just curious what your take on how the lash settings have gone full circle so to speak....

Al,

There are 2 different factors at play in current designs.
First----Lash is to allow for expansion of valve train components. Hydraulic lifters do this with their plunger. Much better materials and better attention to head cooling keeps valve expansion to very low levels, compared to the 60s. I have never had any trouble with my .014" lash NR inverted flank rollers as far as heat expansion is concerned. One thing is these are all short-duration cams, with more time on the seat to allow heat transfer. Those old long-duration factory racing cams had less time for heat transfer, etc, etc, etc.
Second--- The lash also depends on how fast the designer wants to open or shut the valves, which is a critical part of cam design, possibly the main part..... It is sometimes physically impossible to open a valve very fast at a given ramp height: You just don't have enough time to accelerate the lifter/valve before opening the valve. Imagine trying to get a Pro-Stocker to run 200 MPH in the 1/8th, something currently easy to do in the 1/4th.
Those Cranes with the .012" valve lash were symmetrical Cam Dynamics designs, with brutal accelerations and opening/closing rates, extremely hard on parts.....
The 'Old Masters' didn't need wide valve lash. They didn't really open the valves very fast, they didn't have a lot of duration(except for those Herbert/Howard/Engle 380° to 400° Fueler cams...), and they didn't make a lot of horsepower. We have engines as daily street-drivers that could have won the Nationals in the 1950s......

Hope this answers your questions, if not, ask away.....

UDHarold

Thank You Harold...

So as far as the competition stuff goes the trend in the early 60's twords wider lash was as much to compensate for poor materials , cooling systems and the lack of good valve springs....

Then not to excede any given limit on how fast then tappet can be traveling at the point of contact where the lash it taken up and everythings stacks solid... the wide lash idea pretty much went with a long CV ramp to allow the extra wide lash.... but if a given base circle diameter if you use a long gentle CV ramp it seems that in order to not be real abusive the area right after the CV ramp couldnt be real steep or fast acting... where the blend from the CV curve into the initial flank curve comes together.....thus perhaps creating a long duration easy on parts piece.... IE acceleration and jerk ect numbers on the milder side ?... makes for easy on parts with lower tappet velocities.....

But I impagine some felt the need to expiriment with the same wide lash but with a shorter fast ramp but much higher contact velocities and slam bang action ??.... Perhaps the old CD with .035 intake lash ??


If you went with a shorter steeper faster low lash ramp since the curve is perhaps steeper off the BC anyway within reason it's easier to continue with a faster tappet velocity in that merge area from the ramp to the flank without slamming parts togther ?

Is that the general idea or am I way off base ?

Al,

The ramp velocity, acceration, and jerk, all at the junction of ramp and flank, govern how the flank's curve acts. Low numbers, slow reaction, fast numbers, fast reaction, too fast numbers, too fast destruction.
When I went to
General Kinetics in 1974, Don Teweles, the owner, had some cams(He was famous for them) with .033-.036 lash for small blocks, .035-.040 lash for big blocks. The wider of the two made the best power, but tore everything up.
I took my trusty TI and designed him a .026 lash ramp at opened the valve at the EXACT SAME velocity, but a lower acceleration rate. He pointed out that he and his partner, Herb, both had M.E.s from Wayne State University, and I was a Jr College drop-out.
That ramp is still available now from Comp Cams on some of their catalog cams, ones that I designed in the late 70s and are still in use, like their 23-758-9, 440W 306B-8, the 3rd unsymmetrical cam that I designed.
In the mid-80s, Pontiac put one of my 302/310Rs in a 151 Fiero-based sports car. It only lasted 8 hours and 15 minutes before a tip came off a valve, 5-speed, 8500 rpm..... I do not make a cam profile any faster off the seat, and I have not noticed any from other cam companies.
Opening ramps have never minded how fast they're opened, but closing ramps are very sensitive. Too fast, and they beat up seats and guides, and destroy springs. Too slow, and the engine is not as responsive to the throttle, and down on overall torque. We could make cams that let springs live for 5 years, but the 1st company who has one that lives for only 4 years will start winning everything, and there you go......
There is also merit to tight-lash, in that the pushrod hits the same spot in the rocker, reducing shimmy, which effects spring life.....
Very wide-lash cams cause the pushrod to hit all over the place, and the resulting vibrations/shimmy is translated to the valve and spring.

UDHarold