> And you haven't given *any* "proof" that valve lash is significant to valve
"...be careful because the valve lash is the "Cooling Time" that the valve needs on each cycle. If you have no lash you take a chance of burning the valves." http://www.centuryperformance.com/valveadjustment.htm
"...if the valve clearance is insufficient, the valves will open early and close late, resulting in inadequate cooling and compression loss." '70-'71 beetle and kg bentley manual.
the purpose of this article is to address the controversy regarding valve lash settings when running high performance chromoly pushrods in the vw type 1 engine.
the acvw type 1 engine came from the factory with aluminum pushrods that were designed to match the expansion rate of the rest of the engine when it is at operating temperature. this fact is stated on page 26 of the '66-'69 bentley: "...(valve) clearance increases when the engine first warms up, but returns to the original measurement when the engine becomes hot."
when the hi-po engine builder substitutes chromoly pushrods for the factory aluminum pushrods, he introduces a problem with valve lash, because the chromoly pushrods expand less at engine operating temperature. james lindsey, in his vintagvw post, gave us some math that showed a guesstimation of the lash difference: "...valve clearances when hot will be 0.010" greater with chromoly pushrods..."
therefore, many hi-po engine builders feel they have to make a choice between either setting the valves at the factory-recommended cold .006" lash, or trying to decrease this lash setting to compensate for the "shorter" chromoly pushrods... one approach many of them take is to tighten the cold valve lash until there is zero clearance, but you can still rotate the pushrod between your fingers. others will set the cold valve lash at .001"-.002", all of which decreases the hot running valve lash.
it can only be guessed that some of the thought processes behind zero valve lash settings revolve around the idea that the engine builder will see more horsepower due to more valve lift. this concept is known as "tuning the cam" with valve lash, and for race cars it may be valid. in fact, there are many tracks/organizations that specify the exact valve lash you must run for certain classes, so that you don't have a perceived horsepower advantage over your competitor... but what if that required lash is insufficient?:
"...The trend in recent years has been toward greater operating clearance for high performance (solid lifter) camshafts....little change in clearance occurs between cold and hot in the modern (water-cooled) OHV engine. Starting the engine from cold, the valve lash will vary considerably during the warm-up period, but when the engine is fully warmed up and temperature stabilized, the clearance will be within .002" to .003" of the cold setting....most camshafts used clearances of .012" to .014", which were assumed to be sufficient to accommodate all variations that take place in the valvetrain of the OHV engine."
"Designs utilizing these small clearances performed adequately for passenger car use, but consistently burned exhaust valves when used for extended full-throttle operation. To determine the cause, dynamometer tests were conducted utilizing a specially designed machine that could measure valve lash with the engine running at high RPM, at full throttle, and under load. These tests indicated that the exhaust valve stem would expand sufficiently to eliminate all of the valve lash and hold the valve off the seat...."
"...it follows that as soon as the exhaust valve fails to seat properly, heat buildup increases at an accelerated rate. This in turn aggravates the valve stem growth, causes pre-ignition, valve burning and can contribute to ultimate engine failure. We have found that a valve lash of .030" to .032" is sufficient to prevent the exhaust valve from being held off the seat in the most severe competition applications." http://www.mrgasket.com/camsence.htm
so we see that a chromoly pushrod with hot-running valve lash estimated to be .0016", or even .0020", is well under those recommendations... how hard will you be driving your hi-po vw engine? is that extra chromoly valve lash really a problem, or is it a benefit?
one problem for zero-lash proponents is the risk of overheating the exhaust valve at cold engine startup. to begin with, we know that the exhaust valve gets much hotter than the intake valve during normal engine operation:
"...Exhaust valves rely on the head casting to transfer the majority of the valve temperature...Exhaust valves usually have to endure temperatures that are generally in the range of 1000-1500° F." http://www.aera.org/tech/tb1733.htm
"...through the use of "temperature check" valves, we have accumulated data on the actual operating temperatures experienced by the (exhaust)valve. these temperatures are not the same as exhaust gas temperatures (egt's). valves typically run 150-250 degrees(f) less than egt... valve temperature can vary greatly depending on the type of fuel, combustion chamber design, spark advance, and compression ratio while the egt remains constant." http://www.martelbros.com/manley/p103.htm
since the exhaust valve normally runs very hot, it will also heat up much quicker than the intake valve at engine startup... in other words, the length of the valve stem will immediately increase with cold engine startup, taking up part or all of the cold valve lash setting... those high temperatures mean that it is more critical to maintain the valve lash for the exhaust valve, in order to prevent failure of the valve and it's associated seal with the valve seat.
for some engines, like the high-compression diesel, this problem requires special procedures:
"...Exhaust valves typically have a greater lash than intakes due to greater expansion as the engine warms up." http://www.ccjmagazine.com/Deslglos.htm
what all this indicates is that if the exhaust valve is hanging open during cold engine startup, it is getting very hot very quickly, because there is no way for it to cool off:
"...valve operating temperatures can reach 1500°F under heavy load...about 70% of heat transferred from a valve goes out through the seat contact area" - very good theoretical discussion. http://www.sbintl.com/techart2.htm
"...insufficient valve lash can also prevent good valve-to-seat contact and allow excessive heat to build up in the valves...Eventually there’s little or no lash left and the valve makes poor contact with the seat, overheats and burns...maintaining adequate stem-to-quide clearance and valvelash...will go a long ways towards preventing repeat valve failures and comebacks." http://members.aol.com/carleyware/library/ar1192.htm
> "seat time". None.
it is true that most of these links refer to insufficient valve lash at normal engine operating temperatures, where the greatest damage will occur... in the case of our zero-lash proponent, the main concern is at cold engine startup, because the exhaust valve will be fully hot in a matter of seconds... and if the lash isn't enough to allow for adequate heat sinking, the exhaust valve will be stressed.
it is normal for vw valves to tighten up slightly over time, but our zero-lash engine builder must monitor his valve lash far more frequently than the usual 3,000 mile interval... especially if he drives the engine hard. if his cold-start zero-lash adjustment is the slightest bit tight during engine startup, the exhaust valve would be left hanging open, with no heat-sinking capability, until the rest of the engine expanded enough to restore the lost valve lash. is this potential damage to engine parts really worth the risk?
while doing the research for this article, the author visited dozens of automotive/truck/airplane/go cart/etc. websites... but nowhere was there any mention of anyone setting their solid-lifter valves at zero lash, or even .001"-.002" lash, for that matter... it appears that the vw zero/low-lash proponent is unique to our 4-cylinder, air-cooled world? We did recently receive email indicating that some Harley motorcycle engines had some of the same issues being discussed here.
more links of interest on the subject of valve lash:
"...Increased valve lash is equivalent to closing the valves sooner--"milding" the cam timing. Increase in valve noise is minimal. Valves will run cooler." "...Increased valve lash and advanced timing (9 degrees) netted an average torque increase of 3.1 and an average horsepower increase of 4 over stock valve lash and stock timing (6 degrees)." http://www.ibmwr.org/tech/r11tech/R11Manual/r11vallsh.htm http://www.ibmwr.org/tech/r11tech/R11Manual/r11results.htm
"...The data show an increase(2%-10%) in maximum permissible indicated mean effective pressure due to cooling the valves(with water or sodium)" - nasa study, 1942(!), presented here just for fun! http://naca.larc.nasa.gov/reports/1942/naca-tn-861/
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