Valve lash recommendations for forced induction.

[Sam Powell]

The general rule on NA slants is to set the lash wider that the book calls for. How does forced induction effect this thinking? I know the idea with the larger lash is to allow the cylinders to build more pressure by reducing overlap. Is this still appropriate for the forced induction engine? What settings are you guys out there running with turbos and superchargers?

Sam

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[stuntmanmike]

I'm no expert on the mater and anything I say should be taken with a grain of salt so here goes...
The lash settings as far as I'm aware are to compensate for the thermal expansion of the valves due to heat dissipation of the ignition cycle when hot gasses in the combustion chamber dissipate into the surroundings (eg pistons, valves, cylinder head, cylinder wall, etc).
Some racing applications may have a camshaft of varying lift/duration/lobe centreline and may require lash settings recommended by the cam manufacturer that differ to the work shop manual.
Playing with the lash settings boils down to valvetrain reliability/performance.
I feel that too much lash delays the cam opening the valve by a millionth of a second and does not really provide any true tunability other then calibrating the minimum gap required between the pushrod and the lifter + rocker arm.
Others can correct me if this is nonsense.

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[Shaker223]

Something like .010" intake .018" exhaust.

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[turbo66valiant]

.010 intake and .015 exhaust cold and the car picked up a bit.

Later
Ryan

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[Dart270]

I doubt it will make a significant difference, assuming you want the car to idle well at high temps and be "streetable." Loosening does not sound like a good idea either.

Lou

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[Doc]

There is no "one right answer" for this...
The lash clearance is pretty much set by the cam's lobe profile(s)... it's "lash ramp" area.
This is a specially engineered area at the base of the lobes were the lash clearance is slowly "taken-up", after that, the acceleration increases quickly. Use too much clearance and the lifter never touches this area and hits the flank of the lobe (faster acceleration area) with the noted "tick" sound.
Tighten-up too much and the valve never fully seats when "red hot" and at full expansion at WOT. (exhaust valves)
The only reason to "play" with the setting is to effect the Intake Closing (IC) point. An engine does not make compression until the intake valve fully closes so this is an important piece of the DCR calculation.
Overlap is the #2 event the last setting(s) influance.

Long story short, the amount of lash adjustability we have on any cam is limited and tied to the individual lobe profile(s) used. (the lobe shape ground into the cam)
You can have a small impact on the DCR with lash changes, it's not a lot but sometimes it's enough to help-out a "mis-matched" combination of parts.
But in the end, it is best to have a well matched combination of parts, the correct cam to compression relationship for the intended use... and to keep the lash setting(s) close to the intended design of the cam's lash ramp.
DD