Technical > Matt Shaff's Engine Shop

duration change with lash change

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wideopen231:
Anyone know of rule of thumb as to how much duration change with lash change.Lets say a 290* @ .050 cam specs .024 lash and you go to .014 lash netting .010 tighter how much duration would you gain? As said rule of thumb is fine since not every cam will be the same.

Roger:
From Comp Cams:

"The general rule of thumb when adjusting valve lash is that 0.004” of adjustment from the valve lash specified in the cam card equates to a 2° difference in duration when measured at 0.050” lift."

wideopen231:
Thanks Roger,

 That would mean if I loosed mine up .012 I should loose 6*,which is not problem since I normal run my stuff tight around .010 or so,thats cold. most specs are closer to .024 anyway. Want to drop exhaust side on cam I have to test a theory .

THE FRENCHTOWN FLYER:
Going significantly looser might move the take-up point past the "clearance ramp" on the lobe and make for a harsh impact on the lobe. What constitutes "significantly" I can't say.

wideopen231:
agree that too loose abuses valve train.As stated since I run lash a lot tighter than spec,loosing up just gets me closer to spec. I have run .004.Two reason tighter has to be better on valve train and I wanted as much duration as I could get at the time due to lousy track and hoping to kill little bottomend power.

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