FrontEngineDragsters.Org Forum

Technical => Matt Shaff's Engine Shop => Topic started by: wideopen231 on February 07, 2018, 07:30:07 AM

Title: duration change with lash change
Post by: wideopen231 on February 07, 2018, 07:30:07 AM

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.

Title: Re: duration change with lash change
Post by: Roger on February 07, 2018, 11:43:03 AM

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."

Title: Re: duration change with lash change
Post by: wideopen231 on February 07, 2018, 02:44:17 PM

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 .

Title: Re: duration change with lash change
Post by: THE FRENCHTOWN FLYER on February 07, 2018, 04:26:17 PM

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.

Title: Re: duration change with lash change
Post by: wideopen231 on February 07, 2018, 04:36:56 PM

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.

Title: Re: duration change with lash change
Post by: THE FRENCHTOWN FLYER on February 07, 2018, 05:15:26 PM

I also run tighter than spec.

I have friends who set their lash, ran the engine up on the dyno @ WOT to simulate race conditions, and quickly shut it down and checked the lash.
It grew by .030". This on an iron block, billet aluminum head. Yikes

Title: Re: duration change with lash change
Post by: wideopen231 on February 07, 2018, 08:08:45 PM

.030 sounds like other issues. I have run our Top alcohol stuff up to 200 and checked valves.Normal was .008 to .015 growth on aluminum block and billet heads.