Corrected Cam Specs with Different Ratio Rockers and Valve Lash

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Corrected Cam Specs with Different Ratio Rockers and Valve Lash

Post by BlackChevelleSS on Sun Dec 21, 2008 4:30 am

I found out how to calculate gains in increasing rocker ratios.

You take the lobe lift of your cam and multiply it by the rocker ratio
Also increasing Rocker ratio only increases cam lift, it does not affect the duration.

Heres an example
My old cam
0.540 lift at valve via cam card with 1.7 ratio rockers
Cam card states the Lobe lift is 0.3176

1.7 Ratio Rockers = 0.3176 X 1.7 = 0.53992 Valve lift
1.8 Ratio Rockers = 0.3176 X 1.8 = 0.57168 Valve lift

Thats an increase of 0.03176 Valve Lift

Valve Lift with Valve Lash
Now if you run a solid cam and have valve lash you will have to subtract the lash from the lift to get the corrected valve lift.
But Valve lash does affect the duration of the cam, so those will also have to be taken into account.
Does anyone know if you are supposed to subtract this lash from the lobe lift or the valve lift?
Car Craft says that you subtract the valve lash from the valve lift.
So if you use the same 0.540 lift cam with 1.7 ratio rockers and a 0.016 valve lash,
0.53992- 0.016 = 0.5239
Your Corrected valve lift will be 0.524"

Duration with valve lash
To calculate your duration with valve lash you subtract 1/2 degree of duration per 0.001 inch of valve lash
So if your duration is 304* and you have 0.016 of valve lash your corrected duration will be 296*



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Re: Corrected Cam Specs with Different Ratio Rockers and Valve Lash

Post by jonny o on Wed Jan 21, 2009 2:29 pm

Don't forget to mock up your new rockers and check your valve tip for the correct length pushrods.
And be careful with big lift cams and higher ratio rockers... always check for piston clearance.

jonny o
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