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#1304748 Tue Mar 26 2019 07:12 PM
Joined: Dec 2008
Posts: 1,708
panic Offline OP
I've been asked "I have the valve opening & closing points of a cam, but how do I calculate the intake and exhaust centerlines, and the LSA or lobe center angle?"
Using the McGurk #18 data:

For the intake lobe: (intake closing - intake opening) / 2, then add 90 degrees
Example for 10-55 timing: 55 - 10 = 45, divide by 2 = 22.5, add 90 = 112.5 degrees ATDC

For the exhaust lobe: (exhaust opening - exhaust closing) / 2, then add 90 degrees
Example for 72-32 timing: 72 - 32 = 40, divide by 2 = 20, add 90 = 110 degrees BTDC

LSA: (intake CL + exhaust CL) / 2
112.5 + 110 = 222.5 / 2 = 111.25 degrees

I just wrote an Excel spreadsheet that calculates this automatically from your input, e-mail me for a free copy: jdiamond (at)

panic #1304857 Wed Mar 27 2019 01:55 PM
Joined: Jan 2013
Posts: 1,679
Good school Panic, only thing missing is how to determine cam advance ground into the cam.

1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration
panic #1304861 Wed Mar 27 2019 02:08 PM
Joined: Feb 2000
Posts: 4,488
Use this site, to build your custom degree wheel. Just add the data from the cam card, and you have a degree wheel just for that cam!

panic #1304871 Wed Mar 27 2019 03:22 PM
Joined: Feb 2004
Posts: 24,744
Kettle Custodian (pot stirrer)
Thanks, Joe! Added to my "favorites" list!

BTW- - - -has anybody else thought about drilling and tapping the snout of a stovebolt cam and using a harmonic balancer installer to push the timing gear on?

"Freedom's just another word for nothing left to lose"
Kris Kristofferson

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self.
Ernest Hemingway

panic #1304889 Wed Mar 27 2019 05:10 PM
Joined: Dec 2008
Posts: 1,708
panic Offline OP
Typically the suggested (or already selected) intake lobe center is 4 degrees advanced from the CL.
More advance closes up the intake valve-to-piston clearance, more retard closes up the exhaust-valve-to-piston clearance.
Overlap and LSA are constants (at least, in single cam engines!).

Moderated by  Justhorsenround, KCMongo 

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