# cam math by Randy GT350HR

September 2 2013 at 12:22 PM

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Response to Re: Is degreeing REALLY necessary?

original post here,

.050 lift figures are taken off of the proper style lifter for the cam or the end of the pushrod without the rocker installed to make it easier to setup the indicator. Seat to seat duration can be checked at the retainer and that's done with the correct lash and spring.

Opening and closing events can be calculated IF you know to things, 1 duration @.050 and 2 the actual lobe seperation ground into the cam.
Example, 240 @ .050 w/110 lsa would be I.O.=10*BTDC-I.C.=50* ABDC.

This is because the IO and IC + 180* has to = the total duration @ .050. A symetrical cam would flip flop the numbers and read EO @ 50*BBDC and EC @ 10*ATDC. So how the hell did I figure the 10 and 50? Ok here's a quick formula-------
IO=1/2 duration @ .050 minus lsa. 1/2 of 240=120-110=10
IC=duration @ .050 minus IO minus 180. 240-10-180=50 (10+180+50=240)
EC is the same as IO
EO is the same as IC

So now your a math scholar right? What about cam advance and retard and those damn dual pattern cams? Let's use the simple single pattern example with 4* advance. Advancing the cam changes the INSTALLED LSA to 106 this causes the exhaust to READ 114. WHY? Because the lobes are ground into the cam blank and will ALWAYS stay 110 on our theoretical cam, or whatever the grinder chooses on your blank. UNLESS you have dual overhead cams that's the fact.
So using the formula,
IO 240/2=120-106=14
IC=240-14-180=46
EC=240/2=120 114=6
EO=240-6-180=54
Now we're really getting somewhere!

Final exam is a dual pattern, advanced cam on a tight lsa (JOE'S favorite) 278-286 on 106 lsa +4* adv.
IO 278/2=139-102=37
IC=278-37-180=61
EC=286/2=143-110=33
EO=286-33-180=73
Again because the cam is advanced 4* the int is "in @ 102 s the exhaust has to be figured @ 110 since 102+110=212 divided by 2=106 the actual LSA ground on the cam...

Randy

there'll be phantoms, there'll be fires on the road... and the white man dancing

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