Rocker ratios: I'm doing it wrong (?)

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clutchdust

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So I'm debating between stepping up a cam size or two when I do the head swap, or just switching to 1.6 ratio rockers.
What I have now is a Comp Cam rated at 276/282 with .502/.510 lift. I'm thinking of going to either their 282 or 288 cam when I swap out the old iron 2.02 heads for these AFR 2.05" (210cfm) heads. (Engine is +.030" 350, domed hyper pistons, shooting for a CR of just about 10:1)
But I figured it should be pretty easy to figure out what the old cam would look like with 1.6 rockers. Now the Comp website doesn't list the actual cam specs, just "advertised", or in other words, total based on 1.5 ratio rockers.
So me, being the mathematical equivalent to Barack Obama on how to run a business, I think I should be able to figure out what the actual cam specs are by taking the advertised number of 276 and dividing by 1.5. My thinking is that should give me the actual cam duration, comes out to 184*. Then if I multiply that by 1.6, I'm thinking it should give me the "new" duration, but that comes up to 294*.
That seems awfully high.
I can't seem to buy that the duration would change that much. Now lift seems to be much more reasonable. Using the same process my lift goes from .502" with the 1.5 ratio to .535" with the 1.6 ratio.
So I'm wondering if I'm doing this wrong. Does duration change with ratio? Or just lift? Doesn't seem like it can change that much.
If that's the case, I'm looking at just using the old cam/rockers with the new heads, or just changing the rockers, or changing the cam.
 
I think the effective duration will only change very slightly, if at all. I believe that stated duration is a function of lift at .05 inches and higher (can't remember if the .05 is right). so the change from 1.52 to 1.6 would mean that the threshold is met slightly sooner and later in the opening and closing. The cam spec wouldn't change though because it will assume a 1.52 stock rocker.
 
I think Mike has it pretty close- rocker ratio mostly affects lift, but when lift starts earlier and ends later the effective duration gets changed a bit too. How to figure the exact duration numbers I don't know. And I think most lift numbers are at the lifter, so you'd need to multiply that by the ratio for true lift. Maybe I'm wrong, but that's how I see it all working.
 
My valve springs are good for .575" lift but the camshafts that are close to that number have significant more duration than I want... my cam has 230/236 dur (advertized at .050"lift) and .510"/.520" valve lift with 1.5 ratio rockers, with 1.6 rockers that would be .544" and .554"
The 1.6 rockers are on my "to do" list :cool:
 
According to CompCams and one other cam company (name slips my mind) in mag interviews both say effective duration increases about 3 degrees when moving from 1.5s to 1.6s.

To be specific would require using a positive stop indicator tool, a degree wheel, dial indicator, etc.

Calculating valve lift is a lot easier though since it's simple division and multiplication. If you know the lobe lift, simply multiply it by the new rocker arm ratio.

If you don't have the lobe lift but have the advertised valve lift, simply divide the advertised valve lift by the rocker ratio used to get that lift. That'll give you the lobe lift. Now all you need to do is multiply that lobe lift by the new rocker arm ratio. BINGO!

Hope this helps.

Jake
 
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